How do I check if a string is a number float

Question

What is the best possible way to check if a string can be represented as a number in Python    The function I currently have right now is   def is number s       try          float s          return True     except ValueError          return False   Which  not only is ugly and slow  seems clunky  However I haven t found a better method because calling float in the main function is even worse

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how about this    3 14  replace        1  isdigit     which will return true only if there is one or no     in the string of digits    3 14 5  replace        1  isdigit     will return false  edit  just saw another comment     adding a  replace badstuff    maxnum badstuff  for other cases can be done  if you are passing salt and not arbitrary condiments  ref xkcd 974  this will do fine  P

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import re def is number num       pattern   re compile r   -    -0-9  d    d   -       0-9  d         result   pattern match num      if result          return True     else          return False     gt  gt  gt   is number  1   True   gt  gt  gt   is number  111   True   gt  gt  gt   is number  11 1   True   gt  gt  gt   is number  -11 1   True   gt  gt  gt   is number  inf   False   gt  gt  gt   is number  -inf   False

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str isnumeric       Return True if all characters in the string are numeric characters    and there is at least one character  False otherwise  Numeric   characters include digit characters  and all characters that have the   Unicode numeric value property  e g  U 2155  VULGAR FRACTION ONE   FIFTH  Formally  numeric characters are those with the property value   Numeric Type Digit  Numeric Type Decimal or Numeric Type Numeric    str isdecimal       Return True if all characters in the string are decimal characters and   there is at least one character  False otherwise  Decimal characters   are those that can be used to form numbers in base 10  e g  U 0660    ARABIC-INDIC DIGIT ZERO  Formally a decimal character is a character   in the Unicode General Category    Nd       Both available for string types from Python 3 0

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Just Mimic C   In C  there are two different functions that handle parsing of scalar values    Float Parse   Float TryParse     float parse     def parse string       try          return float string      except Exception          throw TypeError   Note  If you re wondering why I changed the exception to a TypeError  here s the documentation   float try parse     def try parse string  fail None       try          return float string      except Exception          return fail    Note  You don t want to return the boolean  False  because that s still a value type  None is better because it indicates failure  Of course  if you want something different you can change the fail parameter to whatever you want   To extend float to include the  parse    and  try parse    you ll need to monkeypatch the  float  class to add these methods   If you want respect pre-existing functions the code should be something like   def monkey patch        if  hasattr float   parse             float parse   parse     if  hasattr float   try parse             float try parse   try parse   SideNote  I personally prefer to call it Monkey Punching because it feels like I m abusing the language when I do this but YMMV   Usage   float parse  giggity      throws TypeException float parse  54 3      returns the scalar value 54 3 float tryParse  twank      returns None float tryParse  32 2      returns the scalar value 32 2   And the great Sage Pythonas said to the Holy See Sharpisus   Anything you can do I can do better  I can do anything better than you

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Sorry for the Zombie thread post - just wanted to round out the code for completeness      is number   function - Uses re   regex library   Should handle all normal and complex numbers   Does not accept trailing spaces     Note  accepts both engineering  quot j quot  and math  quot i quot  but only the imaginary part  quot  bi quot  of a complex number a bi   Also accepts inf or NaN   Thanks to the earlier responders for most the regex fu  import re  ISNUM REGEXP   re compile r   -     0-9    0-9     0-9     eE  -    0-9     ij       def is number str    change order if you have a lot of NaN or inf to parse     if ISNUM REGEXP match str  or str     quot NaN quot  or str     quot inf quot            return True      else          return False   A couple test numbers    42 42e-42j   -42 42E 42i  print  Is it a number    is number input  Gimme any number        Gimme any number   42 42e-42j Is it a number  True

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The input may be as follows   a  50  b 50 c 50 1 d  50 1     1-General input   The input of this function can be everything   Finds whether the given variable is numeric  Numeric strings consist of optional sign  any number of digits  optional decimal part and optional exponential part  Thus  0123 45e6 is a valid numeric value  Hexadecimal  e g  0xf4c3b00c  and binary  e g  0b10100111001  notation is not allowed   is numeric function  import ast import numbers               def is numeric obj       if isinstance obj  numbers Number           return True     elif isinstance obj  str           nodes   list ast walk ast parse obj    1           if not isinstance nodes 0   ast Expr               return False         if not isinstance nodes -1   ast Num               return False         nodes   nodes 1 -1          for i in range len nodes                 if used   or - in digit               if i   2    0                  if not isinstance nodes i   ast UnaryOp                       return False             else                  if not isinstance nodes i    ast USub  ast UAdd                        return False         return True     else          return False   test    gt  gt  gt  is numeric  54   True  gt  gt  gt  is numeric  54 545   True  gt  gt  gt  is numeric  0x45   True   is float function  Finds whether the given variable is float  float strings consist of optional sign  any number of digits       import ast  def is float obj       if isinstance obj  float           return True     if isinstance obj  int           return False     elif isinstance obj  str           nodes   list ast walk ast parse obj    1           if not isinstance nodes 0   ast Expr               return False         if not isinstance nodes -1   ast Num               return False         if not isinstance nodes -1  n  float               return False         nodes   nodes 1 -1          for i in range len nodes                if i   2    0                  if not isinstance nodes i   ast UnaryOp                       return False             else                  if not isinstance nodes i    ast USub  ast UAdd                        return False         return True     else          return False   test    gt  gt  gt  is float  5 4   True  gt  gt  gt  is float  5   False  gt  gt  gt  is float 5  False  gt  gt  gt  is float  5   False  gt  gt  gt  is float   5 4   True   what is ast     2- If you are confident that the variable content is String   use str isdigit   method   gt  gt  gt  a 454  gt  gt  gt  a isdigit   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError   int  object has no attribute  isdigit   gt  gt  gt  a  454   gt  gt  gt  a isdigit   True     3-Numerical input   detect int value    gt  gt  gt  isinstance  54   int  False  gt  gt  gt  isinstance 54  int  True  gt  gt  gt     detect float    gt  gt  gt  isinstance  45 1   float  False  gt  gt  gt  isinstance 45 1  float  True

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In case you are looking for parsing  positive  unsigned  integers instead of floats  you can use the isdigit   function for string objects    gt  gt  gt  a    03523   gt  gt  gt  a isdigit   True  gt  gt  gt  b    963spam   gt  gt  gt  b isdigit   False   String Methods - isdigit    Python2  Python3  There s also something on Unicode strings  which I m not too familiar with Unicode - Is decimal decimal

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This code handles the exponents  floats  and integers  wihtout using regex   return True if str1 lstrip  -   replace        1  isdigit   or float str1  else False

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I wanted to see which method is fastest  Overall the best and most consistent results were given by the check replace function  The fastest results were given by the check exception function  but only if there was no exception fired - meaning its code is the most efficient  but the overhead of throwing an exception is quite large   Please note that checking for a successful cast is the only method which is accurate  for example  this works with check exception but the other two test functions will return False for a valid float   huge number   float  1e 100     Here is the benchmark code   import time  re  random  string  ITERATIONS   10000000  class Timer          def   enter   self           self start   time clock           return self     def   exit   self   args           self end   time clock           self interval   self end - self start  def check regexp x       return re compile    d     d     match x  is not None  def check replace x       return x replace        1  isdigit    def check exception s       try          float s          return True     except ValueError          return False  to check    check regexp  check replace  check exception   print  preparing data      good numbers         str random random     random random         for x in range ITERATIONS    bad numbers          x for x in good numbers   strings            join random choice string ascii uppercase   string digits  for   in range random randint 1 10        for x in range ITERATIONS    print  running test      for func in to check      with Timer   as t          for x in good numbers              res   func x      print   s with good floats   s     func   name    t interval       with Timer   as t          for x in bad numbers              res   func x      print   s with bad floats   s     func   name    t interval       with Timer   as t          for x in strings              res   func x      print   s with strings   s     func   name    t interval     Here are the results with Python 2 7 10 on a 2017 MacBook Pro 13   check regexp with good floats  12 688639 check regexp with bad floats  11 624862 check regexp with strings  11 349414 check replace with good floats  4 419841 check replace with bad floats  4 294909 check replace with strings  4 086358 check exception with good floats  3 276668 check exception with bad floats  13 843092 check exception with strings  15 786169   Here are the results with Python 3 6 5 on a 2017 MacBook Pro 13   check regexp with good floats  13 472906000000009 check regexp with bad floats  12 977665000000016 check regexp with strings  12 417542999999995 check replace with good floats  6 011045999999993 check replace with bad floats  4 849356 check replace with strings  4 282754000000011 check exception with good floats  6 039081999999979 check exception with bad floats  9 322753000000006 check exception with strings  9 952595000000002   Here are the results with PyPy 2 7 13 on a 2017 MacBook Pro 13   check regexp with good floats  2 693217 check regexp with bad floats  2 744819 check regexp with strings  2 532414 check replace with good floats  0 604367 check replace with bad floats  0 538169 check replace with strings  0 598664 check exception with good floats  1 944103 check exception with bad floats  2 449182 check exception with strings  2 200056

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The input may be as follows   a  50  b 50 c 50 1 d  50 1     1-General input   The input of this function can be everything   Finds whether the given variable is numeric  Numeric strings consist of optional sign  any number of digits  optional decimal part and optional exponential part  Thus  0123 45e6 is a valid numeric value  Hexadecimal  e g  0xf4c3b00c  and binary  e g  0b10100111001  notation is not allowed   is numeric function  import ast import numbers               def is numeric obj       if isinstance obj  numbers Number           return True     elif isinstance obj  str           nodes   list ast walk ast parse obj    1           if not isinstance nodes 0   ast Expr               return False         if not isinstance nodes -1   ast Num               return False         nodes   nodes 1 -1          for i in range len nodes                 if used   or - in digit               if i   2    0                  if not isinstance nodes i   ast UnaryOp                       return False             else                  if not isinstance nodes i    ast USub  ast UAdd                        return False         return True     else          return False   test    gt  gt  gt  is numeric  54   True  gt  gt  gt  is numeric  54 545   True  gt  gt  gt  is numeric  0x45   True   is float function  Finds whether the given variable is float  float strings consist of optional sign  any number of digits       import ast  def is float obj       if isinstance obj  float           return True     if isinstance obj  int           return False     elif isinstance obj  str           nodes   list ast walk ast parse obj    1           if not isinstance nodes 0   ast Expr               return False         if not isinstance nodes -1   ast Num               return False         if not isinstance nodes -1  n  float               return False         nodes   nodes 1 -1          for i in range len nodes                if i   2    0                  if not isinstance nodes i   ast UnaryOp                       return False             else                  if not isinstance nodes i    ast USub  ast UAdd                        return False         return True     else          return False   test    gt  gt  gt  is float  5 4   True  gt  gt  gt  is float  5   False  gt  gt  gt  is float 5  False  gt  gt  gt  is float  5   False  gt  gt  gt  is float   5 4   True   what is ast     2- If you are confident that the variable content is String   use str isdigit   method   gt  gt  gt  a 454  gt  gt  gt  a isdigit   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError   int  object has no attribute  isdigit   gt  gt  gt  a  454   gt  gt  gt  a isdigit   True     3-Numerical input   detect int value    gt  gt  gt  isinstance  54   int  False  gt  gt  gt  isinstance 54  int  True  gt  gt  gt     detect float    gt  gt  gt  isinstance  45 1   float  False  gt  gt  gt  isinstance 45 1  float  True

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Just Mimic C   In C  there are two different functions that handle parsing of scalar values    Float Parse   Float TryParse     float parse     def parse string       try          return float string      except Exception          throw TypeError   Note  If you re wondering why I changed the exception to a TypeError  here s the documentation   float try parse     def try parse string  fail None       try          return float string      except Exception          return fail    Note  You don t want to return the boolean  False  because that s still a value type  None is better because it indicates failure  Of course  if you want something different you can change the fail parameter to whatever you want   To extend float to include the  parse    and  try parse    you ll need to monkeypatch the  float  class to add these methods   If you want respect pre-existing functions the code should be something like   def monkey patch        if  hasattr float   parse             float parse   parse     if  hasattr float   try parse             float try parse   try parse   SideNote  I personally prefer to call it Monkey Punching because it feels like I m abusing the language when I do this but YMMV   Usage   float parse  giggity      throws TypeException float parse  54 3      returns the scalar value 54 3 float tryParse  twank      returns None float tryParse  32 2      returns the scalar value 32 2   And the great Sage Pythonas said to the Holy See Sharpisus   Anything you can do I can do better  I can do anything better than you

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I think your solution is fine  but there is a correct regexp implementation   There does seem to be a lot of regexp hate towards these answers which I think is unjustified  regexps can be reasonably clean and correct and fast   It really depends on what you re trying to do   The original question was how can you  check if a string can be represented as a number  float    as per your title    Presumably you would want to use the numeric float value once you ve checked that it s valid  in which case your try except makes a lot of sense   But if  for some reason  you just want to validate that a string is a number then a regex also works fine  but it s hard to get correct   I think most of the regex answers so far  for example  do not properly parse strings without an integer part  such as   7   which is a float as far as python is concerned   And that s slightly tricky to check for in a single regex where the fractional portion is not required   I ve included two regex to show this   It does raise the interesting question as to what a  number  is   Do you include  inf  which is valid as a float in python   Or do you include numbers that are  numbers  but maybe can t be represented in python  such as numbers that are larger than the float max    There s also ambiguities in how you parse numbers   For example  what about  --20    Is this a  number    Is this a legal way to represent  20    Python will let you do  var   --20  and set it to 20  though really this is because it treats it as an expression   but float  --20   does not work   Anyways  without more info  here s a regex that I believe covers all the ints and floats as python parses them     Doesn t properly handle floats missing the integer part  such as   7  SIMPLE FLOAT REGEXP   re compile r   -    0-9      0-9    eE  -    0-9          Example  -12 34E 56         sign  -                                    integer  12                                          mantissa  34                                                   exponent  E 56     Should handle all floats FLOAT REGEXP   re compile r   -     0-9    0-9     0-9     eE  -    0-9          Example  -12 34E 56         sign  -                                    integer  12                                          OR                                           int mantissa  12 34                                                           exponent  E 56   def is float str     return True if FLOAT REGEXP match str  else False   Some example test values   True   lt -  42 True   lt -  42 42 False  lt -  42 42 22 True   lt -  42 42e22 True   lt -  42 42E-22 False  lt -  42 42e-22 8 True   lt -  42 False  lt - 42nope   Running the benchmarking code in  ron-reiter s answer shows that this regex is actually faster than the normal regex and is much faster at handling bad values than the exception  which makes some sense   Results   check regexp with good floats  18 001921 check regexp with bad floats  17 861423 check regexp with strings  17 558862 check correct regexp with good floats  11 04428 check correct regexp with bad floats  8 71211 check correct regexp with strings  8 144161 check replace with good floats  6 020597 check replace with bad floats  5 343049 check replace with strings  5 091642 check exception with good floats  5 201605 check exception with bad floats  23 921864 check exception with strings  23 755481

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I think your solution is fine  but there is a correct regexp implementation   There does seem to be a lot of regexp hate towards these answers which I think is unjustified  regexps can be reasonably clean and correct and fast   It really depends on what you re trying to do   The original question was how can you  check if a string can be represented as a number  float    as per your title    Presumably you would want to use the numeric float value once you ve checked that it s valid  in which case your try except makes a lot of sense   But if  for some reason  you just want to validate that a string is a number then a regex also works fine  but it s hard to get correct   I think most of the regex answers so far  for example  do not properly parse strings without an integer part  such as   7   which is a float as far as python is concerned   And that s slightly tricky to check for in a single regex where the fractional portion is not required   I ve included two regex to show this   It does raise the interesting question as to what a  number  is   Do you include  inf  which is valid as a float in python   Or do you include numbers that are  numbers  but maybe can t be represented in python  such as numbers that are larger than the float max    There s also ambiguities in how you parse numbers   For example  what about  --20    Is this a  number    Is this a legal way to represent  20    Python will let you do  var   --20  and set it to 20  though really this is because it treats it as an expression   but float  --20   does not work   Anyways  without more info  here s a regex that I believe covers all the ints and floats as python parses them     Doesn t properly handle floats missing the integer part  such as   7  SIMPLE FLOAT REGEXP   re compile r   -    0-9      0-9    eE  -    0-9          Example  -12 34E 56         sign  -                                    integer  12                                          mantissa  34                                                   exponent  E 56     Should handle all floats FLOAT REGEXP   re compile r   -     0-9    0-9     0-9     eE  -    0-9          Example  -12 34E 56         sign  -                                    integer  12                                          OR                                           int mantissa  12 34                                                           exponent  E 56   def is float str     return True if FLOAT REGEXP match str  else False   Some example test values   True   lt -  42 True   lt -  42 42 False  lt -  42 42 22 True   lt -  42 42e22 True   lt -  42 42E-22 False  lt -  42 42e-22 8 True   lt -  42 False  lt - 42nope   Running the benchmarking code in  ron-reiter s answer shows that this regex is actually faster than the normal regex and is much faster at handling bad values than the exception  which makes some sense   Results   check regexp with good floats  18 001921 check regexp with bad floats  17 861423 check regexp with strings  17 558862 check correct regexp with good floats  11 04428 check correct regexp with bad floats  8 71211 check correct regexp with strings  8 144161 check replace with good floats  6 020597 check replace with bad floats  5 343049 check replace with strings  5 091642 check exception with good floats  5 201605 check exception with bad floats  23 921864 check exception with strings  23 755481

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Which  not only is ugly and slow  seems clunky    It may take some getting used to  but this is the pythonic way of doing it   As has been already pointed out  the alternatives are worse   But there is one other advantage of doing things this way   polymorphism   The central idea behind duck typing is that  if it walks and talks like a duck  then it s a duck    What if you decide that you need to subclass string so that you can change how you determine if something can be converted into a float   Or what if you decide to test some other object entirely   You can do these things without having to change the above code   Other languages solve these problems by using interfaces   I ll save the analysis of which solution is better for another thread   The point  though  is that python is decidedly on the duck typing side of the equation  and you re probably going to have to get used to syntax like this if you plan on doing much programming in Python  but that doesn t mean you have to like it of course    One other thing you might want to take into consideration  Python is pretty fast in throwing and catching exceptions compared to a lot of other languages  30x faster than  Net for instance    Heck  the language itself even throws exceptions to communicate non-exceptional  normal program conditions  every time you use a for loop    Thus  I wouldn t worry too much about the performance aspects of this code until you notice a significant problem

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So to put it all together  checking for Nan  infinity and complex numbers  it would seem they are specified with j  not i  i e  1 2j  it results in   def is number s       try          n str float s           if n     nan  or n   inf  or n   -inf    return False     except ValueError          try              complex s    for complex         except ValueError              return False     return True

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I know this is particularly old but I would add an answer I believe covers the information missing from the highest voted answer that could be very valuable to any who find this   For each of the following methods connect them with a count if you need any input to be accepted   Assuming we are using vocal definitions of integers rather than 0-255  etc    x isdigit   works well for checking if x is an integer   x replace  -      isdigit   works well for checking if x is a negative  Check - in first position   x replace         isdigit   works well for checking if x is a decimal   x replace         isdigit   works well for checking if x is a ratio   x replace        1  isdigit   works well for checking if x is a fraction

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I have a similar problem  Instead of defining a isNumber function  I want to convert a list of strings to floats  something that in high-level terms would be     float s  for s in list if isFloat s     It is a given we can not really separate the float s  from the isFloat s  functions  these two results should be returned by the same function   Also  if float s  fails  the whole process fails  instead of just ignoring the faulty element  Plus   0  is a valid number and should be included in the list  When filtering out bad elements  be certain not to exclude 0   Therefore  the above comprehension must be modified somehow to    if any element in the list cannot be converted  ignore it and don t throw an exception avoid calling float s  more than once for each element  one for the conversion  the other for the test  if the converted value is 0  it should still be present in the final list    I propose a solution inspired in the Nullable numerical types of C   These types are internally represented by a struct that has the numerical value and adds a boolean indicating if the value is valid   def tryParseFloat s       try          return float s   True      except          return None  False   tupleList    tryParseFloat x  for x in list  floats    v for v b in tupleList if b

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def is float s       if s is None          return False      if len s     0          return False      digits count   0     dots count   0     signs count   0      for c in s          if  0   lt   c  lt    9               digits count    1         elif c                     dots count    1         elif c     -  or c                     signs count    1         else              return False      if digits count    0          return False      if dots count  gt  1          return False      if signs count  gt  1          return False      return True

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You can generalize the exception technique in a useful way by returning more useful values than True and False   For example this function puts quotes round strings but leaves numbers alone   Which is just what I needed for a quick and dirty filter to make some variable definitions for R    import sys  def fix quotes s       try          float s          return s     except ValueError          return    0    format s   for line in sys stdin      input   line split       print input 0     lt - c        join fix quotes c  for c in input 1

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Try this    def is number var       try         if var    int var               return True     except Exception          return False

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You can use Unicode strings  they have a method to do just what you want    gt  gt  gt  s   u 345   gt  gt  gt  s isnumeric   True   Or    gt  gt  gt  s    345   gt  gt  gt  u   unicode s   gt  gt  gt  u isnumeric   True   http   www tutorialspoint com python string isnumeric htm  http   docs python org 2 howto unicode html

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I needed to determine if a string cast into basic types  float int str bool   After not finding anything on the internet I created this   def str to type  s           Get possible cast type for a string      Parameters     ----------     s   string      Returns     -------     float int str bool   type         Depending on what it can be cast to                  try                          f   float s                  if     not in s              return int         return float     except ValueError          value   s upper           if value     TRUE  or value     FALSE               return bool         return type s    Example  str to type  true     bool str to type  6 0     float str to type  6     int str to type  6abc     str str to type u 6abc     unicode          You can capture the type and use it   s    6 0  type    str to type s    float f   type  s

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I also used the function you mentioned  but soon I notice that strings as  Nan    Inf  and it s variation are considered as number  So I propose you improved version of your function  that will return false on those type of input and will not fail  1e3  variants   def is float text       try          float text            check for nan infinity etc          if text isalpha                return False         return True     except ValueError          return False

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Casting to float and catching ValueError is probably the fastest way  since float   is specifically meant for just that  Anything else that requires string parsing  regex  etc  will likely be slower due to the fact that it s not tuned for this operation  My  0 02

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I was working on a problem that led me to this thread  namely how to convert a collection of data to strings and numbers in the most intuitive way   I realized after reading the original code that what I needed was different in two ways   1 - I wanted an integer result if the string represented an integer  2 - I wanted a number or a string result to stick into a data structure  so I adapted the original code to produce this derivative   def string or number s       try          z   int s          return z     except ValueError          try              z   float s              return z         except ValueError              return s

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In case you are looking for parsing  positive  unsigned  integers instead of floats  you can use the isdigit   function for string objects    gt  gt  gt  a    03523   gt  gt  gt  a isdigit   True  gt  gt  gt  b    963spam   gt  gt  gt  b isdigit   False   String Methods - isdigit    Python2  Python3  There s also something on Unicode strings  which I m not too familiar with Unicode - Is decimal decimal

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Which  not only is ugly and slow   I d dispute both   A regex or other string parsing method would be uglier and slower     I m not sure that anything much could be faster than the above   It calls the function and returns   Try Catch doesn t introduce much overhead because the most common exception is caught without an extensive search of stack frames   The issue is that any numeric conversion function has two kinds of results   A number  if the number is valid A status code  e g   via errno  or exception to show that no valid number could be parsed    C  as an example  hacks around this a number of ways   Python lays it out clearly and explicitly   I think your code for doing this is perfect

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I have a similar problem  Instead of defining a isNumber function  I want to convert a list of strings to floats  something that in high-level terms would be     float s  for s in list if isFloat s     It is a given we can not really separate the float s  from the isFloat s  functions  these two results should be returned by the same function   Also  if float s  fails  the whole process fails  instead of just ignoring the faulty element  Plus   0  is a valid number and should be included in the list  When filtering out bad elements  be certain not to exclude 0   Therefore  the above comprehension must be modified somehow to    if any element in the list cannot be converted  ignore it and don t throw an exception avoid calling float s  more than once for each element  one for the conversion  the other for the test  if the converted value is 0  it should still be present in the final list    I propose a solution inspired in the Nullable numerical types of C   These types are internally represented by a struct that has the numerical value and adds a boolean indicating if the value is valid   def tryParseFloat s       try          return float s   True      except          return None  False   tupleList    tryParseFloat x  for x in list  floats    v for v b in tupleList if b

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Casting to float and catching ValueError is probably the fastest way  since float   is specifically meant for just that  Anything else that requires string parsing  regex  etc  will likely be slower due to the fact that it s not tuned for this operation  My  0 02

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I did some speed test  Lets say that if the string is likely to be a number the try except strategy is the fastest possible If the string is not likely to be a number and you are interested in Integer check  it worths to do some test  isdigit plus heading  -     If you are interested to check float number  you have to use the try except code whitout escape

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Which  not only is ugly and slow   I d dispute both   A regex or other string parsing method would be uglier and slower     I m not sure that anything much could be faster than the above   It calls the function and returns   Try Catch doesn t introduce much overhead because the most common exception is caught without an extensive search of stack frames   The issue is that any numeric conversion function has two kinds of results   A number  if the number is valid A status code  e g   via errno  or exception to show that no valid number could be parsed    C  as an example  hacks around this a number of ways   Python lays it out clearly and explicitly   I think your code for doing this is perfect

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For int use this    gt  gt  gt   1221323  isdigit   True   But for float we need some tricks  -   Every float number has one point      gt  gt  gt   12 34  isdigit   False  gt  gt  gt   12 34  replace        1  isdigit   True  gt  gt  gt   12 3 4  replace        1  isdigit   False   Also for negative numbers just add lstrip      gt  gt  gt   -12  lstrip  -    12    And now we get a universal way    gt  gt  gt   -12 34  lstrip  -   replace        1  isdigit   True  gt  gt  gt    -234  lstrip  -   replace        1  isdigit   False

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Try this    def is number var       try         if var    int var               return True     except Exception          return False

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I also used the function you mentioned  but soon I notice that strings as  Nan    Inf  and it s variation are considered as number  So I propose you improved version of your function  that will return false on those type of input and will not fail  1e3  variants   def is float text       try          float text            check for nan infinity etc          if text isalpha                return False         return True     except ValueError          return False

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For strings of non-numbers  try  except  is actually slower than regular expressions   For strings of valid numbers  regex is slower   So  the appropriate method depends on your input    If you find that you are in a performance bind  you can use a new third-party module called fastnumbers that provides a function called isfloat   Full disclosure  I am the author   I have included its results in the timings below     from   future   import print function import timeit  prep base        x    invalid  y    5402  z    4 754e3       prep try method        def is number try val       try          float val          return True     except ValueError          return False       prep re method        import re float match   re compile r  -    d     d     eE  -    d       match def is number re val       return bool float match val         fn method        from fastnumbers import isfloat       print  Try with non-number strings   timeit timeit  is number try x        prep base   prep try method    seconds   print  Try with integer strings   timeit timeit  is number try y        prep base   prep try method    seconds   print  Try with float strings   timeit timeit  is number try z        prep base   prep try method    seconds   print   print  Regex with non-number strings   timeit timeit  is number re x        prep base   prep re method    seconds   print  Regex with integer strings   timeit timeit  is number re y        prep base   prep re method    seconds   print  Regex with float strings   timeit timeit  is number re z        prep base   prep re method    seconds   print   print  fastnumbers with non-number strings   timeit timeit  isfloat x        prep base    from fastnumbers import isfloat     seconds   print  fastnumbers with integer strings   timeit timeit  isfloat y        prep base    from fastnumbers import isfloat     seconds   print  fastnumbers with float strings   timeit timeit  isfloat z        prep base    from fastnumbers import isfloat     seconds   print       Try with non-number strings 2 39108395576 seconds Try with integer strings 0 375686168671 seconds Try with float strings 0 369210958481 seconds  Regex with non-number strings 0 748660802841 seconds Regex with integer strings 1 02021503448 seconds Regex with float strings 1 08564686775 seconds  fastnumbers with non-number strings 0 174362897873 seconds fastnumbers with integer strings 0 179651021957 seconds fastnumbers with float strings 0 20222902298 seconds   As you can see   try  except  was fast for numeric input but very slow for an invalid input regex is very efficient when the input is invalid fastnumbers wins in both cases

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Which  not only is ugly and slow   I d dispute both   A regex or other string parsing method would be uglier and slower     I m not sure that anything much could be faster than the above   It calls the function and returns   Try Catch doesn t introduce much overhead because the most common exception is caught without an extensive search of stack frames   The issue is that any numeric conversion function has two kinds of results   A number  if the number is valid A status code  e g   via errno  or exception to show that no valid number could be parsed    C  as an example  hacks around this a number of ways   Python lays it out clearly and explicitly   I think your code for doing this is perfect

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TL DR The best solution is s replace        1  isdigit    I did some benchmarks comparing the different approaches  def is number tryexcept s           Returns True is string is a number          try          float s          return True     except ValueError          return False  import re     def is number regex s           Returns True is string is a number          if re match    d     d      s  is None          return s isdigit       return True   def is number repl isdigit s           Returns True is string is a number          return s replace        1  isdigit     If the string is not a number  the except-block is quite slow  But more importantly  the try-except method is the only approach that handles scientific notations correctly   funcs               is number tryexcept             is number regex            is number repl isdigit              a float     1234   print  Float notation   1234  is not supported by    for f in funcs      if not f a float           print   t -   f   name      Float notation   1234  is not supported by  - is number regex    scientific1    1 000000e 50  scientific2    1e50    print  Scientific notation  1 000000e 50  is not supported by    for f in funcs      if not f scientific1           print   t -   f   name        print  Scientific notation  1e50  is not supported by    for f in funcs      if not f scientific2           print   t -   f   name      Scientific notation  1 000000e 50  is not supported by  - is number regex - is number repl isdigit Scientific notation  1e50  is not supported by  - is number regex - is number repl isdigit    EDIT  The benchmark results  import timeit  test cases     1 12345    1 12 345    abc12345    12345   times n    f   name      for f in funcs   for t in test cases      for f in funcs          f   f   name           times n f  append min timeit Timer   s t    f                          from   main   import  s  t   f                                 repeat repeat 3  number 1000000      where the following functions were tested  from re import match as re match from re import compile as re compile  def is number tryexcept s           Returns True is string is a number          try          float s          return True     except ValueError          return False  def is number regex s           Returns True is string is a number          if re match    d     d      s  is None          return s isdigit       return True   comp   re compile    d     d           def compiled regex s           Returns True is string is a number          if comp match s  is None          return s isdigit       return True   def is number repl isdigit s           Returns True is string is a number          return s replace        1  isdigit

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I was working on a problem that led me to this thread  namely how to convert a collection of data to strings and numbers in the most intuitive way   I realized after reading the original code that what I needed was different in two ways   1 - I wanted an integer result if the string represented an integer  2 - I wanted a number or a string result to stick into a data structure  so I adapted the original code to produce this derivative   def string or number s       try          z   int s          return z     except ValueError          try              z   float s              return z         except ValueError              return s

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I did some speed test  Lets say that if the string is likely to be a number the try except strategy is the fastest possible If the string is not likely to be a number and you are interested in Integer check  it worths to do some test  isdigit plus heading  -     If you are interested to check float number  you have to use the try except code whitout escape

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In a most general case for a float  one would like to take care of integers and decimals  Let s take the string  quot 1 1 quot  as an example  I would try one of the following  1  gt   isnumeric   word    quot 1 1 quot    quot  quot  join word split  quot   quot    isnumeric    gt  gt  gt  True  2  gt   isdigit   word    quot 1 1 quot    quot  quot  join word split  quot   quot    isdigit    gt  gt  gt  True  3  gt   isdecimal   word    quot 1 1 quot    quot  quot  join word split  quot   quot    isdecimal    gt  gt  gt  True  Speed    All the aforementioned methods have similar speeds   timeit  quot  quot  join word split  quot   quot    isnumeric    gt  gt  gt  257 ns    12 ns per loop  mean    std  dev  of 7 runs  1000000 loops each    timeit  quot  quot  join word split  quot   quot    isdigit    gt  gt  gt  252 ns    11 ns per loop  mean    std  dev  of 7 runs  1000000 loops each    timeit  quot  quot  join word split  quot   quot    isdecimal    gt  gt  gt  244 ns    7 17 ns per loop  mean    std  dev  of 7 runs  1000000 loops each

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str isnumeric       Return True if all characters in the string are numeric characters    and there is at least one character  False otherwise  Numeric   characters include digit characters  and all characters that have the   Unicode numeric value property  e g  U 2155  VULGAR FRACTION ONE   FIFTH  Formally  numeric characters are those with the property value   Numeric Type Digit  Numeric Type Decimal or Numeric Type Numeric    str isdecimal       Return True if all characters in the string are decimal characters and   there is at least one character  False otherwise  Decimal characters   are those that can be used to form numbers in base 10  e g  U 0660    ARABIC-INDIC DIGIT ZERO  Formally a decimal character is a character   in the Unicode General Category    Nd       Both available for string types from Python 3 0

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use following it handles all cases -  import re a re match     d      d         d         2 3    a re match     d      d         d         2    a re match     d      d         d          3   a re match     d      d         d         2 3sd   a re match     d      d         d         2 3

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import re def is number num       pattern   re compile r   -    -0-9  d    d   -       0-9  d         result   pattern match num      if result          return True     else          return False     gt  gt  gt   is number  1   True   gt  gt  gt   is number  111   True   gt  gt  gt   is number  11 1   True   gt  gt  gt   is number  -11 1   True   gt  gt  gt   is number  inf   False   gt  gt  gt   is number  -inf   False

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This code handles the exponents  floats  and integers  wihtout using regex   return True if str1 lstrip  -   replace        1  isdigit   or float str1  else False

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There is one exception that you may want to take into account  the string  NaN   If you want is number to return FALSE for  NaN  this code will not work as Python converts it to its representation of a number that is not a number  talk about identity issues     gt  gt  gt  float  NaN   nan   Otherwise  I should actually thank you for the piece of code I now use extensively      G

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def is float s       if s is None          return False      if len s     0          return False      digits count   0     dots count   0     signs count   0      for c in s          if  0   lt   c  lt    9               digits count    1         elif c                     dots count    1         elif c     -  or c                     signs count    1         else              return False      if digits count    0          return False      if dots count  gt  1          return False      if signs count  gt  1          return False      return True

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Sorry for the Zombie thread post - just wanted to round out the code for completeness      is number   function - Uses re   regex library   Should handle all normal and complex numbers   Does not accept trailing spaces     Note  accepts both engineering  quot j quot  and math  quot i quot  but only the imaginary part  quot  bi quot  of a complex number a bi   Also accepts inf or NaN   Thanks to the earlier responders for most the regex fu  import re  ISNUM REGEXP   re compile r   -     0-9    0-9     0-9     eE  -    0-9     ij       def is number str    change order if you have a lot of NaN or inf to parse     if ISNUM REGEXP match str  or str     quot NaN quot  or str     quot inf quot            return True      else          return False   A couple test numbers    42 42e-42j   -42 42E 42i  print  Is it a number    is number input  Gimme any number        Gimme any number   42 42e-42j Is it a number  True

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I wanted to see which method is fastest  Overall the best and most consistent results were given by the check replace function  The fastest results were given by the check exception function  but only if there was no exception fired - meaning its code is the most efficient  but the overhead of throwing an exception is quite large   Please note that checking for a successful cast is the only method which is accurate  for example  this works with check exception but the other two test functions will return False for a valid float   huge number   float  1e 100     Here is the benchmark code   import time  re  random  string  ITERATIONS   10000000  class Timer          def   enter   self           self start   time clock           return self     def   exit   self   args           self end   time clock           self interval   self end - self start  def check regexp x       return re compile    d     d     match x  is not None  def check replace x       return x replace        1  isdigit    def check exception s       try          float s          return True     except ValueError          return False  to check    check regexp  check replace  check exception   print  preparing data      good numbers         str random random     random random         for x in range ITERATIONS    bad numbers          x for x in good numbers   strings            join random choice string ascii uppercase   string digits  for   in range random randint 1 10        for x in range ITERATIONS    print  running test      for func in to check      with Timer   as t          for x in good numbers              res   func x      print   s with good floats   s     func   name    t interval       with Timer   as t          for x in bad numbers              res   func x      print   s with bad floats   s     func   name    t interval       with Timer   as t          for x in strings              res   func x      print   s with strings   s     func   name    t interval     Here are the results with Python 2 7 10 on a 2017 MacBook Pro 13   check regexp with good floats  12 688639 check regexp with bad floats  11 624862 check regexp with strings  11 349414 check replace with good floats  4 419841 check replace with bad floats  4 294909 check replace with strings  4 086358 check exception with good floats  3 276668 check exception with bad floats  13 843092 check exception with strings  15 786169   Here are the results with Python 3 6 5 on a 2017 MacBook Pro 13   check regexp with good floats  13 472906000000009 check regexp with bad floats  12 977665000000016 check regexp with strings  12 417542999999995 check replace with good floats  6 011045999999993 check replace with bad floats  4 849356 check replace with strings  4 282754000000011 check exception with good floats  6 039081999999979 check exception with bad floats  9 322753000000006 check exception with strings  9 952595000000002   Here are the results with PyPy 2 7 13 on a 2017 MacBook Pro 13   check regexp with good floats  2 693217 check regexp with bad floats  2 744819 check regexp with strings  2 532414 check replace with good floats  0 604367 check replace with bad floats  0 538169 check replace with strings  0 598664 check exception with good floats  1 944103 check exception with bad floats  2 449182 check exception with strings  2 200056

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So to put it all together  checking for Nan  infinity and complex numbers  it would seem they are specified with j  not i  i e  1 2j  it results in   def is number s       try          n str float s           if n     nan  or n   inf  or n   -inf    return False     except ValueError          try              complex s    for complex         except ValueError              return False     return True

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You can use Unicode strings  they have a method to do just what you want    gt  gt  gt  s   u 345   gt  gt  gt  s isnumeric   True   Or    gt  gt  gt  s    345   gt  gt  gt  u   unicode s   gt  gt  gt  u isnumeric   True   http   www tutorialspoint com python string isnumeric htm  http   docs python org 2 howto unicode html

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Updated after Alfe pointed out you don t need to check for float separately as complex handles both   def is number s       try          complex s    for int  long  float and complex     except ValueError          return False      return True     Previously said  Is some rare cases you might also need to check for complex numbers  e g  1 2i   which can not be represented by a float   def is number s       try          float s    for int  long and float     except ValueError          try              complex s    for complex         except ValueError              return False      return True

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For int use this    gt  gt  gt   1221323  isdigit   True   But for float we need some tricks  -   Every float number has one point      gt  gt  gt   12 34  isdigit   False  gt  gt  gt   12 34  replace        1  isdigit   True  gt  gt  gt   12 3 4  replace        1  isdigit   False   Also for negative numbers just add lstrip      gt  gt  gt   -12  lstrip  -    12    And now we get a universal way    gt  gt  gt   -12 34  lstrip  -   replace        1  isdigit   True  gt  gt  gt    -234  lstrip  -   replace        1  isdigit   False

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This answer provides step by step guide having function with examples to find the string is   Positive integer Positive negative - integer float How to discard  quot NaN quot   not a number  strings while checking for number   Check if string is positive integer You may use str isdigit   to check whether given string is positive integer  Sample Results    For digit  gt  gt  gt   1  isdigit   True  gt  gt  gt   1  isalpha   False  Check for string as positive negative - integer float str isdigit   returns False if the string is a negative number or a float number  For example    returns  False  for float  gt  gt  gt   123 3  isdigit   False   returns  False  for negative number  gt  gt  gt   -123  isdigit   False  If you want to also check for the negative integers and float  then you may write a custom function to check for it as  def is number n       try          float n      Type-casting the string to  float                        If string is not a valid  float                         it ll raise  ValueError  exception     except ValueError          return False     return True  Sample Run   gt  gt  gt  is number  123        positive integer number True   gt  gt  gt  is number  123 4      positive float number True    gt  gt  gt  is number  -123       negative integer number True   gt  gt  gt  is number  -123 4     negative  float  number True   gt  gt  gt  is number  abc         False  for  quot some random quot  string False  Discard  quot NaN quot   not a number  strings while checking for number The above functions will return True for the  quot NAN quot   Not a number  string because for Python it is valid float representing it is not a number  For example   gt  gt  gt  is number  NaN   True  In order to check whether the number is  quot NaN quot   you may use math isnan   as   gt  gt  gt  import math  gt  gt  gt  nan num   float  nan     gt  gt  gt  math isnan nan num  True  Or if you don t want to import additional library to check this  then you may simply check it via comparing it with itself using     Python returns False when nan float is compared with itself  For example     nan num  variable is taken from above example  gt  gt  gt  nan num    nan num False  Hence  above function is number can be updated to return False for  quot NaN quot  as  def is number n       is number   True     try          num   float n            check for  quot nan quot  floats         is number   num    num     or use  math isnan num       except ValueError          is number   False     return is number  Sample Run   gt  gt  gt  is number  Nan       not a number  quot Nan quot  string False   gt  gt  gt  is number  nan       not a number string  quot nan quot  with all lower cased False   gt  gt  gt  is number  123       positive integer True   gt  gt  gt  is number  -123      negative integer True   gt  gt  gt  is number  -1 12     negative  float  True   gt  gt  gt  is number  abc        quot some random quot  string False  PS  Each operation for each check depending on the type of number comes with additional overhead  Choose the version of is number function which fits your requirement

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You can generalize the exception technique in a useful way by returning more useful values than True and False   For example this function puts quotes round strings but leaves numbers alone   Which is just what I needed for a quick and dirty filter to make some variable definitions for R    import sys  def fix quotes s       try          float s          return s     except ValueError          return    0    format s   for line in sys stdin      input   line split       print input 0     lt - c        join fix quotes c  for c in input 1

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In a most general case for a float  one would like to take care of integers and decimals  Let s take the string  quot 1 1 quot  as an example  I would try one of the following  1  gt   isnumeric   word    quot 1 1 quot    quot  quot  join word split  quot   quot    isnumeric    gt  gt  gt  True  2  gt   isdigit   word    quot 1 1 quot    quot  quot  join word split  quot   quot    isdigit    gt  gt  gt  True  3  gt   isdecimal   word    quot 1 1 quot    quot  quot  join word split  quot   quot    isdecimal    gt  gt  gt  True  Speed    All the aforementioned methods have similar speeds   timeit  quot  quot  join word split  quot   quot    isnumeric    gt  gt  gt  257 ns    12 ns per loop  mean    std  dev  of 7 runs  1000000 loops each    timeit  quot  quot  join word split  quot   quot    isdigit    gt  gt  gt  252 ns    11 ns per loop  mean    std  dev  of 7 runs  1000000 loops each    timeit  quot  quot  join word split  quot   quot    isdecimal    gt  gt  gt  244 ns    7 17 ns per loop  mean    std  dev  of 7 runs  1000000 loops each

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In case you are looking for parsing  positive  unsigned  integers instead of floats  you can use the isdigit   function for string objects    gt  gt  gt  a    03523   gt  gt  gt  a isdigit   True  gt  gt  gt  b    963spam   gt  gt  gt  b isdigit   False   String Methods - isdigit    Python2  Python3  There s also something on Unicode strings  which I m not too familiar with Unicode - Is decimal decimal

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RyanN suggests     If you want to return False for a NaN and Inf  change line to x   float s   return  x    x  and  x - 1    x   This should return True for all floats except Inf and NaN   But this doesn t quite work  because for sufficiently large floats  x-1    x returns true  For example  2 0  54 - 1    2 0  54

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Casting to float and catching ValueError is probably the fastest way  since float   is specifically meant for just that  Anything else that requires string parsing  regex  etc  will likely be slower due to the fact that it s not tuned for this operation  My  0 02

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TL DR The best solution is s replace        1  isdigit    I did some benchmarks comparing the different approaches  def is number tryexcept s           Returns True is string is a number          try          float s          return True     except ValueError          return False  import re     def is number regex s           Returns True is string is a number          if re match    d     d      s  is None          return s isdigit       return True   def is number repl isdigit s           Returns True is string is a number          return s replace        1  isdigit     If the string is not a number  the except-block is quite slow  But more importantly  the try-except method is the only approach that handles scientific notations correctly   funcs               is number tryexcept             is number regex            is number repl isdigit              a float     1234   print  Float notation   1234  is not supported by    for f in funcs      if not f a float           print   t -   f   name      Float notation   1234  is not supported by  - is number regex    scientific1    1 000000e 50  scientific2    1e50    print  Scientific notation  1 000000e 50  is not supported by    for f in funcs      if not f scientific1           print   t -   f   name        print  Scientific notation  1e50  is not supported by    for f in funcs      if not f scientific2           print   t -   f   name      Scientific notation  1 000000e 50  is not supported by  - is number regex - is number repl isdigit Scientific notation  1e50  is not supported by  - is number regex - is number repl isdigit    EDIT  The benchmark results  import timeit  test cases     1 12345    1 12 345    abc12345    12345   times n    f   name      for f in funcs   for t in test cases      for f in funcs          f   f   name           times n f  append min timeit Timer   s t    f                          from   main   import  s  t   f                                 repeat repeat 3  number 1000000      where the following functions were tested  from re import match as re match from re import compile as re compile  def is number tryexcept s           Returns True is string is a number          try          float s          return True     except ValueError          return False  def is number regex s           Returns True is string is a number          if re match    d     d      s  is None          return s isdigit       return True   comp   re compile    d     d           def compiled regex s           Returns True is string is a number          if comp match s  is None          return s isdigit       return True   def is number repl isdigit s           Returns True is string is a number          return s replace        1  isdigit

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User helper function   def if ok fn  string     try      return fn string    except Exception as e      return None   then   if ok int  my str  or if ok float  my str  or if ok complex  my str  is number   lambda s  any  if ok fn  s  for fn in  int  float  complex

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In case you are looking for parsing  positive  unsigned  integers instead of floats  you can use the isdigit   function for string objects    gt  gt  gt  a    03523   gt  gt  gt  a isdigit   True  gt  gt  gt  b    963spam   gt  gt  gt  b isdigit   False   String Methods - isdigit    Python2  Python3  There s also something on Unicode strings  which I m not too familiar with Unicode - Is decimal decimal

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I know this is particularly old but I would add an answer I believe covers the information missing from the highest voted answer that could be very valuable to any who find this   For each of the following methods connect them with a count if you need any input to be accepted   Assuming we are using vocal definitions of integers rather than 0-255  etc    x isdigit   works well for checking if x is an integer   x replace  -      isdigit   works well for checking if x is a negative  Check - in first position   x replace         isdigit   works well for checking if x is a decimal   x replace         isdigit   works well for checking if x is a ratio   x replace        1  isdigit   works well for checking if x is a fraction

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For strings of non-numbers  try  except  is actually slower than regular expressions   For strings of valid numbers  regex is slower   So  the appropriate method depends on your input    If you find that you are in a performance bind  you can use a new third-party module called fastnumbers that provides a function called isfloat   Full disclosure  I am the author   I have included its results in the timings below     from   future   import print function import timeit  prep base        x    invalid  y    5402  z    4 754e3       prep try method        def is number try val       try          float val          return True     except ValueError          return False       prep re method        import re float match   re compile r  -    d     d     eE  -    d       match def is number re val       return bool float match val         fn method        from fastnumbers import isfloat       print  Try with non-number strings   timeit timeit  is number try x        prep base   prep try method    seconds   print  Try with integer strings   timeit timeit  is number try y        prep base   prep try method    seconds   print  Try with float strings   timeit timeit  is number try z        prep base   prep try method    seconds   print   print  Regex with non-number strings   timeit timeit  is number re x        prep base   prep re method    seconds   print  Regex with integer strings   timeit timeit  is number re y        prep base   prep re method    seconds   print  Regex with float strings   timeit timeit  is number re z        prep base   prep re method    seconds   print   print  fastnumbers with non-number strings   timeit timeit  isfloat x        prep base    from fastnumbers import isfloat     seconds   print  fastnumbers with integer strings   timeit timeit  isfloat y        prep base    from fastnumbers import isfloat     seconds   print  fastnumbers with float strings   timeit timeit  isfloat z        prep base    from fastnumbers import isfloat     seconds   print       Try with non-number strings 2 39108395576 seconds Try with integer strings 0 375686168671 seconds Try with float strings 0 369210958481 seconds  Regex with non-number strings 0 748660802841 seconds Regex with integer strings 1 02021503448 seconds Regex with float strings 1 08564686775 seconds  fastnumbers with non-number strings 0 174362897873 seconds fastnumbers with integer strings 0 179651021957 seconds fastnumbers with float strings 0 20222902298 seconds   As you can see   try  except  was fast for numeric input but very slow for an invalid input regex is very efficient when the input is invalid fastnumbers wins in both cases

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Which  not only is ugly and slow  seems clunky    It may take some getting used to  but this is the pythonic way of doing it   As has been already pointed out  the alternatives are worse   But there is one other advantage of doing things this way   polymorphism   The central idea behind duck typing is that  if it walks and talks like a duck  then it s a duck    What if you decide that you need to subclass string so that you can change how you determine if something can be converted into a float   Or what if you decide to test some other object entirely   You can do these things without having to change the above code   Other languages solve these problems by using interfaces   I ll save the analysis of which solution is better for another thread   The point  though  is that python is decidedly on the duck typing side of the equation  and you re probably going to have to get used to syntax like this if you plan on doing much programming in Python  but that doesn t mean you have to like it of course    One other thing you might want to take into consideration  Python is pretty fast in throwing and catching exceptions compared to a lot of other languages  30x faster than  Net for instance    Heck  the language itself even throws exceptions to communicate non-exceptional  normal program conditions  every time you use a for loop    Thus  I wouldn t worry too much about the performance aspects of this code until you notice a significant problem

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how about this    3 14  replace        1  isdigit     which will return true only if there is one or no     in the string of digits    3 14 5  replace        1  isdigit     will return false  edit  just saw another comment     adding a  replace badstuff    maxnum badstuff  for other cases can be done  if you are passing salt and not arbitrary condiments  ref xkcd 974  this will do fine  P

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RyanN suggests     If you want to return False for a NaN and Inf  change line to x   float s   return  x    x  and  x - 1    x   This should return True for all floats except Inf and NaN   But this doesn t quite work  because for sufficiently large floats  x-1    x returns true  For example  2 0  54 - 1    2 0  54

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Which  not only is ugly and slow  seems clunky    It may take some getting used to  but this is the pythonic way of doing it   As has been already pointed out  the alternatives are worse   But there is one other advantage of doing things this way   polymorphism   The central idea behind duck typing is that  if it walks and talks like a duck  then it s a duck    What if you decide that you need to subclass string so that you can change how you determine if something can be converted into a float   Or what if you decide to test some other object entirely   You can do these things without having to change the above code   Other languages solve these problems by using interfaces   I ll save the analysis of which solution is better for another thread   The point  though  is that python is decidedly on the duck typing side of the equation  and you re probably going to have to get used to syntax like this if you plan on doing much programming in Python  but that doesn t mean you have to like it of course    One other thing you might want to take into consideration  Python is pretty fast in throwing and catching exceptions compared to a lot of other languages  30x faster than  Net for instance    Heck  the language itself even throws exceptions to communicate non-exceptional  normal program conditions  every time you use a for loop    Thus  I wouldn t worry too much about the performance aspects of this code until you notice a significant problem

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Here s my simple way of doing it  Let s say that I m looping through some strings and I want to add them to an array if they turn out to be numbers   try      myvar append  float string to check    except      continue   Replace the myvar apppend with whatever operation you want to do with the string if it turns out to be a number  The idea is to try to use a float   operation and use the returned error to determine whether or not the string is a number

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Updated after Alfe pointed out you don t need to check for float separately as complex handles both   def is number s       try          complex s    for int  long  float and complex     except ValueError          return False      return True     Previously said  Is some rare cases you might also need to check for complex numbers  e g  1 2i   which can not be represented by a float   def is number s       try          float s    for int  long and float     except ValueError          try              complex s    for complex         except ValueError              return False      return True

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use following it handles all cases -  import re a re match     d      d         d         2 3    a re match     d      d         d         2    a re match     d      d         d          3   a re match     d      d         d         2 3sd   a re match     d      d         d         2 3

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I needed to determine if a string cast into basic types  float int str bool   After not finding anything on the internet I created this   def str to type  s           Get possible cast type for a string      Parameters     ----------     s   string      Returns     -------     float int str bool   type         Depending on what it can be cast to                  try                          f   float s                  if     not in s              return int         return float     except ValueError          value   s upper           if value     TRUE  or value     FALSE               return bool         return type s    Example  str to type  true     bool str to type  6 0     float str to type  6     int str to type  6abc     str str to type u 6abc     unicode          You can capture the type and use it   s    6 0  type    str to type s    float f   type  s

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Which  not only is ugly and slow  seems clunky    It may take some getting used to  but this is the pythonic way of doing it   As has been already pointed out  the alternatives are worse   But there is one other advantage of doing things this way   polymorphism   The central idea behind duck typing is that  if it walks and talks like a duck  then it s a duck    What if you decide that you need to subclass string so that you can change how you determine if something can be converted into a float   Or what if you decide to test some other object entirely   You can do these things without having to change the above code   Other languages solve these problems by using interfaces   I ll save the analysis of which solution is better for another thread   The point  though  is that python is decidedly on the duck typing side of the equation  and you re probably going to have to get used to syntax like this if you plan on doing much programming in Python  but that doesn t mean you have to like it of course    One other thing you might want to take into consideration  Python is pretty fast in throwing and catching exceptions compared to a lot of other languages  30x faster than  Net for instance    Heck  the language itself even throws exceptions to communicate non-exceptional  normal program conditions  every time you use a for loop    Thus  I wouldn t worry too much about the performance aspects of this code until you notice a significant problem

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User helper function   def if ok fn  string     try      return fn string    except Exception as e      return None   then   if ok int  my str  or if ok float  my str  or if ok complex  my str  is number   lambda s  any  if ok fn  s  for fn in  int  float  complex

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This answer provides step by step guide having function with examples to find the string is   Positive integer Positive negative - integer float How to discard  quot NaN quot   not a number  strings while checking for number   Check if string is positive integer You may use str isdigit   to check whether given string is positive integer  Sample Results    For digit  gt  gt  gt   1  isdigit   True  gt  gt  gt   1  isalpha   False  Check for string as positive negative - integer float str isdigit   returns False if the string is a negative number or a float number  For example    returns  False  for float  gt  gt  gt   123 3  isdigit   False   returns  False  for negative number  gt  gt  gt   -123  isdigit   False  If you want to also check for the negative integers and float  then you may write a custom function to check for it as  def is number n       try          float n      Type-casting the string to  float                        If string is not a valid  float                         it ll raise  ValueError  exception     except ValueError          return False     return True  Sample Run   gt  gt  gt  is number  123        positive integer number True   gt  gt  gt  is number  123 4      positive float number True    gt  gt  gt  is number  -123       negative integer number True   gt  gt  gt  is number  -123 4     negative  float  number True   gt  gt  gt  is number  abc         False  for  quot some random quot  string False  Discard  quot NaN quot   not a number  strings while checking for number The above functions will return True for the  quot NAN quot   Not a number  string because for Python it is valid float representing it is not a number  For example   gt  gt  gt  is number  NaN   True  In order to check whether the number is  quot NaN quot   you may use math isnan   as   gt  gt  gt  import math  gt  gt  gt  nan num   float  nan     gt  gt  gt  math isnan nan num  True  Or if you don t want to import additional library to check this  then you may simply check it via comparing it with itself using     Python returns False when nan float is compared with itself  For example     nan num  variable is taken from above example  gt  gt  gt  nan num    nan num False  Hence  above function is number can be updated to return False for  quot NaN quot  as  def is number n       is number   True     try          num   float n            check for  quot nan quot  floats         is number   num    num     or use  math isnan num       except ValueError          is number   False     return is number  Sample Run   gt  gt  gt  is number  Nan       not a number  quot Nan quot  string False   gt  gt  gt  is number  nan       not a number string  quot nan quot  with all lower cased False   gt  gt  gt  is number  123       positive integer True   gt  gt  gt  is number  -123      negative integer True   gt  gt  gt  is number  -1 12     negative  float  True   gt  gt  gt  is number  abc        quot some random quot  string False  PS  Each operation for each check depending on the type of number comes with additional overhead  Choose the version of is number function which fits your requirement

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Which  not only is ugly and slow   I d dispute both   A regex or other string parsing method would be uglier and slower     I m not sure that anything much could be faster than the above   It calls the function and returns   Try Catch doesn t introduce much overhead because the most common exception is caught without an extensive search of stack frames   The issue is that any numeric conversion function has two kinds of results   A number  if the number is valid A status code  e g   via errno  or exception to show that no valid number could be parsed    C  as an example  hacks around this a number of ways   Python lays it out clearly and explicitly   I think your code for doing this is perfect

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Casting to float and catching ValueError is probably the fastest way  since float   is specifically meant for just that  Anything else that requires string parsing  regex  etc  will likely be slower due to the fact that it s not tuned for this operation  My  0 02

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Here s my simple way of doing it  Let s say that I m looping through some strings and I want to add them to an array if they turn out to be numbers   try      myvar append  float string to check    except      continue   Replace the myvar apppend with whatever operation you want to do with the string if it turns out to be a number  The idea is to try to use a float   operation and use the returned error to determine whether or not the string is a number

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There is one exception that you may want to take into account  the string  NaN   If you want is number to return FALSE for  NaN  this code will not work as Python converts it to its representation of a number that is not a number  talk about identity issues     gt  gt  gt  float  NaN   nan   Otherwise  I should actually thank you for the piece of code I now use extensively      G

[python] How do I check if a string is a number (float)?

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