Checking if a string can be converted to float in Python

Question

I ve got some Python code that runs through a list of strings and converts them to integers or floating point numbers if possible   Doing this for integers is pretty easy  if element isdigit      newelement   int element    Floating point numbers are more difficult  Right now I m using partition      to split the string and checking to make sure that one or both sides are digits   partition   element partition      if  partition 0  isdigit   and partition 1         and partition 2  isdigit         or  partition 0        and partition 1         and partition 2  isdigit         or  partition 0  isdigit   and partition 1         and partition 2            newelement   float element    This works  but obviously the if statement for that is a bit of a bear   The other solution I considered is to just wrap the conversion in a try catch block and see if it succeeds  as described in this question   Anyone have any other ideas  Opinions on the relative merits of the partition and try catch approaches

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I used the function already 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 the function  that will return false on those type of input and will not fail  1e3  variants   def is float text         check for nan infinity etc      if text isalpha            return False     try          float text          return True     except ValueError          return False

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If you don t need to worry about scientific or other expressions of numbers and are only working with strings that could be numbers with or without a period   Function  def is float s       result   False     if s count         1          if s replace          isdigit                result   True     return result   Lambda version  is float   lambda x  x replace        1  isdigit   and     in x   Example  if is float some string       some string   float some string  elif some string isdigit        some string   int some string  else      print  Does not convert to int or float     This way you aren t accidentally converting what should be an int  into a float

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We can use regex as   import re if re match   0-9     0-9      lt your string gt    print  quot Its a float int quot   else  print  quot Its something alien quot    let me explain the regex in english     - gt  0 or more occurence   - gt  1 or more occurence   - gt  0 1 occurence  now  lets convert    0-9   - gt  let there be 0 or more occurence of digits in between 0-9     - gt  followed by a 0 or one     if you need to check if it can be int float else we can also use instead of    use  1    0-9   - gt  followed by 0 or more occurence of digits in between 0-9

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Passing dictionary as argument it will convert strings which can be converted to float and will leave others      def covertDict float data           for i in data              if data i  split      0  isdigit                    try                      data i    float data i                   except                      continue         return data

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Python method to check for float   def isfloat value     try      float value      return True   except ValueError      return False   Don t get bit by the goblins hiding in the float boat   DO UNIT TESTING   What is  and is not a float may surprise you   Command to parse                        Is it a float   Comment --------------------------------------  --------------- ------------ print isfloat                           False print isfloat  1234567                  True  print isfloat  NaN                      True            nan is also float print isfloat  NaNananana BATMAN        False print isfloat  123 456                  True print isfloat  123 E4                   True print isfloat   1                       True print isfloat  1 234                    False print isfloat  NULL                     False           case insensitive print isfloat   1                       False            print isfloat  123 EE4                  False            print isfloat  6 523537535629999e-07    True print isfloat  6e777777                 True            This is same as Inf print isfloat  -iNF                     True print isfloat  1 797693e 308            True print isfloat  infinity                 True print isfloat  infinity and BEYOND      False print isfloat  12 34 56                 False           Two dots not allowed  print isfloat   56                      False print isfloat  56                       False print isfloat  0E0                      True print isfloat  x86E0                    False print isfloat  86-5                     False print isfloat  True                     False           Boolean is not a float     print isfloat True                      True            Boolean is a float print isfloat   1e1 5                   False print isfloat   1e1                     True print isfloat   1e1 3                   False print isfloat   1 3P1                   False print isfloat  - 1                      False print isfloat   1                       False           brackets not interpreted

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It seems many regex given miss one thing or another  This has been working for me so far    i   s   -     inf inity   nan     d     d     d     e  -   d     s    It allows for infinity  or inf  with sign  nan  no digit before the decimal  and leading trailing spaces  if desired   The   and   are needed to keep from partially matching something like 1 2f-2 as 1 2  You could use  ed  instead of just e if you need to parse some files where D is used for double-precision scientific notation  You would want to replace it afterward or just replace them before checking since the float   function won t allow it

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Simplified version of the function is digit str   which suffices in most cases  doesn t consider exponential notation and  NaN  value    def is digit str       return str lstrip  -   replace          isdigit

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Try to convert to float   If there is an error  print the ValueError exception   try      x   float  1 23       print  val   x      y   float  abc       print  val   y  except ValueError as err      print  floatErr   err    Output   val  1 23 floatErr  could not convert string to float   abc

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You can use the try-except-else clause   this will catch any conversion  value errors raised when the value passed cannot be converted to a float    def try parse float item         result   None       try          float item        except          pass       else          result   float item        return result

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TL DR    If your input is mostly strings that can be converted to floats  the try  except  method is the best native Python method  If your input is mostly strings that cannot be converted to floats  regular expressions or the partition method will be better  If you are 1  unsure of your input or need more speed and 2  don t mind and can install a third-party C-extension  fastnumbers works very well      There is another method available via a third-party module called fastnumbers  disclosure  I am the author   it provides a function called isfloat   I have taken the unittest example outlined by Jacob Gabrielson in this answer  but added the fastnumbers isfloat method   I should also note that Jacob s example did not do justice to the regex option because most of the time in that example was spent in global lookups because of the dot operator    I have modified that function to give a fairer comparison to try  except        def is float try str       try          float str          return True     except ValueError          return False  import re  float regexp   re compile r   -       b 0-9        0-9        0-9   b     eE  -    0-9   b      match def is float re str       return True if  float regexp str  else False  def is float partition element       partition element partition          if  partition 0  isdigit   and partition 1       and partition 2  isdigit    or  partition 0      and partition 1       and partition 2  isdigit    or  partition 0  isdigit   and partition 1       and partition 2                return True     else          return False  from fastnumbers import isfloat   if   name         main         import unittest     import timeit      class ConvertTests unittest TestCase            def test re perf self               print             print  re sad    timeit Timer  ttest is float re  12 2x      import ttest   timeit               print  re happy    timeit Timer  ttest is float re  12 2      import ttest   timeit            def test try perf self               print             print  try sad    timeit Timer  ttest is float try  12 2x      import ttest   timeit               print  try happy    timeit Timer  ttest is float try  12 2      import ttest   timeit            def test fn perf self               print             print  fn sad    timeit Timer  ttest isfloat  12 2x      import ttest   timeit               print  fn happy    timeit Timer  ttest isfloat  12 2      import ttest   timeit             def test part perf self               print             print  part sad    timeit Timer  ttest is float partition  12 2x      import ttest   timeit               print  part happy    timeit Timer  ttest is float partition  12 2      import ttest   timeit        unittest main       On my machine  the output is   fn sad  0 220988988876 fn happy  0 212214946747   part sad  1 2219619751 part happy  0 754667043686   re sad  1 50515985489 re happy  1 01107215881   try sad  2 40243887901 try happy  0 425730228424   ---------------------------------------------------------------------- Ran 4 tests in 7 761s  OK   As you can see  regex is actually not as bad as it originally seemed  and if you have a real need for speed  the fastnumbers method is quite good

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I would just use   try      float element  except ValueError      print  quot Not a float quot     it s simple  and it works  Note that it will still throw OverflowError if element is e g  1 lt  lt 1024  Another option would be a regular expression  import re if re match r  -  d       d      element  is None      print  quot Not float quot

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I was looking for some similar code  but it looks like using try excepts is the best way  Here is the code I m using  It includes a retry function if the input is invalid  I needed to check if the input was greater than 0 and if so convert it to a float   def cleanInput question retry False        inputValue   input   n nOnly positive numbers can be entered  please re-enter the value  n n    format question   if retry else input question      try          if float inputValue   lt   0   raise ValueError           else   return float inputValue       except ValueError   return cleanInput question retry True     willbefloat   cleanInput  Give me the number

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If you cared about performance  and I m not suggesting you should   the try-based approach is the clear winner  compared with your partition-based approach or the regexp approach   as long as you don t expect a lot of invalid strings  in which case it s potentially slower  presumably due to the cost of exception handling    Again  I m not suggesting you care about performance  just giving you the data in case you re doing this 10 billion times a second  or something   Also  the partition-based code doesn t handle at least one valid string        floatstr py F   partition sad  3 1102449894 partition happy  2 09208488464    re sad  7 76906108856 re happy  7 09421992302    try sad  12 1525540352 try happy  1 44165301323                                                                          FAIL  test partition    main   ConvertTests  ---------------------------------------------------------------------- Traceback  most recent call last     File    floatstr py   line 48  in test partition     self failUnless is float partition  20e2    AssertionError  ---------------------------------------------------------------------- Ran 8 tests in 33 670s  FAILED  failures 1    Here s the code  Python 2 6  regexp taken from John Gietzen s answer    def is float try str       try          float str          return True     except ValueError          return False  import re  float regexp   re compile r   -       b 0-9        0-9        0-9   b     eE  -    0-9   b      def is float re str       return re match  float regexp  str    def is float partition element       partition element partition          if  partition 0  isdigit   and partition 1       and partition 2  isdigit    or  partition 0      and partition 1       and pa  rtition 2  isdigit    or  partition 0  isdigit   and partition 1       and partition 2                return True  if   name         main         import unittest     import timeit      class ConvertTests unittest TestCase           def test re self               self failUnless is float re  20e2             def test try self               self failUnless is float try  20e2             def test re perf self               print             print  re sad    timeit Timer  floatstr is float re  12 2x      import floatstr   timeit               print  re happy    timeit Timer  floatstr is float re  12 2      import floatstr   timeit            def test try perf self               print             print  try sad    timeit Timer  floatstr is float try  12 2x      import floatstr   timeit               print  try happy    timeit Timer  floatstr is float try  12 2      import floatstr   timeit            def test partition perf self               print             print  partition sad    timeit Timer  floatstr is float partition  12 2x      import floatstr   timeit               print  partition happy    timeit Timer  floatstr is float partition  12 2      import floatstr   timeit            def test partition self               self failUnless is float partition  20e2             def test partition2 self               self failUnless is float partition   2             def test partition3 self               self failIf is float partition  1234x 2         unittest main

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str strval  isdigit     seems to be simple     Handles values stored in as a string or int or float

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This regex will check for scientific floating point numbers     -       b 0-9        0-9        0-9   b     eE  -    0-9   b      However  I believe that your best bet is to use the parser in a try

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1 43  replace        1  isdigit     which will return true only if there is one or no     in the string of digits     1 4 3  replace        1  isdigit     will return false    1 ww  replace        1  isdigit     will return false

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Just for variety here is another method to do it    gt  gt  gt  all  i isnumeric   for i in  1 2  split     1    True  gt  gt  gt  all  i isnumeric   for i in  2  split     1    True  gt  gt  gt  all  i isnumeric   for i in  2 f  split     1    False   Edit  Im sure it will not hold up to all cases of float though especially when there is an exponent  To solve that it looks like this  This will return True only val is a float and False for int but is probably less performant than regex     gt  gt  gt  def isfloat val           return all    any  i isnumeric    i in       e     for i in val    len val split          2         gt  gt  gt  isfloat  1   False  gt  gt  gt  isfloat  1 2   True  gt  gt  gt  isfloat  1 2e3   True  gt  gt  gt  isfloat  12e3   False

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I tried some of the above simple options  using a try test around converting to a float  and found that there is a problem in most of the replies   Simple test  along the lines of above answers    entry   ttk Entry self  validate  key   entry  validatecommand      entry register  test num     P    def  test num P       try           float P          return True     except ValueError          return False   The problem comes when    You enter  -  to start a negative number    You are then trying float  -   which fails   You enter a number  but then try to delete all the digits   You are then trying float     which likewise also fails  The quick solution I had is   def  test num P       if P       or P     -   return True     try           float P          return True     except ValueError          return False

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