Detect whether a Python string is a number or a letter

Question

How can I detect either numbers or letters in a string  I am aware you use the ASCII codes  but what functions take advantage of them

User · Answer

For a string of length 1 you can simply perform isdigit() or isalpha()

If your string length is greater than 1, you can make a function something like..

def isinteger(a):
    try:
        int(a)
        return True
    except ValueError:
        return False

User · Answer

Check if string is positive digit  integer  and alphabet You may use str isdigit   and str isalpha   to check whether given string is positive integer and alphabet respectively  Sample Results    For alphabet  gt  gt  gt   A  isdigit   False  gt  gt  gt   A  isalpha   True    For digit  gt  gt  gt   1  isdigit   True  gt  gt  gt   1  isalpha   False  Check for strings 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  Allow Complex Number like  quot 1 2j quot  to be treated as valid number The above function will still return you False for the complex numbers  If you want your is number function to treat complex numbers as valid number  then you need to type cast your passed string to complex   instead of float    Then your is number function will look like  def is number n       is number   True     try                 v type-casting the number here as  complex   instead of  float          num   complex n          is number   num    num     except ValueError          is number   False     return is number  Sample Run   gt  gt  gt  is number  1 2j        Valid  True                              complex number    gt  gt  gt  is number  1  2j       Invalid  False                             string with space in complex number represetantion                                   is treated as invalid complex number   gt  gt  gt  is number  123         Valid True                              positive integer   gt  gt  gt  is number  -123        Valid  True                              negative integer   gt  gt  gt  is number  abc         Invalid  False                             some random string  not a valid number   gt  gt  gt  is number  nan         Invalid False                             not a number  quot nan quot  string  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

[python] Detect whether a Python string is a number or a letter

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