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

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?

Answer 1

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 

Answer 2

Check if string is nonnegative digit (integer) and alphabet

You may use str.isdigit() and str.isalpha() to check whether a given string is a nonnegative integer (0 or greater) and alphabetical character, respectively.

Sample Results:

# For alphabet >>> 'A'.isdigit() False >>> 'A'.isalpha() True  # For digit >>> '1'.isdigit() True >>> '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 >>> '123.3'.isdigit() False # returns `False` for negative number >>> '-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:

>>> is_number('123')    # positive integer number True  >>> is_number('123.4')  # positive float number True   >>> is_number('-123')   # negative integer number True  >>> is_number('-123.4') # negative `float` number True  >>> is_number('abc')    # `False` for "some random" string False 

Discard "NaN" (not a number) strings while checking for number

The above functions will return True for the "NAN" (Not a number) string because for Python it is valid float representing it is not a number. For example:

>>> is_number('NaN') True 

In order to check whether the number is "NaN", you may use math.isnan() as:

>>> import math >>> nan_num = float('nan')  >>> 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 >>> nan_num == nan_num False 

Hence, above function is_number can be updated to return False for "NaN" as:

def is_number(n):     is_number = True     try:         num = float(n)         # check for "nan" floats         is_number = num == num   # or use `math.isnan(num)`     except ValueError:         is_number = False     return is_number 

Sample Run:

>>> is_number('Nan')   # not a number "Nan" string False  >>> is_number('nan')   # not a number string "nan" with all lower cased False  >>> is_number('123')   # positive integer True  >>> is_number('-123')  # negative integer True  >>> is_number('-1.12') # negative `float` True  >>> is_number('abc')   # "some random" string False 

Allow Complex Number like "1+2j" 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:

>>> is_number('1+2j')    # Valid  True                     #      : complex number   >>> is_number('1+ 2j')   # Invalid  False                    #      : string with space in complex number represetantion                          #        is treated as invalid complex number  >>> is_number('123')     # Valid True                     #      : positive integer  >>> is_number('-123')    # Valid  True                     #      : negative integer  >>> is_number('abc')     # Invalid  False                    #      : some random string, not a valid number  >>> is_number('nan')     # Invalid False                    #      : not a number "nan" 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.