Some unexpected characters are numeric or digits

@July 21, 2023

We found an interesting trait of str.isnumeric() and str.isdigit() that we didn’t expect…

Let’s first look at the help texts (Python 3.10.6)

isdigit(self, /)

Copy

Return True if the string is a digit string, False otherwise.

A string is a digit string if all characters in the string are digits and there
is at least one character in the string.

and conversely

isnumeric(self, /)

Copy

Return True if the string is a numeric string, False otherwise.

A string is numeric if all characters in the string are numeric and there
is at least one character in the string.

This can lead to the impression that if a string consists of only digits, it must be a number and indeed, in most cases this works as expected:

>>> num_str = “1234”
>>> num_str.isdigit()
True
>>> num_str.isnumeric()
True
>>> int(num_str)
1234

but there is now the case of raised numbers and fraction ASCII characters that we didn’t expect and that you should consider:

>>> pow3 = '³'
>>> pow3.isdigit()
True
>>> pow3.isnumeric()
True
>>> int(pow3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '³'

>>> quarter = '¼'
>>> quarter.isdigit()
False
>>> quarter.isnumeric()
True
>>> float(quarter)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: could not convert string to float: '¼'

So consider using good old (and slower) regular expressions for input validation that is then fed to int() or float()

import re

re.match('\d+', pow3)  # this pretend-digit is no match for regex :D