If you want to check whether a Python string is an integer, you can try casting to an int with int() and catching the ValueError if it’s not an integer:

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def is_integer(value: str, *, base: int=10) -> bool:
    try:
        int(value, base=base)
        return True
    except ValueError:
        return False

To check for nonnegative integers, you can use the str.is_digit() method. It will “return true if all characters in the string are digits and there is at least one character, false otherwise:

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>>> "123".isdigit()
True
>>> "-123".isdigit()
False

Thanks to Jeremy Kahn for reminding me that isdigit only detects positive integers.