TheAlgorithms/Python · error · TypeError

Input must be an integer

Error message

Input must be an integer

What it means

Raised by num_digits() in maths/number_of_digits.py when n is not an int. The function counts decimal digits with integer floor-division (n = n // 10), an operation that only terminates correctly for ints; floats, strings, and other types are rejected with TypeError before the loop.

Source

Thrown at maths/number_of_digits.py:26

    >>> num_digits(12345)
    5
    >>> num_digits(123)
    3
    >>> num_digits(0)
    1
    >>> num_digits(-1)
    1
    >>> num_digits(-123456)
    6
    >>> num_digits('123')  # Raises a TypeError for non-integer input
    Traceback (most recent call last):
        ...
    TypeError: Input must be an integer
    """

    if not isinstance(n, int):
        raise TypeError("Input must be an integer")

    digits = 0
    n = abs(n)
    while True:
        n = n // 10
        digits += 1
        if n == 0:
            break
    return digits


def num_digits_fast(n: int) -> int:
    """
    Find the number of digits in a number.
    abs() is used as logarithm for negative numbers is not defined.

    >>> num_digits_fast(12345)
    5

View on GitHub (pinned to f5988cc097)

Solutions

1. Convert first: num_digits(int(value)).
2. Use argparse type=int or int(input()) so inputs arrive as ints.
3. If accepting floats, decide policy (count digits of int part) and convert explicitly: num_digits(int(abs(x))).

Example fix

# before
num_digits('123')

# after
num_digits(int('123'))

n = int(n)  # raises early with your own context if n is a bad string

def is_int(x) -> bool:
    return isinstance(x, int) and not isinstance(x, bool)

When it happens

Trigger: num_digits('123') as in the doctest, num_digits(12.0), num_digits(None), or num_digits(True) (which passes, since bool subclasses int).

Common situations: Values from input()/CLI remaining strings, JSON numbers parsed as float, or passing the wrong variable (a label or flag instead of the count).

Related errors

• Expected a list of numbers as input, found {type(item).__nam
• Expected a list of numbers as input, found {type(point).__na
• Input value must be a positive integer
• Input value must be a 'int' type
• Input must be an integer

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/561b4a84119dbf0e. Report an issue: GitHub.