TheAlgorithms/Python · error · TypeError

Input must be a string

Error message

Input must be a string

What it means

Raised by count_vowels in strings/count_vowels.py when s is not a str. The function counts characters from the fixed vowel set 'aeiouAEIOU' by iterating the string, so any non-string (int, list, None) is rejected up front with TypeError. Unlike several neighbors in this repo, this one correctly uses TypeError for a type mismatch.

Source

Thrown at strings/count_vowels.py:25

    Examples:
    >>> count_vowels("hello world")
    3
    >>> count_vowels("HELLO WORLD")
    3
    >>> count_vowels("123 hello world")
    3
    >>> count_vowels("")
    0
    >>> count_vowels("a quick brown fox")
    5
    >>> count_vowels("the quick BROWN fox")
    5
    >>> count_vowels("PYTHON")
    1
    """
    if not isinstance(s, str):
        raise TypeError("Input must be a string")

    vowels = "aeiouAEIOU"
    return sum(1 for char in s if char in vowels)


if __name__ == "__main__":
    from doctest import testmod

    testmod()

View on GitHub (pinned to f5988cc097)

Solutions

1. Convert at the boundary: count_vowels(str(text)).
2. Handle None explicitly: count_vowels(text or '').
3. Decode bytes first: count_vowels(blob.decode('utf-8')) if handling raw byte input.

Example fix

# before
count_vowels(payload.get('comment'))  # comment missing -> None

# after
count_vowels(payload.get('comment') or '')

if not isinstance(s, str):
    raise TypeError(f'expected str, got {type(s).__name__}')
count = count_vowels(s)

def is_str(value) -> bool:
    return isinstance(value, str)

try:
    n = count_vowels(text)
except TypeError:
    n = 0 if text is None else count_vowels(str(text))

When it happens

Trigger: count_vowels(123); count_vowels(['a', 'e']); count_vowels(None) when an optional text field was never provided.

Common situations: Optional fields defaulting to None; JSON numbers where text was expected; passing bytes instead of str (bytes is also rejected).

Related errors

• Expected string as input, found {type(input_str)}
• 'float' object cannot be interpreted as an integer
• 'float' object cannot be interpreted as an integer
• iterations must be defined as integers
• All weights must be integers but got weight of type {type(wt

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/25808e12ec003827. Report an issue: GitHub.