TheAlgorithms/Python · error · ValueError

Input list must be a non empty list

Error message

Input list must be a non empty list

What it means

Raised by is_harmonic_series() in maths/series/harmonic.py when the input is a list with zero elements. An empty series has no reciprocals to compare, so the common-difference test on the reciprocal sequence is impossible and the function rejects the input early with a clear message.

Source

Thrown at maths/series/harmonic.py:43

        ...
    ValueError: Input series is not valid, valid series - [1, 2/3, 2]
    >>> is_harmonic_series([])
    Traceback (most recent call last):
        ...
    ValueError: Input list must be a non empty list
    >>> is_harmonic_series([0])
    Traceback (most recent call last):
        ...
    ValueError: Input series cannot have 0 as an element
    >>> is_harmonic_series([1,2,0,6])
    Traceback (most recent call last):
        ...
    ValueError: Input series cannot have 0 as an element
    """
    if not isinstance(series, list):
        raise ValueError("Input series is not valid, valid series - [1, 2/3, 2]")
    if len(series) == 0:
        raise ValueError("Input list must be a non empty list")
    if len(series) == 1 and series[0] != 0:
        return True
    rec_series = []
    series_len = len(series)
    for i in range(series_len):
        if series[i] == 0:
            raise ValueError("Input series cannot have 0 as an element")
        rec_series.append(1 / series[i])
    common_diff = rec_series[1] - rec_series[0]
    for index in range(2, series_len):
        if rec_series[index] - rec_series[index - 1] != common_diff:
            return False
    return True


def harmonic_mean(series: list) -> float:
    """
    return the harmonic mean of series

View on GitHub (pinned to f5988cc097)

Solutions

1. Check truthiness before calling: if series and is_harmonic_series(series)
2. Validate data length at ingestion time and reject empty input upstream
3. Provide a sensible default or skip processing when empty

Example fix

# before
result = is_harmonic_series(data)

# after
result = bool(data) and is_harmonic_series(data)

result = bool(series) and is_harmonic_series(series)

def is_non_empty_list(value: object) -> bool:
    return isinstance(value, list) and len(value) > 0

When it happens

Trigger: Calling is_harmonic_series([]). Any code path that constructs the series dynamically (parsing, filtering, slicing) and yields an empty list will hit this.

Common situations: Parsing '1,2,3' style input where the string was empty; slicing a list with bad indices; filtering all elements out. Single-element lists are fine ([x] with x != 0 returns True).

Related errors

• Input list must be a non empty list
• Length must be a positive integer.
• {num}: Invalid input, please enter a positive integer.
• max_set_length must be non-negative
• Number {n} must instead be a positive integer

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/a53da48d191219d1. Report an issue: GitHub.