TheAlgorithms/Python · error · ValueError

power_factor must be a valid float value between -1 and 1.

Error message

power_factor must be a valid float value between -1 and 1.

What it means

Thrown by real_power() in electronics/real_and_reactive_power.py when power_factor is not an int/float or falls outside [-1, 1]. Real power P = S * pf is only defined for a cosine-like power factor in the closed range [-1, 1]; the isinstance check also means booleans pass (bool is an int subclass) but strings/None fail even if they look numeric.

Source

Thrown at electronics/real_and_reactive_power.py:21

def real_power(apparent_power: float, power_factor: float) -> float:
    """
    Calculate real power from apparent power and power factor.

    Examples:
    >>> real_power(100, 0.9)
    90.0
    >>> real_power(0, 0.8)
    0.0
    >>> real_power(100, -0.9)
    -90.0
    """
    if (
        not isinstance(power_factor, (int, float))
        or power_factor < -1
        or power_factor > 1
    ):
        raise ValueError("power_factor must be a valid float value between -1 and 1.")
    return apparent_power * power_factor


def reactive_power(apparent_power: float, power_factor: float) -> float:
    """
    Calculate reactive power from apparent power and power factor.

    Examples:
    >>> reactive_power(100, 0.9)
    43.58898943540673
    >>> reactive_power(0, 0.8)
    0.0
    >>> reactive_power(100, -0.9)
    43.58898943540673
    """
    if (
        not isinstance(power_factor, (int, float))
        or power_factor < -1

View on GitHub (pinned to f5988cc097)

Solutions

1. Pass power_factor as a float in [-1, 1] — e.g. real_power(100, 0.9) -> 90.0.
2. Convert numeric strings first: float(value); divide percentages by 100.
3. Clamp floating-point drift: max(-1.0, min(1.0, pf)) when the value is computed, not user-entered.

Example fix

# before
real_power(100, '0.9')   # ValueError
real_power(100, 90)      # percentage passed raw -> ValueError

# after
real_power(100, float('0.9'))          # 90.0
real_power(100, 90 / 100)              # 90.0

if not isinstance(power_factor, (int, float)) or not -1 <= power_factor <= 1:
    raise ValueError('power_factor must be a number in [-1, 1]')

def valid_power_factor(pf: object) -> bool:
    return isinstance(pf, (int, float)) and not isinstance(pf, bool) and -1 <= pf <= 1

try:
    p = real_power(s, pf)
except ValueError as exc:
    if 'power_factor' in str(exc):
        pf = float(pf) if isinstance(pf, str) else max(-1.0, min(1.0, pf))
        p = real_power(s, pf)
    else:
        raise

When it happens

Trigger: real_power(100, 1.5); real_power(100, '0.9') (string not isinstance of (int, float)); real_power(100, None); any pf where not isinstance(power_factor, (int,float)) or pf < -1 or pf > 1.

Common situations: Reading power factor from config/JSON as a string and passing it unconverted; forgetting that pf can be expressed as a percentage (90 instead of 0.9); calculations that drift slightly outside the range due to floating-point rounding (e.g. 1.0000000000000002).

Related errors

• Power cannot be negative in any electrical/electronics syste
• One and only one argument must be 0
• One and only one argument must be 0
• Inductance cannot be negative
• Frequency cannot be negative

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/ec13d6dfa2f3288e. Report an issue: GitHub.