TheAlgorithms/Python · error · ValueError

Resistance cannot be negative

Error message

Resistance cannot be negative

What it means

Thrown by ohms_law() when resistance < 0. Resistance is a magnitude in ohms and cannot be negative, so after the one-zero sentinel check the function rejects negative resistance before computing. Voltage and current are allowed to be negative (direction/sign conventions), resistance is not.

Source

Thrown at electronics/ohms_law.py:28

    >>> ohms_law(voltage=10, resistance=5, current=0)
    {'current': 2.0}
    >>> ohms_law(voltage=0, current=0, resistance=10)
    Traceback (most recent call last):
      ...
    ValueError: One and only one argument must be 0
    >>> ohms_law(voltage=0, current=1, resistance=-2)
    Traceback (most recent call last):
      ...
    ValueError: Resistance cannot be negative
    >>> ohms_law(resistance=0, voltage=-10, current=1)
    {'resistance': -10.0}
    >>> ohms_law(voltage=0, current=-1.5, resistance=2)
    {'voltage': -3.0}
    """
    if (voltage, current, resistance).count(0) != 1:
        raise ValueError("One and only one argument must be 0")
    if resistance < 0:
        raise ValueError("Resistance cannot be negative")
    if voltage == 0:
        return {"voltage": float(current * resistance)}
    elif current == 0:
        return {"current": voltage / resistance}
    elif resistance == 0:
        return {"resistance": voltage / current}
    else:
        raise ValueError("Exactly one argument must be 0")


if __name__ == "__main__":
    import doctest

    doctest.testmod()

View on GitHub (pinned to f5988cc097)

Solutions

1. Pass resistance as a positive value in ohms.
2. If resistance came from V/I with a tiny I, clamp or re-measure rather than passing the noisy negative result.
3. Use abs() only when the negative sign is known to be spurious.

Example fix

# before
ohms_law(voltage=0, current=1, resistance=-2)  # ValueError: Resistance cannot be negative

# after
ohms_law(voltage=0, current=1, resistance=2)  # {'voltage': 2.0}

if resistance < 0:
    raise ValueError('resistance must be >= 0')

def valid_resistance(r: float) -> bool:
    return r >= 0

try:
    out = ohms_law(v, i, r)
except ValueError as exc:
    if 'Resistance cannot be negative' in str(exc):
        # noisy V/I measurement produced negative R — re-measure or clamp
        ...
    raise

When it happens

Trigger: ohms_law(voltage=0, current=1, resistance=-2); ohms_law(current=0, voltage=-10, resistance=-5) — any call with resistance < 0 and exactly one zero argument.

Common situations: Passing a computed resistance that went negative due to measurement noise or a division by a near-zero current; sign conventions from circuit solvers that use negative resistance for active elements; input typos with a stray minus sign.

Related errors

• One and only one argument must be 0
• 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

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/d4be23097ea9cf50. Report an issue: GitHub.