TheAlgorithms/Python · error · ValueError

One and only one argument must be 0

Error message

One and only one argument must be 0

What it means

Thrown by ohms_law() in electronics/ohms_law.py when the count of zero arguments among (voltage, current, resistance) is not exactly one. The function solves V = I*R for whichever quantity is 0 — 0 is the sentinel for the unknown, not a physical value. Note that voltage and current may legitimately be negative (the doctests show {'resistance': -10.0} and {'voltage': -3.0}); only the count of zeros matters here.

Source

Thrown at electronics/ohms_law.py:26

    and resistance, and then in a Python dict return name/value pair of the zero value.

    >>> 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 exactly one argument as 0: ohms_law(voltage=0, current=2, resistance=5) -> {'voltage': 10.0}.
2. Pre-check (voltage, current, resistance).count(0) == 1 for dynamic inputs.
3. Skip the call entirely when all three values are already known.

Example fix

# before
ohms_law(voltage=2, current=2, resistance=5)  # ValueError

# after
v = ohms_law(voltage=0, current=2, resistance=5)['voltage']  # 10.0

if (voltage, current, resistance).count(0) != 1:
    raise ValueError('exactly one of V/I/R must be 0 (the unknown)')

def has_single_unknown(v: float, i: float, r: float) -> bool:
    return (v, i, r).count(0) == 1

try:
    out = ohms_law(v, i, r)
except ValueError as exc:
    if 'must be 0' in str(exc):
        # wrong sentinel usage — fix the call, don't retry
        ...
    raise

When it happens

Trigger: ohms_law(voltage=2, current=2, resistance=5) (all known); ohms_law(voltage=0, current=0, resistance=5) (two zeros); using None instead of 0 to mark the unknown.

Common situations: Assuming the function validates a fully-specified triple; porting from APIs that used None as a placeholder; batch-processing rows where none or several fields are zero.

Related errors

• Resistance cannot be negative
• 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/e21cde404dca0b68. Report an issue: GitHub.