TheAlgorithms/Python · error · ValueError

Invalid 'to_type' value: {to_type!r}. Supported values are:

Error message

Invalid 'to_type' value: {to_type!r}.  Supported values are:
{', '.join(METRIC_CONVERSION)}

What it means

Raised by volume_conversion() in conversions/volume_conversions.py when to_type is not a key in METRIC_CONVERSION (checked after from_type). Same supported set: cubic meter, litre, kilolitre, gallon, cubic yard, cubic foot, cup. Message lists all valid values.

Source

Thrown at conversions/volume_conversions.py:69

    >>> volume_conversion(2, "cubic yard", "litre")
    1529.1
    >>> volume_conversion(4, "cubic foot", "cup")
    473.396
    >>> volume_conversion(1, "cup", "kilolitre")
    0.000236588
    >>> volume_conversion(4, "wrongUnit", "litre")
    Traceback (most recent call last):
        ...
    ValueError: Invalid 'from_type' value: 'wrongUnit'  Supported values are:
    cubic meter, litre, kilolitre, gallon, cubic yard, cubic foot, cup
    """
    if from_type not in METRIC_CONVERSION:
        raise ValueError(
            f"Invalid 'from_type' value: {from_type!r}  Supported values are:\n"
            + ", ".join(METRIC_CONVERSION)
        )
    if to_type not in METRIC_CONVERSION:
        raise ValueError(
            f"Invalid 'to_type' value: {to_type!r}.  Supported values are:\n"
            + ", ".join(METRIC_CONVERSION)
        )
    return (
        value
        * METRIC_CONVERSION[from_type].from_factor
        * METRIC_CONVERSION[to_type].to_factor
    )


if __name__ == "__main__":
    import doctest

    doctest.testmod()

View on GitHub (pinned to f5988cc097)

Solutions

Restrict targets to the seven supported keys.
Post-convert for missing units: get litres from the library, then multiply by the needed factor (1 litre = 1.05669 US quarts).
Maintain one canonical unit vocabulary in your app and translate at the library boundary.

Example fix

# before
volume_conversion(1, 'litre', 'quart')  # ValueError: Invalid 'to_type' value: 'quart'

# after
litres = volume_conversion(1, 'litre', 'litre')
quarts = litres * 1.0566882094325937  # US liquid quart

Defensive patterns

Strategy: validation

Validate before calling

from conversions.volume_conversions import METRIC_CONVERSION

to_type = ALIASES.get(to_type.lower(), to_type)
if to_type not in METRIC_CONVERSION:
    raise ValueError(f'unsupported target volume unit {to_type!r}')
volume_conversion(value, from_type, to_type)

Type guard

from conversions.volume_conversions import METRIC_CONVERSION

def is_volume_unit(u: object) -> bool:
    return isinstance(u, str) and u in METRIC_CONVERSION

Try / catch

try:
    volume_conversion(v, f, t)
except ValueError as e:
    if 'to_type' in str(e):
        litres = volume_conversion(v, f, 'litre')
        result = litres * CUSTOM_FACTOR.get(t, 1)
    else:
        raise

Prevention

Only seven units supported; convert exotic targets via litre yourself
Watch for pluralized labels from NLP or user text
Keep a canonical unit vocabulary and translate once at the edge

When it happens

Trigger: Calling volume_conversion(1, 'litre', 'millilitre'), or targeting 'quart', 'pint', 'tablespoon' — none are in the chart.

Common situations: Cooking/measurement apps needing tsp/tbsp/fl oz, which this library lacks; pluralized unit names from natural-language parsing; output unit driven by user preference strings.

Related errors

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/5763589979fa2155. Report an issue: GitHub.