TheAlgorithms/Python · error · ValueError

Expecting {number_of_bands} colors, provided {len(colors)} c

Error message

Expecting {number_of_bands} colors, provided {len(colors)} colors

What it means

Raised by check_validity() in electronics/resistor_color_code.py when len(colors) differs from number_of_bands (after number_of_bands has already passed the 3-6 range check). A resistor's color list must contain exactly one color per band: 3 bands means 3 colors, 4 means 4, etc.

Source

Thrown at electronics/resistor_color_code.py:289

      ...
    ValueError: Expecting 4 colors, provided 3 colors

    >>> check_validity(3, ["Cyan","Red","Yellow"])
    Traceback (most recent call last):
      ...
    ValueError: Cyan is not a valid color

    """
    if number_of_bands >= 3 and number_of_bands <= 6:
        if number_of_bands == len(colors):
            for color in colors:
                if color not in valid_colors:
                    msg = f"{color} is not a valid color"
                    raise ValueError(msg)
            return True
        else:
            msg = f"Expecting {number_of_bands} colors, provided {len(colors)} colors"
            raise ValueError(msg)
    else:
        msg = "Invalid number of bands. Resistor bands must be 3 to 6"
        raise ValueError(msg)


def calculate_resistance(number_of_bands: int, color_code_list: list) -> dict:
    """
    Function calculates the total resistance of the resistor using the color codes.
    Function takes number_of_bands, color_code_list as input and returns
    resistance

    >>> calculate_resistance(3, ["Black","Blue","Orange"])
    {'resistance': '6000Ω ±20% '}

    >>> calculate_resistance(4, ["Orange","Green","Blue","Gold"])
    {'resistance': '35000000Ω ±5% '}

    >>> calculate_resistance(5, ["Violet","Brown","Grey","Silver","Green"])

View on GitHub (pinned to f5988cc097)

Solutions

Count the resistor's physical bands first and pass exactly that many colors, in band order (significant digits, multiplier, tolerance, temp coefficient).
Validate len(colors) == number_of_bands before calling check_validity or calculate_resistance.
For a 4-band resistor remember the layout is digit-digit-multiplier-tolerance (4 colors), not 3.

Example fix

# before
calculate_resistance(4, ["Violet", "Brown", "Grey", "Silver", "Green"])  # 5 colors

# after
calculate_resistance(5, ["Violet", "Brown", "Grey", "Silver", "Green"])  # 5 bands, 5 colors

Defensive patterns

Strategy: validation

Validate before calling

if len(color_code_list) != number_of_bands:
    raise UserInputError(
        f"Need exactly {number_of_bands} colors, got {len(color_code_list)}"
    )

Try / catch

try:
    result = calculate_resistance(number_of_bands, color_code_list)
except ValueError as exc:
    logger.warning("Resistor input rejected: %s", exc)
    result = None

Prevention

Derive number_of_bands from len(color_code_list) instead of passing both independently.
Remember band layout: N bands means exactly N colors including tolerance and temp-coefficient bands.
Lint user input lengths in forms before submission.

When it happens

Trigger: calculate_resistance(4, ['Violet','Brown','Grey','Silver','Green']) — 5 colors for a 4-band resistor; forgetting that a 4-band code includes the tolerance band; dropping or adding a band when transcribing a code.

Common situations: Transcribing resistor codes by hand and missing the tolerance band; parsing banded strings and splitting incorrectly; assuming the multiplier band is not part of the count.

Related errors

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/fb82b7a77fe9d227. Report an issue: GitHub.