TheAlgorithms/Python · error · ValueError

{color} is not a valid color

Error message

{color} is not a valid color

What it means

Raised by check_validity() in electronics/resistor_color_code.py when one of the colors in the colors list is not in the module-level valid_colors collection. valid_colors enumerates the accepted resistor-band color names (Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White, Gold, Silver, plus temperature-coefficient colors for 6-band use). Matching is case-sensitive and exact.

Source

Thrown at electronics/resistor_color_code.py:285

    True

    >>> check_validity(4, ["Black","Blue","Orange"])
    Traceback (most recent call last):
      ...
    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% '}

View on GitHub (pinned to f5988cc097)

Solutions

1. Pass exact color names with the casing used in valid_colors (e.g. 'Black', 'Blue', 'Orange').
2. Normalize input before calling: title-case the string and verify membership in valid_colors (import it from electronics.resistor_color_code).
3. Map common aliases (UK 'Grey' vs 'Gray', abbreviations) to canonical names before validation.
4. Catch ValueError and reprompt the user with the list of allowed colors.

Example fix

# before
check_validity(3, ["cyan", "Red", "Yellow"])  # ValueError

# after
from electronics.resistor_color_code import valid_colors
from electronics.resistor_color_code import check_validity
colors = ["cyan", "Red", "Yellow"]
canonical = [c.capitalize() for c in colors]
assert all(c in valid_colors for c in canonical)
check_validity(3, canonical)

from electronics.resistor_color_code import valid_colors

canonical = [c.strip().capitalize() for c in colors]
if not all(c in valid_colors for c in canonical):
    bad = [c for c in canonical if c not in valid_colors]
    raise UserInputError(f"Unknown colors: {bad}; allowed: {sorted(valid_colors)}")

def are_valid_colors(colors: list[str]) -> bool:
    return all(isinstance(c, str) and c in valid_colors for c in colors)

try:
    check_validity(bands, colors)
except ValueError as exc:
    # reprompt with the allowed color list
    show_error(str(exc), allowed=sorted(valid_colors))

When it happens

Trigger: check_validity(3, ['Cyan', 'Red', 'Yellow']) — Cyan is not a resistor band color; passing lowercase or abbreviated colors like 'blk', 'black', or 'GRY'; passing a hex color string.

Common situations: Colors parsed from free-text user input or scraped from a datasheet table; case mismatches ('black' vs 'Black'); colors valid for one position but not the global list (e.g. 'None').

Related errors

• {color} is not a valid color for significant figure bands
• {total_number_of_bands} is not a valid number of bands
• {color} is not a valid color for multiplier band
• {color} is not a valid color for tolerance band
• {color} is not a valid color for temperature coeffecient ban

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/4867728906ca9886. Report an issue: GitHub.