Original PHP script from this stackoverflow question : https://stackoverflow.com/a/42921358/8439435 then updated to Python and following the WCAG 2.0 - G18.

color_contrast.py

BLACK_COLOR = "#000000"
WHITE_COLOR = "#FFFFFF"

def get_contrast_color(background_color: str) -> str:
    """
    Util function to determine what's the best color between black or white to choose for a text
    depending on the background color given in parameter.
    Based on algorythm from WCAG 2.0 explained here : https://www.w3.org/TR/WCAG20-TECHS/G18.html#G18-tests

    :param background_color: the background color in HEX format (eg: #9D412B)
    :return: A black or a white color in HEX format
    """
    def calc_luminosity_contrast(color: str) -> float:
        extract_colors = (color[i:i+2] for i in range(1, len(color), 2))
        colors_8bit = (int(c, 16) / 255 for c in extract_colors)

        def relative_luminance(value: float) -> float:
            return (value / 12.92) if value <= 0.03928 else pow((value + 0.055) / 1.055, 2.4)

        red, green, blue = (relative_luminance(c) for c in colors_8bit)
        return 0.2126 * red + 0.7152 * green + 0.0722 * blue

    l1 = calc_luminosity_contrast(background_color) + 0.05
    l2 = calc_luminosity_contrast(BLACK_COLOR) + 0.05

    contrast_ratio = l1 / l2 if l1 > l2 else l2 / l1

    return BLACK_COLOR if contrast_ratio >= 4.5 else WHITE_COLOR

Beware: the luminosity contrast algorithm used here is based on an early working draft of WCAG 2.0 techniques. The formula and required ratio changed in the final version.

@fuzzbomb Thanks for the notice, I've updated my code !