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Web colors

A simple library for working with the color names and color codes defined by the HTML and CSS specifications.

An overview of HTML and CSS colors

Colors on the Web are specified in the sRGB color space, where each color is made up of a red component, a green component and a blue component. This is useful because it maps (fairly) cleanly to the red, green and blue components of pixels on a computer display, and to the cone cells of a human eye, which come in three sets roughly corresponding to the wavelengths of light associated with red, green and blue.

The HTML 4 standard defines two ways to specify sRGB colors:

• A hash mark ('#') followed by three pairs of hexdecimal digits, specifying values for red, green and blue components in that order; for example, #0099cc. Since each pair of hexadecimal digits can express 256 different values, this allows up to 256**3 or 16,777,216 unique colors to be specified (though, due to differences in display technology, not all of these colors may be clearly distinguished on any given physical display).
• A set of predefined color names which correspond to specific hexadecimal values; for example, white. HTML 4 defines sixteen such colors.

The CSS 2 standard allows any valid HTML 4 color specification, and adds three new ways to specify sRGB colors:

• A hash mark followed by three hexadecimal digits, which is expanded into three hexadecimal pairs by repeating each digit; thus #09c is equivalent to #0099cc.
• The string 'rgb', followed by parentheses, between which are three numeric values each between 0 and 255, inclusive, which are taken to be the values of the red, green and blue components in that order; for example, rgb(0, 153, 204).
• The same as above, except using percentages instead of numeric values; for example, rgb(0%, 60%, 80%).

The CSS 2.1 revision does not add any new methods of specifying sRGB colors, but does add one additional named color.

The CSS 3 color module (currently a W3C Candidate Recommendation) adds one new way to specify sRGB colors:

• A hue-saturation-lightness triple (HSL), using the construct hsl().

It also adds support for variable opacity of colors, by allowing the specification of alpha-channel information, through the rgba() and hsla() constructs, which are identical to rgb() and hsl() with one exception: a fourth value is supplied, indicating the level of opacity from 0.0 (completely transparent) to 1.0 (completely opaque). Though not technically a color, the keyword transparent is also made available in lieu of a color value, and corresponds to rgba(0,0,0,0).

Additionally, CSS3 defines a new set of color names; this set is taken directly from the named colors defined for SVG (Scalable Vector Graphics) markup, and is a proper superset of the named colors defined in CSS 2.1. This set also has significant overlap with traditional X11 color sets as defined by the rgb.txt file on many Unix and Unix-like operating systems, though the correspondence is not exact; the set of X11 colors is not standardized, and the set of CSS3 colors contains some definitions which diverge significantly from customary X11 definitions (for example, CSS3's green is not equivalent to X11's green; the value which X11 designates green is designated lime in CSS3).

What this module supports

The mappings and functions within this module support the following methods of specifying sRGB colors, and conversions between them:

• Integer rgb() triplet.
• Percentage rgb() triplet.
• Varying selections of predefined color names (see below).

This module does not support hsl() triplets, nor does it support opacity/alpha-channel information via rgba() or hsla().

If you need to convert between RGB-specified colors and HSL-specified colors, or colors specified via other means, consult the colorsys module in the Python standard library, which can perform conversions amongst several common color spaces.

Normalization

For colors specified via hexadecimal values, this module will accept input in the following formats:

• A hash mark (#) followed by three hexadecimal digits, where letters may be upper- or lower-case.
• A hash mark (#) followed by six hexadecimal digits, where letters may be upper- or lower-case.

For output which consists of a color specified via hexadecimal values, and for functions which perform intermediate conversion to hexadecimal before returning a result in another format, this module always normalizes such values to the following format:

• A hash mark (#) followed by six hexadecimal digits, with letters forced to lower-case.

The function normalize_hex() in this module can be used to perform this normalization manually if desired; see its documentation for an explanation of the normalization process.

For colors specified via predefined names, this module will accept input in the following formats:

• An entirely lower-case name, such as aliceblue.
• A name using initial capitals, such as AliceBlue.

For output which consists of a color specified via a predefined name, and for functions which perform intermediate conversion to a predefined name before returning a result in another format, this module always normalizes such values to be entirely lower-case.

Mappings of color names

For each set of defined color names -- HTML 4, CSS 2, CSS 2.1 and CSS 3 -- this module exports two mappings: one of normalized color names to normalized hexadecimal values, and one of normalized hexadecimal values to normalized color names. These eight mappings are as follows:

html4_names_to_hex
Mapping of normalized HTML 4 color names to normalized hexadecimal values.
html4_hex_to_names
Mapping of normalized hexadecimal values to normalized HTML 4 color names.
css2_names_to_hex
Mapping of normalized CSS 2 color names to normalized hexadecimal values. Because CSS 2 defines the same set of named colors as HTML 4, this is merely an alias for html4_names_to_hex.
css2_hex_to_names
Mapping of normalized hexadecimal values to normalized CSS 2 color nams. For the reasons described above, this is merely an alias for html4_hex_to_names.
css21_names_to_hex
Mapping of normalized CSS 2.1 color names to normalized hexadecimal values. This is identical to html4_names_to_hex, except for one addition: orange.
css21_hex_to_names
Mapping of normalized hexadecimal values to normalized CSS 2.1 color names. As above, this is identical to html4_hex_to_names except for the addition of orange.
css3_names_to_hex
Mapping of normalized CSS3 color names to normalized hexadecimal values.
css3_hex_to_names
Mapping of normalized hexadecimal values to normalized CSS3 color names.

Normalization functions

normalize_hex(hex_value)

Normalize a hexadecimal color value to the following form and return the result:

#[a-f0-9]{6}


In other words, the following transformations are applied as needed:

• If the value contains only three hexadecimal digits, it is expanded to six.
• The value is normalized to lower-case.

If the supplied value cannot be interpreted as a hexadecimal color value, ValueError is raised.

Example:

>>> normalize_hex('#09c')
'#0099cc'


Conversions from named colors

name_to_hex(name, spec='css3')

Convert a color name to a normalized hexadecimal color value.

The optional keyword argument spec determines which specification's list of color names will be used; valid values are html4, css2, css21 and css3, and the default is css3.

The color name will be normalized to lower-case before being looked up, and when no color of that name exists in the given specification, ValueError is raised.

Example:

>>> name_to_hex('deepskyblue')
'#00bfff'

name_to_rgb(name, spec='css3')

Convert a color name to a 3-tuple of integers suitable for use in an rgb() triplet specifying that color.

The optional keyword argument spec determines which specification's list of color names will be used; valid values are html4, css2, css21 and css3, and the default is css3.

The color name will be normalized to lower-case before being looked up, and when no color of that name exists in the given specification, ValueError is raised.

Example:

>>> name_to_rgb('navy')
(0, 0, 128)

name_to_rgb_percent(name, spec='css3')

Convert a color name to a 3-tuple of percentages suitable for use in an rgb() triplet specifying that color.

The optional keyword argument spec determines which specification's list of color names will be used; valid values are html4, css2, css21 and css3, and the default is css3.

The color name will be normalized to lower-case before being looked up, and when no color of that name exists in the given specification, ValueError is raised.

Example:

>>> name_to_rgb_percent('navy')
('0%', '0%', '50%')


hex_to_name(hex_value, spec='css3')

Convert a hexadecimal color value to its corresponding normalized color name, if any such name exists.

The optional keyword argument spec determines which specification's list of color names will be used; valid values are html4, css2, css21 and css3, and the default is css3.

The hexadecimal value will be normalized before being looked up, and when no color name for the value is found in the given specification, ValueError is raised.

Example:

>>> hex_to_name('#000080')
'navy'

hex_to_rgb(hex_value)

Convert a hexadecimal color value to a 3-tuple of integers suitable for use in an rgb() triplet specifying that color.

The hexadecimal value will be normalized before being converted.

Example:

>>> hex_to_rgb('#000080')
(0, 0, 128)

hex_to_rgb_percent(hex_value)

Convert a hexadecimal color value to a 3-tuple of percentages suitable for use in an rgb() triplet representing that color.

The hexadecimal value will be normalized before converting.

Example:

>>> hex_to_rgb_percent('#ffffff')
('100%', '100%', '100%')


Conversions from integer rgb() triplets

rgb_to_name(rgb_triplet, spec='css3')

Convert a 3-tuple of integers, suitable for use in an rgb() color triplet, to its corresponding normalized color name, if any such name exists.

The optional keyword argument spec determines which specification's list of color names will be used; valid values are html4, css2, css21 and css3, and the default is css3.

If there is no matching name, ValueError is raised.

Example:

>>> rgb_to_name((0, 0, 0))
'black'

rgb_to_hex(rgb_triplet)

Convert a 3-tuple of integers, suitable for use in an rgb() color triplet, to a normalized hexadecimal value for that color.

Example:

>>> rgb_to_hex((255, 255, 255))
'#ffffff'

rgb_to_rgb_percent(rgb_triplet)

Convert a 3-tuple of integers, suitable for use in an rgb() color triplet, to a 3-tuple of percentages suitable for use in representing that color.

This function makes some trade-offs in terms of the accuracy of the final representation; for some common integer values, special-case logic is used to ensure a precise result (e.g., integer 128 will always convert to '50%', integer 32 will always convert to '12.5%'), but for all other values a standard Python float is used and rounded to two decimal places, which may result in a loss of precision for some values.

Examples:

>>> rgb_to_rgb_percent((255, 255, 255))
('100%', '100%', '100%')
>>> rgb_to_rgb_percent((0, 0, 128))
('0%', '0%', '50%')
>>> rgb_to_rgb_percent((33, 56, 192))
('12.94%', '21.96%', '75.29%')
>>> rgb_to_rgb_percent((64, 32, 16))
('25%', '12.5%', '6.25%')


Conversions from percentage rgb() triplets

rgb_percent_to_name(rgb_percent_triplet, spec='css3')

Convert a 3-tuple of percentages, suitable for use in an rgb() color triplet, to its corresponding normalized color name, if any such name exists.

The optional keyword argument spec determines which specification's list of color names will be used; valid values are html4, css2, css21 and css3, and the default is css3.

If there is no matching name, ValueError is raised.

Example:

>>> rgb_percent_to_name(('0%', '0%', '50%'))
'navy'

rgb_percent_to_hex(rgb_percent_triplet)

Convert a 3-tuple of percentages, suitable for use in an rgb() color triplet, to a normalized hexadecimal color value for that color.

Example:

>>> rgb_percent_to_hex(('100%', '100%', '0%'))
'#ffff00'

rgb_percent_to_rgb(rgb_percent_triplet)

Convert a 3-tuple of percentages, suitable for use in an rgb() color triplet, to a 3-tuple of integers suitable for use in representing that color.

Some precision may be lost in this conversion. See the note regarding precision for rgb_to_rgb_percent() for details; generally speaking, the following is true for any 3-tuple t of integers in the range 0...255 inclusive:

t == rgb_percent_to_rgb(rgb_to_rgb_percent(t))


Examples:

>>> rgb_percent_to_rgb(('100%', '100%', '100%'))
(255, 255, 255)
>>> rgb_percent_to_rgb(('0%', '0%', '50%'))
(0, 0, 128)
>>> rgb_percent_to_rgb(('25%', '12.5%', '6.25%'))
(64, 32, 16)
>>> rgb_percent_to_rgb(('12.94%', '21.96%', '75.29%'))
(33, 56, 192)