Normalization and conventions¶
Since the various formats used to specify colors in Web documents do not always map cleanly to Python data types, and some variation is permitted in how to use each format in a Web document, webcolors applies a set of conventions for representing color names and values, and for normalizing them.
Python string types¶
The webcolors module is written to be compatible with both Python 2 and Python 3, which have different approaches to strings:
- On Python 2, a sequence of bytes in a particular encoding (a “byte
string”) is represented by the type
str, and Unicode strings are represented by the type
unicode. Promiscuous mixing of
unicodeis possible in Python 2, but not recommended as it is a frequent source of bugs.
- On Python 3, a sequence of bytes in a particular encoding is
represented by the type
bytes, and Unicode strings are represented by the type
str. Promiscuous mixing of
bytesis not permitted in Python 3, and will usually raise exceptions.
The approach to string types in webcolors is as follows:
- On Python 3, use of Unicode strings –
str– is mandatory for all string arguments to functions in webcolors. Use of
bytesvalues is forbidden.
- All mappings from color names to hexadecimal values (and vice versa)
are dictionaries whose keys and values are Unicode strings (
stron Python 3 and
unicodeon Python 2). This permits promiscuous use of byte strings on Python 2, but ensures that results will be Unicode strings.
- All functions whose return values include strings will use Unicode
unicodeon Python 2 and
stron Python 3).
- All functions whose arguments include string values, except for
the HTML5 color algorithms (see below), will accept a sequence of
str) on Python 2, but will convert to Unicode strings (
unicode) for output.
Because the HTML5 Recommendation specifies its color algorithms in terms of Unicode strings only (and in some cases, requires exact identification of Unicode code points to determine behavior), the following constraint applies to the functions implementing these algorithms:
- Any string arguments must be Unicode strings (
unicodeon Python 2 or
stron Python 3). Use of
stron Python 2 or
byteson Python 3 will raise a
Use of Unicode strings whenever possible is strongly preferred. To
encourage this, all documentation for webcolors uses the
prefix for string literals. Use of the
u prefix is required on
Python 2 to mark a string literal as Unicode; on Python 3.3 and later,
use of this prefix is permitted but not necessary (as all un-prefixed
string literals on Python 3 are Unicode strings).
Due to use of the
u prefix, using webcolors on Python 3 will
require at least Python 3.3.
Hexadecimal color values¶
For colors specified via hexadecimal values, webcolors will accept strings in the following formats:
- The character
#followed by three hexadecimal digits, where digits A-F may be upper- or lowercase.
- The character
#followed by six hexadecimal digits, where digits A-F may be upper- or lowercase (i.e., what HTML5 designates a “valid simple color” when all digits are uppercase, and a “valid lowercase simple color” when all digits are lowercase).
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, webcolors always normalizes such values to a string in the following format:
- The character
#followed by six hexadecimal digits, with digits A-F forced to lowercase (what HTML5 designates a “valid lowercase simple color”).
normalize_hex() can be used to perform
this normalization manually if desired.
Integer and percentage
Functions which work with integer
rgb() triplets accept and return
them as a 3-tuple of Python
int. Functions which work with
rgb() triplets accept them as 3-tuple of Python strings
unicode is permitted on Python 2; only
is permitted on Python 3) and return them as a 3-tuple of Python
Unicode strings (
str depending on Python version).
float is used in some conversions to and from
the triplet representations; for each function which may have the
precision of its results affected by this, a note is provided in the
For colors specified via
rgb() triplets, values contained in the
triplets will be normalized via clipping in accordance with CSS:
- Integer values less than 0 will be normalized to 0, and percentage values less than 0% will be normalized to 0%.
- Integer values greater than 255 will be normalized to 255, and percentage values greater than 100% will be normalized to 100%.
- The “negative zero” values -0 and -0% will be normalized to 0 and 0%, respectively.
For colors specified via predefined names, webcolors will accept
strings containing names case-insensitively, so long as they contain
no spaces or non-alphabetic characters. Thus, for example,
u'aliceblue' are both accepted, and both will
refer to the same color (namely,
rgb(240, 248, 255)).
For output which consists of a color name, and for functions which perform intermediate conversion to a predefined name before returning a result in another format, webcolors always normalizes such values to be entirely lowercase.
Identifying sets of named colors¶
For purposes of identifying the specification from which to draw the selection of defined color names, webcolors recognizes the following strings as identifiers:
- The HTML 4 named colors.
- The CSS 2 named colors.
- The CSS 2.1 named colors.
- The CSS 3/SVG named colors. For all functions for which the set of color names is relevant, this is the default set used.
The CSS 1 named colors are not represented here, as CSS 1 merely “suggested” a set of color names, and declined to provide values for them. The CSS 2 “system colors” are also not represented here, as they had no fixed defined values and are now deprecated.