Conformance and testing#
Much of the behavior of webcolors is dictated by the relevant web standards, which define the acceptable color formats, how to determine valid values for each format and the values corresponding to defined color names. Maintaining correct conversions and conformance to those standards is crucial.
The source distribution of webcolors (the
.tar.gz file you can download
from the Python Package Index) includes a
tests/ directory containing a
normal test suite as well as supplemental test files which perform more
The normal test suite#
The normal test suite for webcolors aims for 100% coverage of code paths, but does not aim for 100% coverage of possible color value inputs and outputs. Instead, it uses a small number of test values to routinely exercise various functions.
The test values used in most test functions are chosen to provide, where applicable, at least one of each of the following types of values:
An endpoint of the acceptable range of values (i.e.,
A value beyond the high end of the acceptable range (i.e., greater than 255 in an integer triplet, or greater than 100% for a percentage triplet).
A value beyond the low end of the acceptable range (i.e., less than 0 in an integer triplet, or less than 0% for a percentage triplet).
A “negative zero” value (-0 in an integer triplet, or -0% in a percentage triplet).
An arbitrary value not from an endpoint of the acceptable range (usually
#000080, chosen because the author likes navy blue).
A value which corresponds to a named color in CSS3/SVG but not in earlier standards (usually
#daa520, which is
Since this covers the cases most likely to produce problems, this test suite provides good basic confidence in the correctness of the tested functions. It runs on every commit to the repository, and on every release tag. You can see the results of test runs online at GitHub.
However, the normal test suite cannot guarantee that the color definitions included in webcolors correspond to those in the relevant standards, and cannot provide guarantees of correct conversions for all possible values. For that, additional tests are required.
Full verification tests#
These tests are contained in two files which are not executed during normal
tests/full_colors.py. They are not
run as part of the normal test suite, but are run prior to each release of
Verifying color definitions#
definitions.py test file verifies that the color definitions in
webcolors are correct. It does this by downloading the relevant standards
documents as HTML, parsing out the color definitions in them, and comparing
them to the definitions in webcolors. That consists of:
Parsing out the names and hexadecimal values of the 16 named colors in the HTML 4 standard, and checking that the names and values in
Parsing out the names and hexadecimal values of the 17 named colors in the CSS2.1 standard, and checking that the names and values in
Parsing out the names and hexadecimal and integer values of the 147 named colors in the CSS3 color module (although the color set is taken from SVG, CSS3 provides both hexadecimal and integer values for them, while the SVG standard provides only integer values), and checking that the names and values in
CSS3_NAMES_TO_HEXmatch, and that
name_to_rgb()returns the correct integer values.
Fully verifying correctness of conversions#
The full_colors.py test file exercises
rgb_percent_to_rgb() as fully as is practical.
For conversions between hexadecimal and integer
rgb(), it generates all
16,777,216 possible color values for each format in order (starting at
(0,0,0) and incrementing), and verifies that each one
converts to the corresponding value in the other format. Thus, it is possible
to be confident that webcolors provides correct conversions between all
possible color values in those formats.
Testing the correctness of conversion to and from percentage
however, is more difficult, and a full test is not provided, for two reasons:
rgb()values can make use of floating-point values, and because standard floating-point types in most common programming languages (Python included) are inherently imprecise, exact verification is not possible.
The only rigorous definition of the format of a percentage value is in CSS2, which declares a percentage to be “a <number> immediately followed by ‘%’”. The CSS2 definition of a number places no limit on the length past the decimal point, and appears to be declaring any real number as a valid value, though percentage triplets clip their inputs to the range 0.0-100.0. As the subset of reals in the range 0.0 to 100.0 is uncountably infinite, testing all legal values is not possible on current hardware in any reasonable amount of time.
Since precise correctness and completeness are not achievable, webcolors
instead aims to achieve consistency in conversions. Specifically, the
full_colors.py test generates all 16,777,216 integer
and for each such triplet t verifies that the following assertion holds:
t == rgb_percent_to_rgb(rgb_to_rgb_percent(t))
Running the tests#
The standard test runner for webcolors is nox,
which supports testing against multiple Python versions and executing a variety
of different test tasks. The source distribution of webcolors includes its
noxfile.py file. To run the tests, install nox (
pip install nox), then
download and unpack a git checkout or source package of webcolors.
To run the normal test suite against the complete set of supported Python versions:
python -m pip install nox python -m nox
py -m pip install nox py -m nox
This requires that you have each supported version of Python (for webcolors
1.13, this is 3.7, 3.8, 3.9, 3.10, and 3.11) available. To test only
against a specific version of Python, use the
--python flag and pass the
version to test. For example, to test on Python 3.10:
python -m nox --python "3.10"
py -m nox --python "3.10"
To run the full verification tests for definition correctness and conversions, specify the “release” test keyword instead (so named because these tests are usually run only prior to a new release of webcolors):
python -m nox --keyword release
py -m nox --keyword release
Note that this requires an internet connection, and is CPU-intensive.