Prohibit contrast degradation for styles via test (#1919)

Web accessibility is important. Unfortunately currently many pygments
styles have rules with poor contrasts. This commit introduces a test
case that fails if the minimum contrast of a style gets worse, e.g:

E  AssertionError: contrast degradation for style 'borland'
E    The following rules have a contrast lower than the required 2.9:
E
E    * 1.90 Token.Text.Whitespace
E    * 2.80 Token.Generic.Heading
E    * 2.30 Token.Generic.Subheading
E
E  assert not 1.9 < 2.9

This is accomplished by storing the current minimum contrasts in
./tests/contrast/min_contrasts.json.

When you improve a minimum contrast the test fails with:

E  AssertionError: congrats, you improved a contrast! please run ./scripts/update_contrasts.py
E  assert not 1.9 > 0.9

Running the script as instructed updates the JSON file, making the test pass.

New styles are required to meet the WCAG AA contrast minimum of 4.5.

First commit to address #1718.

1 files changed, 21 insertions, 0 deletions

diff --git a/scripts/update_contrasts.py b/scripts/update_contrasts.py
new file mode 100755
index 00000000..156bc5cb
--- /dev/null
+++ b/scripts/update_contrasts.py
@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+"""
+    Updates tests/contrast/min_contrasts.json
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+    Whenever you have improved the minimum contrast of a style you should run
+    this script, so that the test_contrasts.py test prevents future degredations.
+"""
+
+import os
+import sys
+
+# always prefer Pygments from source if exists
+srcpath = os.path.join(os.path.dirname(__file__), "..")
+if os.path.isdir(os.path.join(srcpath, "pygments")):
+    sys.path.insert(0, srcpath)
+
+import tests.contrast.test_contrasts
+
+tests.contrast.test_contrasts.test_contrasts(fail_if_improved=False)
+tests.contrast.test_contrasts.update_json()