GitHub matrix job to test against old dependencies

The job pins Python dependencies to oldest supported versions.

.github/workflows/python-package.yml

@@ -18,6 +18,11 @@ jobs:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
         python-version: ['3.6', '3.7', '3.8', '3.9', '3.10']
+        constraints: ['']
+        include:
+        - os: ubuntu-latest
+          python-version: '3.6'
+          constraints: '--constraint constraints-oldest.txt'
 
     steps:
     - uses: actions/checkout@v2
@@ -30,7 +35,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade 'pip>=20.3'  # strict dependency resolution added in pip 20.3
-        pip install -e '.[isort,test]'
+        pip install -e '.[isort,test]' ${{ matrix.constraints }}
     - name: Test with pytest
       run: |
         pytest

constraints-oldest.txt

@@ -0,0 +1,13 @@
+# Constraints for pip to pin dependencies to oldest supported versions.
+# This is used in a GitHub Workflow matrix job which ensures everything
+# still works against oldest supported versions of both the Python
+# interpreter and Python ependencies. Keep this up-to-date with minimum
+# versions in `setup.cfg`.
+black==21.8b0
+mypy==0.910
+pytest==6.1.0
+pytest-flake8==1.0.6
+pytest-isort==1.1.0
+pytest-kwparametrize==0.0.3
+regex==2021.4.4
+types-toml==0.1.3

setup.cfg

@@ -26,10 +26,13 @@ package_dir =
     =src
 packages = find:
 install_requires =
+    # NOTE: remember to keep `constraints-oldest.txt` in sync with these
     black>=21.5b1
     toml
     typing-extensions ; python_version < "3.8"
     dataclasses ; python_version < "3.7"
+# NOTE: remember to keep `.github/workflows/python-package.yml` in sync
+#       with the minimum required Python version
 python_requires = >=3.6
 
 [options.packages.find]
@@ -46,6 +49,7 @@ pygments.lexers =
 isort =
     isort>=5.0.1
 test =
+    # NOTE: remember to keep `constraints-oldest.txt` in sync with these
     black>=21.7b1  # to prevent Mypy error about `gen_python_files`, see issue #189
     flake8<4
     mypy>=0.910

src/darker/tests/test_black_diff.py

@@ -1,6 +1,7 @@
 """Unit tests for `darker.black_diff`"""
 
 import re
+from dataclasses import dataclass, field
 from pathlib import Path
 from unittest.mock import ANY, patch
 
@@ -17,6 +18,20 @@
 from darker.utils import TextDocument
 
 
+@dataclass
+class RegexEquality:
+    """Compare equality to either `re.Pattern` or `regex.Pattern`"""
+
+    pattern: str
+    flags: int = field(default=re.UNICODE)
+
+    def __eq__(self, other):
+        return (
+            other.pattern == self.pattern
+            and other.flags & 0x1FF == re.compile(self.pattern).flags | self.flags
+        )
+
+
 @pytest.mark.kwparametrize(
     dict(
         config_path=None, config_lines=["line-length = 79"], expect={"line_length": 79}
@@ -46,15 +61,19 @@
     dict(config_lines=[r"include = '\.pyi$'"], expect={}),
     dict(
         config_lines=[r"exclude = '\.pyx$'"],
-        expect={"exclude": re.compile("\\.pyx$")},
+        expect={"exclude": RegexEquality("\\.pyx$")},
     ),
     dict(
         config_lines=["extend-exclude = '''", r"^/setup\.py", r"|^/dummy\.py", "'''"],
-        expect={"extend_exclude": re.compile("(?x)^/setup\\.py\n|^/dummy\\.py\n")},
+        expect={
+            "extend_exclude": RegexEquality(
+                "(?x)^/setup\\.py\n|^/dummy\\.py\n", re.VERBOSE
+            )
+        },
     ),
     dict(
         config_lines=["force-exclude = '''", r"^/setup\.py", r"|\.pyc$", "'''"],
-        expect={"force_exclude": re.compile("(?x)^/setup\\.py\n|\\.pyc$\n")},
+        expect={"force_exclude": RegexEquality("(?x)^/setup\\.py\n|\\.pyc$\n")},
     ),
     config_path=None,
 )