Stop Click from crashing Black on invalid environments

Fixes pytest-dev#277

README.md

@@ -833,6 +833,10 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
   * they now work when an indentation block starts with empty lines or misaligned
     comments (#334)
 
+* made Click not fail on invalid environments; note that Click is right but the
+  likelihood we'll need to access non-ASCII file paths when dealing with Python source
+  code is low (#277)
+
 * fixed improper formatting of f-strings with quotes inside interpolated
   expressions (#322)
 

black.py

@@ -3482,5 +3482,28 @@ def write_cache(
         pass
 
 
+def patch_click() -> None:
+    """Make Click not crash.
+
+    On certain misconfigured environments, Python 3 selects the ASCII encoding as the
+    default which restricts paths that it can access during the lifetime of the
+    application.  Click refuses to work in this scenario by raising a RuntimeError.
+
+    In case of Black the likelihood that non-ASCII characters are going to be used in
+    file paths is minimal since it's Python source code.  Moreover, this crash was
+    spurious on Python 3.7 thanks to PEP 538 and PEP 540.
+    """
+    try:
+        from click import core
+        from click import _unicodefun  # type: ignore
+    except ModuleNotFoundError:
+        return
+
+    for module in (core, _unicodefun):
+        if hasattr(module, "_verify_python3_env"):
+            module._verify_python3_env = lambda: None
+
+
 if __name__ == "__main__":
+    patch_click()
     main()

tests/test_black.py

@@ -1213,6 +1213,28 @@ def test_symlink_out_of_root_directory(self) -> None:
         child.is_symlink.assert_called()
         self.assertEqual(child.is_symlink.call_count, 2)
 
+    def test_shhh_click(self) -> None:
+        try:
+            from click import _unicodefun  # type: ignore
+        except ModuleNotFoundError:
+            self.skipTest("Incompatible Click version")
+        if not hasattr(_unicodefun, "_verify_python3_env"):
+            self.skipTest("Incompatible Click version")
+        # First, let's see if Click is crashing with a preferred ASCII charset.
+        with patch("locale.getpreferredencoding") as gpe:
+            gpe.return_value = "ASCII"
+            with self.assertRaises(RuntimeError):
+                _unicodefun._verify_python3_env()
+        # Now, let's silence Click...
+        black.patch_click()
+        # ...and confirm it's silent.
+        with patch("locale.getpreferredencoding") as gpe:
+            gpe.return_value = "ASCII"
+            try:
+                _unicodefun._verify_python3_env()
+            except RuntimeError as re:
+                self.fail(f"`patch_click()` failed, exception still raised: {re}")
+
 
 if __name__ == "__main__":
     unittest.main(module="test_black")