🧪 Make stderr interceptable

Previously, `sys.stderr` was exposed as a module-scoped var, while
`capsys` patches the `stderr` attribute on the `sys` module. So such
patching did not influence the `stderr` variable in the `_cli` module
as it remained linked to the non-patched object.

With this change, the ability to inspect printing to stderr is
recovered.

Author: webknjaz

src/pre_commit_terraform/_cli.py

@@ -1,6 +1,6 @@
 """Outer CLI layer of the app interface."""
 
-from sys import stderr
+import sys
 from typing import cast as cast_to
 
 from ._cli_parsing import initialize_argument_parser
@@ -30,25 +30,25 @@ def invoke_cli_app(cli_args: list[str]) -> ReturnCodeType:
     try:
         return invoke_cli_app(parsed_cli_args)
     except PreCommitTerraformExit as exit_err:
-        print(f'App exiting: {exit_err !s}', file=stderr)
+        print(f'App exiting: {exit_err !s}', file=sys.stderr)
         raise
     except PreCommitTerraformRuntimeError as unhandled_exc:
         print(
             f'App execution took an unexpected turn: {unhandled_exc !s}. '
             'Exiting...',
-            file=stderr,
+            file=sys.stderr,
         )
         return ReturnCode.ERROR
     except PreCommitTerraformBaseError as unhandled_exc:
         print(
             f'A surprising exception happened: {unhandled_exc !s}. Exiting...',
-            file=stderr,
+            file=sys.stderr,
         )
         return ReturnCode.ERROR
     except KeyboardInterrupt as ctrl_c_exc:
         print(
             f'User-initiated interrupt: {ctrl_c_exc !s}. Exiting...',
-            file=stderr,
+            file=sys.stderr,
         )
         return ReturnCode.ERROR
 

tests/pytest/_cli_test.py

@@ -21,24 +21,29 @@
 
 
 @pytest.mark.parametrize(
-    'raised_error',
+    ('raised_error', 'expected_stderr'),
     (
         # pytest.param(PreCommitTerraformExit('sentinel'), 'App exiting: sentinel', id='app-exit'),
         pytest.param(
             PreCommitTerraformRuntimeError('sentinel'),
+            'App execution took an unexpected turn: sentinel. Exiting...',
             id='app-runtime-exc',
         ),
         pytest.param(
             PreCommitTerraformBaseError('sentinel'),
+            'A surprising exception happened: sentinel. Exiting...',
             id='app-base-exc',
         ),
         pytest.param(
             KeyboardInterrupt('sentinel'),
+            'User-initiated interrupt: sentinel. Exiting...',
             id='ctrl-c',
         ),
     ),
 )
 def test_known_interrupts(
+        capsys: pytest.CaptureFixture[str],
+        expected_stderr: str,
         monkeypatch: pytest.MonkeyPatch,
         raised_error: BaseException,
 ) -> None:
@@ -62,8 +67,14 @@ def invoke_cli_app(self, parsed_cli_args: Namespace) -> ReturnCodeType:
 
     assert ReturnCode.ERROR == invoke_cli_app(['sentinel'])
 
+    captured_outputs = capsys.readouterr()
+    assert captured_outputs.err == f'{expected_stderr !s}\n'
 
-def test_app_exit(monkeypatch: pytest.MonkeyPatch) -> None:
+
+def test_app_exit(
+        capsys: pytest.CaptureFixture[str],
+        monkeypatch: pytest.MonkeyPatch,
+) -> None:
     """Check that an exit exception is re-raised."""
     class CustomCmdStub:
         CLI_SUBCOMMAND_NAME = 'sentinel'
@@ -84,3 +95,6 @@ def invoke_cli_app(self, parsed_cli_args: Namespace) -> ReturnCodeType:
 
     with pytest.raises(PreCommitTerraformExit, match='^sentinel$'):
         invoke_cli_app(['sentinel'])
+
+    captured_outputs = capsys.readouterr()
+    assert captured_outputs.err == 'App exiting: sentinel\n'