Convert violation to relative path for subfolder support (#574)

* Convert violation to relative path for subfolder support

* Fix lint error

* Pytest fixes: resetting `GitPathTool` between tests

Author: eacherkan-via

diff_cover/git_path.py

@@ -36,6 +36,10 @@ def relative_path(cls, git_diff_path):
         """
         Returns git_diff_path relative to cwd.
         """
+        # If GitPathTool hasn't been initialized, return the path unchanged
+        if cls._cwd is None or cls._root is None:
+            return git_diff_path
+
         # Remove git_root from src_path for searching the correct filename
         # If cwd is `/home/user/work/diff-cover/diff_cover`
         # and src_path is `diff_cover/violations_reporter.py`

diff_cover/violationsreporters/base.py

@@ -6,6 +6,7 @@
 from collections import defaultdict, namedtuple
 
 from diff_cover.command_runner import execute, run_command_for_code
+from diff_cover.git_path import GitPathTool
 from diff_cover.util import to_unix_path
 
 Violation = namedtuple("Violation", "line, message")
@@ -152,14 +153,19 @@ def violations(self, src_path):
         if not any(src_path.endswith(ext) for ext in self.driver.supported_extensions):
             return []
 
-        if src_path not in self.violations_dict:
+        # `src_path` is relative to the git root. We convert it to be relative to
+        # the current working directory, since quality tools report paths relative
+        # to the current working directory.
+        relative_src_path = to_unix_path(GitPathTool.relative_path(src_path))
+
+        if relative_src_path not in self.violations_dict:
             if self.reports:
                 self.violations_dict = self.driver.parse_reports(self.reports)
-                return self.violations_dict[src_path]
+                return self.violations_dict[relative_src_path]
 
-            if not os.path.exists(src_path):
-                self.violations_dict[src_path] = []
-                return self.violations_dict[src_path]
+            if not os.path.exists(relative_src_path):
+                self.violations_dict[relative_src_path] = []
+                return self.violations_dict[relative_src_path]
 
             if self.driver_tool_installed is None:
                 self.driver_tool_installed = self.driver.installed()
@@ -170,13 +176,13 @@ def violations(self, src_path):
             if self.options:
                 for arg in self.options.split():
                     command.append(arg)
-            command.append(src_path.encode(sys.getfilesystemencoding()))
+            command.append(relative_src_path.encode(sys.getfilesystemencoding()))
 
             stdout, stderr = execute(command, self.driver.exit_codes)
             output = stderr if self.driver.output_stderr else stdout
             self.violations_dict.update(self.driver.parse_reports([output]))
 
-        return self.violations_dict[src_path]
+        return self.violations_dict[relative_src_path]
 
     def measured_lines(self, src_path):
         """

tests/conftest.py

@@ -0,0 +1,17 @@
+import pytest
+
+from diff_cover.git_path import GitPathTool
+
+
+@pytest.fixture(autouse=True)
+def reset_git_path_tool():
+    """Reset GitPathTool before each test to ensure test isolation.
+
+    GitPathTool uses class variables (_cwd and _root) that persist across tests.
+    This fixture ensures each test starts with a clean state.
+    """
+    GitPathTool._cwd = None
+    GitPathTool._root = None
+    yield
+    GitPathTool._cwd = None
+    GitPathTool._root = None

tests/test_violations_reporter.py

@@ -2312,3 +2312,46 @@ def test_parse_report(self):
         driver = ClangFormatDriver()
         actual_violations = driver.parse_reports([report])
         assert actual_violations == expected_violations
+
+
+class TestQualityReporterSubdirectory:
+    """
+    Test that QualityReporter works correctly when running from a subdirectory.
+
+    When running diff-quality from a subdirectory:
+    - Git reports paths relative to the git root (e.g., "subdir/file.py")
+    - Quality tools report paths relative to the current working directory (e.g., "file.py")
+    """
+
+    def test_violations_from_subdirectory(self, mocker, process_patcher):
+        """
+        Test that violations are found when running from a subdirectory.
+
+        Simulates running diff-quality from "subdir/" where:
+        - Git reports the file as "subdir/file.py" (relative to git root)
+        - The quality tool reports violations on "file.py" (relative to cwd)
+        """
+        from diff_cover.git_path import GitPathTool
+
+        # Simulate running from a subdirectory by mocking relative_path
+        # to strip the "subdir/" prefix (as it would when cwd is inside subdir/)
+        mocker.patch.object(
+            GitPathTool, "relative_path", side_effect=lambda x: x.replace("subdir/", "")
+        )
+
+        # Quality tool outputs violations with paths relative to cwd
+        # (without the "subdir/" prefix)
+        tool_output = "file.py:10: error: Something is wrong  [error-code]"
+        process_patcher((tool_output.encode("utf-8"), b""))
+
+        quality = QualityReporter(mypy_driver)
+
+        # Request violations using the git-relative path (with "subdir/" prefix)
+        # This is what diff-quality would pass based on git diff output
+        violations = quality.violations("subdir/file.py")
+
+        # Verify violations are found (the fix makes this work)
+        expected = [
+            Violation(line=10, message="error: Something is wrong  [error-code]")
+        ]
+        assert violations == expected