Use abspath for errors when cwd changes during testing

changelog/6428.bugfix.rst

@@ -0,0 +1,2 @@
+Paths appearing in error messages are now correct in case the current working directory has
+changed since the start of the session.

src/_pytest/nodes.py

@@ -29,6 +29,7 @@
 from _pytest.mark.structures import MarkDecorator
 from _pytest.mark.structures import NodeKeywords
 from _pytest.outcomes import fail
+from _pytest.pathlib import Path
 from _pytest.store import Store
 
 if TYPE_CHECKING:
@@ -361,9 +362,14 @@ def _repr_failure_py(
         else:
             truncate_locals = True
 
+        # excinfo.getrepr() formats paths relative to the CWD if `abspath` is False.
+        # It is possible for a fixture/test to change the CWD while this code runs, which
+        # would then result in the user seeing confusing paths in the failure message.
+        # To fix this, if the CWD changed, always display the full absolute path.
+        # It will be better to just always display paths relative to invocation_dir, but
+        # this requires a lot of plumbing (#6428).
         try:
-            os.getcwd()
-            abspath = False
+            abspath = Path(os.getcwd()) != Path(self.config.invocation_dir)
         except OSError:
             abspath = True
 

testing/test_nodes.py

@@ -58,3 +58,30 @@ class FakeSession:
 
     outside = py.path.local("/outside")
     assert nodes._check_initialpaths_for_relpath(FakeSession, outside) is None
+
+
+def test_failure_with_changed_cwd(testdir):
+    """
+    Test failure lines should use absolute paths if cwd has changed since
+    invocation, so the path is correct (#6428).
+    """
+    p = testdir.makepyfile(
+        """
+        import os
+        import pytest
+
+        @pytest.fixture
+        def private_dir():
+            out_dir = 'ddd'
+            os.mkdir(out_dir)
+            old_dir = os.getcwd()
+            os.chdir(out_dir)
+            yield out_dir
+            os.chdir(old_dir)
+
+        def test_show_wrong_path(private_dir):
+            assert False
+    """
+    )
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines([str(p) + ":*: AssertionError", "*1 failed in *"])