Fix nodes._check_initialpaths_for_relpath for dirs

Ref: #4321 (comment)

Author: blueyed

src/_pytest/nodes.py

@@ -447,6 +447,8 @@ def _prunetraceback(self, excinfo):
 def _check_initialpaths_for_relpath(session, fspath):
     for initial_path in session._initialpaths:
         if fspath.common(initial_path) == initial_path:
+            if initial_path.isdir():
+                return fspath.relto(initial_path)
             return fspath.relto(initial_path.dirname)
 
 

testing/test_nodes.py

@@ -1,3 +1,5 @@
+import py
+
 import pytest
 from _pytest import nodes
 
@@ -29,3 +31,19 @@ def test():
     )
     with pytest.raises(ValueError, match=".*instance of PytestWarning.*"):
         items[0].warn(UserWarning("some warning"))
+
+
+def test__check_initialpaths_for_relpath():
+    """Ensure that it handles dirs, and does not always use dirname."""
+    d = py.path.local()
+
+    class FakeSession:
+        _initialpaths = [d]
+
+    assert nodes._check_initialpaths_for_relpath(FakeSession, d) == ""
+
+    f = d.join("file")
+    assert nodes._check_initialpaths_for_relpath(FakeSession, f) == "file"
+
+    outside = py.path.local("/outside")
+    assert nodes._check_initialpaths_for_relpath(FakeSession, outside) is None