Fix #2775 - running pytest with "--pyargs" will result in Items with empty "parent.nodeid" if run from a different root directory

_pytest/main.py

@@ -520,8 +520,18 @@ class FSCollector(Collector):
         super(FSCollector, self).__init__(name, parent, config, session)
         self.fspath = fspath
 
+    def _check_initialpaths_for_relpath(self):
+        for initialpath in self.session._initialpaths:
+            parent_path = self.fspath
+            for _ in parent_path.parts():
+                if parent_path.samefile(initialpath):
+                    return self.fspath.relto(initialpath.dirname)
+                parent_path = parent_path.__class__(parent_path.dirname)
+
     def _makeid(self):
         relpath = self.fspath.relto(self.config.rootdir)
+        if not relpath:
+            relpath = self._check_initialpaths_for_relpath()
         if os.sep != "/":
             relpath = relpath.replace(os.sep, "/")
         return relpath

changelog/2775.bugfix

@@ -0,0 +1 @@
+Fix the bug where running pytest with "--pyargs" will result in Items with empty "parent.nodeid" if run from a different root directory.

testing/acceptance_test.py

@@ -624,8 +624,10 @@ class TestInvocationVariants(object):
         for p in search_path:
             monkeypatch.syspath_prepend(p)
 
+        os.chdir('world')
         # mixed module and filenames:
-        result = testdir.runpytest("--pyargs", "-v", "ns_pkg.hello", "world/ns_pkg")
+        result = testdir.runpytest("--pyargs", "-v", "ns_pkg.hello", "ns_pkg/world")
+        testdir.chdir()
         assert result.ret == 0
         result.stdout.fnmatch_lines([
             "*test_hello.py::test_hello*PASSED",