monkeypatch.syspath_prepend: invalidate import cache

This was done with testdir only, and uses the fixed monkeypatch method
there now.

changelog/5098.bugfix.rst

@@ -0,0 +1 @@
+Invalidate import caches with ``monkeypatch.syspath_prepend``, which is required with namespace packages being used.

src/_pytest/monkeypatch.py

@@ -271,6 +271,18 @@ class MonkeyPatch(object):
         # https://github.com/pypa/setuptools/blob/d8b901bc/docs/pkg_resources.txt#L162-L171
         fixup_namespace_packages(str(path))
 
+        # A call to syspathinsert() usually means that the caller wants to
+        # import some dynamically created files, thus with python3 we
+        # invalidate its import caches.
+        # This is especially important when any namespace package is in used,
+        # since then the mtime based FileFinder cache (that gets created in
+        # this case already) gets not invalidated when writing the new files
+        # quickly afterwards.
+        if sys.version_info >= (3, 3):
+            from importlib import invalidate_caches
+
+            invalidate_caches()
+
     def chdir(self, path):
         """ Change the current working directory to the specified path.
         Path can be a string or a py.path.local object.

src/_pytest/pytester.py

@@ -616,27 +616,10 @@ class Testdir(object):
         This is undone automatically when this object dies at the end of each
         test.
         """
-        from pkg_resources import fixup_namespace_packages
-
         if path is None:
             path = self.tmpdir
 
-        dirname = str(path)
+        self.monkeypatch.syspath_prepend(str(path))
-        sys.path.insert(0, dirname)
-        fixup_namespace_packages(dirname)
-
-        # a call to syspathinsert() usually means that the caller wants to
-        # import some dynamically created files, thus with python3 we
-        # invalidate its import caches
-        self._possibly_invalidate_import_caches()
-
-    def _possibly_invalidate_import_caches(self):
-        # invalidate caches if we can (py33 and above)
-        try:
-            from importlib import invalidate_caches
-        except ImportError:
-            return
-        invalidate_caches()
 
     def mkdir(self, name):
         """Create a new (sub)directory."""

testing/test_monkeypatch.py

@@ -462,3 +462,10 @@ def test_syspath_prepend_with_namespace_packages(testdir, monkeypatch):
     import ns_pkg.world
 
     assert ns_pkg.world.check() == "world"
+
+    # Should invalidate caches via importlib.invalidate_caches.
+    tmpdir = testdir.tmpdir
+    modules_tmpdir = tmpdir.mkdir("modules_tmpdir")
+    monkeypatch.syspath_prepend(str(modules_tmpdir))
+    modules_tmpdir.join("main_app.py").write("app = True")
+    from main_app import app  # noqa: F401