From 8fd5a658eb7d36e6816d65ee8f81ebef79c1af14 Mon Sep 17 00:00:00 2001
From: Daniel Hahler <git@thequod.de>
Date: Fri, 12 Apr 2019 17:27:36 +0200
Subject: [PATCH] monkeypatch.syspath_prepend: invalidate import cache

This was done with testdir only, and uses the fixed monkeypatch method
there now.
---
 changelog/5098.bugfix.rst   |  1 +
 src/_pytest/monkeypatch.py  | 12 ++++++++++++
 src/_pytest/pytester.py     | 19 +------------------
 testing/test_monkeypatch.py |  7 +++++++
 4 files changed, 21 insertions(+), 18 deletions(-)
 create mode 100644 changelog/5098.bugfix.rst

diff --git a/changelog/5098.bugfix.rst b/changelog/5098.bugfix.rst
new file mode 100644
index 000000000..f0104b24e
--- /dev/null
+++ b/changelog/5098.bugfix.rst
@@ -0,0 +1 @@
+Invalidate import caches with ``monkeypatch.syspath_prepend``, which is required with namespace packages being used.
diff --git a/src/_pytest/monkeypatch.py b/src/_pytest/monkeypatch.py
index f6c134664..3e221d3d9 100644
--- a/src/_pytest/monkeypatch.py
+++ b/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.
diff --git a/src/_pytest/pytester.py b/src/_pytest/pytester.py
index 131876910..d90ef1ce7 100644
--- a/src/_pytest/pytester.py
+++ b/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)
-        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()
+        self.monkeypatch.syspath_prepend(str(path))
 
     def mkdir(self, name):
         """Create a new (sub)directory."""
diff --git a/testing/test_monkeypatch.py b/testing/test_monkeypatch.py
index d43fb6bab..9e45e05c8 100644
--- a/testing/test_monkeypatch.py
+++ b/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