Import distutils.file_util to prevent fake fs caching problem

- closes #501

Author: mrbean-bremen

CHANGES.md

@@ -8,11 +8,13 @@ of 2020. As pyfakefs 4.0 is a major release, we are giving you advance notice of
 the proposed changes so you can be ready.
 
   * pyfakefs 4.0 drops support for Python 2.7. If you still need
-    Python 2.7, you can still use the latest pyfakefs 3.x version. 
+    Python 2.7, you can continue to use the latest pyfakefs 3.x version. 
 
 ## Version 3.7 (as yet unreleased)
 
 ### Fixes
+  * fixed a problem related to patching `distutils` functions
+    ([#501](../../issues/501))
   * correctly handle missing read permission for parent directory
     (see [#496](../../issues/496))
   * raise for `os.scandir` with non-existing directory

pyfakefs/fake_filesystem_unittest.py

@@ -43,6 +43,7 @@
 import unittest
 import warnings
 import zipfile  # noqa: F401 make sure it gets correctly stubbed, see #427
+import distutils.file_util  # noqa: F401 same reason - see #501
 
 from pyfakefs.fake_filesystem import set_uid, set_gid, reset_ids
 

pyfakefs/tests/fake_filesystem_unittest_test.py

@@ -25,6 +25,7 @@
 import sys
 import tempfile
 import unittest
+from distutils.dir_util import copy_tree, remove_tree
 from unittest import TestCase
 
 from pyfakefs import fake_filesystem_unittest, fake_filesystem
@@ -604,5 +605,32 @@ def test_b(self):
         shutil.make_archive('archive', 'zip', root_dir='foo')
 
 
+class TestDistutilsCopyTree(fake_filesystem_unittest.TestCase):
+    """Regression test for #501."""
+
+    def setUp(self):
+        self.setUpPyfakefs()
+        self.fs.create_dir("./test/subdir/")
+        self.fs.create_dir("./test/subdir2/")
+        self.fs.create_file("./test2/subdir/1.txt")
+
+    def test_file_copied(self):
+        copy_tree("./test2/", "./test/")
+        remove_tree("./test2/")
+
+        self.assertTrue(os.path.isfile('./test/subdir/1.txt'))
+        self.assertFalse(os.path.isdir('./test2/'))
+
+    def test_file_copied_again(self):
+        # used to fail because 'test2' could not be found
+        self.assertTrue(os.path.isfile('./test2/subdir/1.txt'))
+
+        copy_tree("./test2/", "./test/")
+        remove_tree("./test2/")
+
+        self.assertTrue(os.path.isfile('./test/subdir/1.txt'))
+        self.assertFalse(os.path.isdir('./test2/'))
+
+
 if __name__ == "__main__":
     unittest.main()