Load external packages pathlib2 and scandir before internal packages

- see #462

Author: mrbean-bremen

CHANGES.md

@@ -3,10 +3,15 @@ The release versions are PyPi releases.
 
 ## Version 3.6 (as yet unreleased)
 
+#### Changes
+  * import external `pathlib2` and `scandir` packages first if present
+    (see [#462](../../issues/462))
+    
 #### New Features
 
 #### Fixes
 
+
 ## [Version 3.5.5](https://pypi.python.org/pypi/pyfakefs/3.5.5)
 
 #### Fixes

extra_requirements.txt

@@ -8,6 +8,6 @@
 # Older versions might work ok, the versions chosen here are just the latest
 # available at the time of writing.
 
-pathlib2>=2.3.2
+pathlib2>=2.3.2;python_version<'3.4'
 
-scandir>=1.8
+scandir>=1.8;python_version<'3.5'

pyfakefs/extra_packages.py

@@ -1,24 +1,40 @@
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Imports external packages that replace or emulate internal packages.
+If the external module is not present, the build-in module is imported.
+"""
+
 try:
-    import pathlib
-    pathlib2 = None
+    import pathlib2
+    pathlib = None
 except ImportError:
     try:
-        import pathlib2
-        pathlib = pathlib2
+        import pathlib
+        pathlib2 = None
     except ImportError:
         pathlib = None
         pathlib2 = None
 
 
 try:
-    from os import scandir
+    import scandir
     use_scandir = True
-    use_scandir_package = False
+    use_scandir_package = True
 except ImportError:
     try:
-        import scandir
+        from os import scandir
         use_scandir = True
-        use_scandir_package = True
+        use_scandir_package = False
     except ImportError:
         use_scandir = False
         use_scandir_package = False