package: read a specific Python version requirement file

This change makes pbr reads a specific requirement file tight to the
major Python version that is used. So you can add a file such as
requirements-py3.txt to have a specific requirement file for Python 3,
or requirements-py2.txt to have a specific requirement file for
Python 2.

Change-Id: I4b548e9830586f11b82539d334dd5cf4b0445a36

doc/source/index.rst

@@ -67,6 +67,21 @@ your project, will parse them and split them up approprirately, and inject
 them into the install_requires and/or tests_require and/or dependency_links
 arguments to setup. Voila!
 
+You can also have a requirement file for each specific major version of
+Python. If you want to have a different package list for Python 3, just drop
+a requirements-py3.txt, and it will be used instead.
+
+The requirement files are tried in that order (N being the Python major
+version number used to install the package):
+
+* requirements-pyN.txt
+* tools/pip-requires-py3
+* requirements.txt
+* tools/pip-requires
+
+Only the first file found is used to install the list of packages it
+contains.
+
 long_description
 ----------------
 

pbr/packaging.py

@@ -57,7 +57,13 @@ def get_requirements_files():
     files = os.environ.get("PBR_REQUIREMENTS_FILES")
     if files:
         return tuple(f.strip() for f in files.split(','))
-    return REQUIREMENTS_FILES
+    # Returns a list composed of:
+    # - REQUIREMENTS_FILES with -py2 or -py3 in the name
+    #   (e.g. requirements-py3.txt)
+    # - REQUIREMENTS_FILES
+    return (list(map(('-py' + str(sys.version_info[0])).join,
+                     map(os.path.splitext, REQUIREMENTS_FILES)))
+            + list(REQUIREMENTS_FILES))
 
 
 def append_text_list(config, key, text_list):

pbr/tests/test_setup.py

@@ -352,6 +352,22 @@ class ParseRequirementsTest(base.BaseTestCase):
         self.assertEqual(['foobar', 'foobaz'],
                          packaging.parse_requirements([self.tmp_file]))
 
+    def test_parse_requirements_python_version(self):
+        with open("requirements-py%d.txt" % sys.version_info[0],
+                  "w") as fh:
+            fh.write("# this is a comment\nfoobar\n# and another one\nfoobaz")
+        self.assertEqual(['foobar', 'foobaz'],
+                         packaging.parse_requirements())
+
+    def test_parse_requirements_right_python_version(self):
+        with open("requirements-py1.txt", "w") as fh:
+            fh.write("thisisatrap")
+        with open("requirements-py%d.txt" % sys.version_info[0],
+                  "w") as fh:
+            fh.write("# this is a comment\nfoobar\n# and another one\nfoobaz")
+        self.assertEqual(['foobar', 'foobaz'],
+                         packaging.parse_requirements())
+
 
 class ParseDependencyLinksTest(base.BaseTestCase):