parse_repository_tag function and accompanying test

docker/utils/__init__.py

 from .utils import (
-    compare_version, convert_port_bindings, mkbuildcontext, ping, tar
+    compare_version, convert_port_bindings, mkbuildcontext, ping, tar, parse_repository_tag
 ) # flake8: noqa

docker/utils/utils.py

@@ -114,3 +114,15 @@ def convert_port_bindings(port_bindings):
         else:
             result[key] = [_convert_port_binding(v)]
     return result
+
+
+def parse_repository_tag(repo):
+    column_index = repo.rfind(':')
+    if column_index < 0:
+        return repo, ""
+    tag = repo[column_index+1:]
+    slash_index = tag.find('/')
+    if slash_index < 0:
+        return repo[:column_index], tag
+
+    return repo, ""

tests/utils_test.py

+import unittest
+
+from docker.utils import parse_repository_tag
+
+
+class UtilsTest(unittest.TestCase):
+
+    def test_parse_repository_tag(self):
+        self.assertEqual(parse_repository_tag("root"), ("root", ""))
+        self.assertEqual(parse_repository_tag("root:tag"), ("root", "tag"))
+        self.assertEqual(parse_repository_tag("user/repo"), ("user/repo", ""))
+        self.assertEqual(parse_repository_tag("user/repo:tag"), ("user/repo", "tag"))
+        self.assertEqual(parse_repository_tag("url:5000/repo"), ("url:5000/repo", ""))
+        self.assertEqual(parse_repository_tag("url:5000/repo:tag"), ("url:5000/repo", "tag"))
+
+
+if __name__ == '__main__':
+    unittest.main()