Enable static type checking with pytype

.gitignore

@@ -35,3 +35,4 @@ tests/data/user-key.json
 # Generated files
 pylintrc
 pylintrc.test
+pytype_output/

.travis.yml

@@ -22,6 +22,8 @@ matrix:
     env: TOXENV=py36-system SYSTEM_TEST=1 SKIP_APP_ENGINE_SYSTEM_TEST=1
   - python: 2.7
     env: TOXENV=py27-system SYSTEM_TEST=1 SKIP_APP_ENGINE_SYSTEM_TEST=1
+  - python: 3.6
+    env: TOXENV=pytype
 cache:
   directories:
   - ${HOME}/.cache

google/auth/_oauth2client.py

@@ -25,6 +25,7 @@
 
 from google.auth import _helpers
 import google.auth.app_engine
+import google.auth.compute_engine
 import google.oauth2.credentials
 import google.oauth2.service_account
 
@@ -37,7 +38,7 @@
         ImportError('oauth2client is not installed.'), caught_exc)
 
 try:
-    import oauth2client.contrib.appengine
+    import oauth2client.contrib.appengine  # pytype: disable=import-error
     _HAS_APPENGINE = True
 except ImportError:
     _HAS_APPENGINE = False

google/auth/app_engine.py

@@ -28,10 +28,12 @@
 from google.auth import credentials
 from google.auth import crypt
 
+# pytype: disable=import-error
 try:
     from google.appengine.api import app_identity
 except ImportError:
     app_identity = None
+# pytype: enable=import-error
 
 
 class Signer(crypt.Signer):

google/auth/jwt.py

@@ -738,7 +738,7 @@ def before_request(self, request, method, url, headers):
         parts = urllib.parse.urlsplit(url)
         # Strip query string and fragment
         audience = urllib.parse.urlunsplit(
-            (parts.scheme, parts.netloc, parts.path, None, None))
+            (parts.scheme, parts.netloc, parts.path, "", ""))
         token = self._get_jwt_for_audience(audience)
         self.apply(headers, token=token)
 

setup.cfg

@@ -1,2 +1,17 @@
 [bdist_wheel]
 universal = 1
+
+[pytype]
+# Where to start analysis.
+inputs = .
+# Some files aren't worth analyzing, namely tests. Hidden directories (e.g.
+# .tox) are automatically filtered out.
+exclude = tests system_tests
+# All pytype output goes here.
+output = pytype_output
+# Python version (major.minor) of the target code.
+python_version = 3.6
+# Paths to source code directories, separated by ':'.
+pythonpath = .
+# Errors to disable.
+disable = pyi-error

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = lint,py27,py34,py35,py36,pypy,cover
+envlist = lint,py27,py34,py35,py36,pypy,cover,pytype
 
 [testenv]
 deps =
@@ -82,3 +82,11 @@ deps =
   flake8
   flake8-import-order
   docutils
+
+[testenv:pytype]
+basepython = python3.6
+commands =
+  pytype
+deps =
+  {[testenv]deps}
+  pytype