Adds walrus test and python 3.7/3.8 tox definitions

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist=py27,py34,py35,py36
+envlist=py27,py34,py35,py36,py37,py38
 
 [testenv]
 commands=

yapf/yapflib/py3compat.py

@@ -20,6 +20,8 @@
 
 PY3 = sys.version_info[0] >= 3
 PY36 = sys.version_info[0] >= 3 and sys.version_info[1] >= 6
+PY37 = sys.version_info[0] >= 3 and sys.version_info[1] >= 7
+PY38 = sys.version_info[0] >= 3 and sys.version_info[1] >= 8
 
 if PY3:
   StringIO = io.StringIO

yapftests/reformatter_basic_test.py

@@ -3033,6 +3033,20 @@ def testForceMultilineDict_False(self):
     finally:
       style.SetGlobalStyle(style.CreateYapfStyle())
 
+  @unittest.skipUnless(py3compat.PY38, 'Requires Python 3.8')
+  def testWalrus(self):
+    unformatted_code = textwrap.dedent("""\
+      if (x  :=  len([1]*1000)>100):
+        print(f'{x} is pretty big' )
+    """)
+    expected = textwrap.dedent("""\
+      if (x := len([1] * 1000) > 100):
+        print(f'{x} is pretty big')
+    """)
+    uwlines = yapf_test_helper.ParseAndUnwrap(unformatted_code)
+    self.assertCodeEqual(expected,
+                         reformatter.Reformat(uwlines))
+
 
 if __name__ == '__main__':
   unittest.main()