Add W606 warning for async and await keywords in Python 3.7

From https://docs.python.org/3/whatsnew/3.6.html#new-keywords

> async and await are not recommended to be used as variable, class,
> function or module names. Introduced by PEP 492 in Python 3.5, they
> will become proper keywords in Python 3.7. Starting in Python 3.6, the
> use of async or await as names will generate a DeprecationWarning.

By adding a warning to pycodestyle.py these future warnings and syntax
errors can be caught during static code analysis.

The await expression tests were taken from PEP-492.

https://www.python.org/dev/peps/pep-0492/#id58

4 files changed, 103 insertions, 0 deletions

diff --git a/CHANGES.txt b/CHANGES.txt
index 0957be8..8fedbaa 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -7,6 +7,8 @@ UNRELEASED
 New checks:
 
 * Add W605 warning for invalid escape sequences in string literals
+* Add W606 warning for 'async' and 'await' reserved keywords being introduced
+  in Python 3.7
 
 2.3.1 (2017-01-31)
 ------------------
diff --git a/docs/intro.rst b/docs/intro.rst
index 3035a2f..4b064d0 100644
--- a/docs/intro.rst
+++ b/docs/intro.rst
@@ -415,6 +415,8 @@ This is the current list of error and warning codes:
 +------------+----------------------------------------------------------------------+
 | W605       | invalid escape sequence '\x'                                         |
 +------------+----------------------------------------------------------------------+
+| W606       | 'async' and 'await' are reserved keywords starting with Python 3.7   |
++------------+----------------------------------------------------------------------+
 
 
 **(*)** In the default configuration, the checks **E121**, **E123**, **E126**,
diff --git a/pycodestyle.py b/pycodestyle.py
index d31ac9e..ad67fb0 100755
--- a/pycodestyle.py
+++ b/pycodestyle.py
@@ -1439,6 +1439,60 @@ def python_3000_invalid_escape_sequence(logical_line, tokens):
                     pos = string.find('\\', pos + 1)
 
 
+@register_check
+def python_3000_async_await_keywords(logical_line, tokens):
+    """'async' and 'await' are reserved keywords starting with Python 3.7
+
+    W606: async = 42
+    W606: await = 42
+    Okay: async def read_data(db):\n    data = await db.fetch('SELECT ...')
+    """
+    # The Python tokenize library before Python 3.5 recognizes async/await as a
+    # NAME token. Therefore, use a state machine to look for the possible
+    # async/await constructs as defined by the Python grammar:
+    # https://docs.python.org/3/reference/grammar.html
+
+    state = None
+    for token_type, text, start, end, line in tokens:
+        error = False
+
+        if state is None:
+            if token_type == tokenize.NAME:
+                if text == 'async':
+                    state = ('async_stmt', start)
+                elif text == 'await':
+                    state = ('await', start)
+        elif state[0] == 'async_stmt':
+            if token_type == tokenize.NAME and text in ('def', 'with', 'for'):
+                # One of funcdef, with_stmt, or for_stmt. Return to looking
+                # for async/await names.
+                state = None
+            else:
+                error = True
+        elif state[0] == 'await':
+            if token_type in (tokenize.NAME, tokenize.NUMBER, tokenize.STRING):
+                # An await expression. Return to looking for async/await names.
+                state = None
+            else:
+                error = True
+
+        if error:
+            yield (
+                state[1],
+                "W606 'async' and 'await' are reserved keywords starting with "
+                "Python 3.7",
+            )
+            state = None
+
+    # Last token
+    if state is not None:
+        yield (
+            state[1],
+            "W606 'async' and 'await' are reserved keywords starting with "
+            "Python 3.7",
+        )
+
+
 ##############################################################################
 # Helper functions
 ##############################################################################
diff --git a/testsuite/W60.py b/testsuite/W60.py
index cbe267d..030bec5 100644
--- a/testsuite/W60.py
+++ b/testsuite/W60.py
@@ -29,3 +29,48 @@ regex = r'''
 \\.png$
 '''
 s = '\\'
+#: W606
+async = 42
+#: W606
+await = 42
+#: W606
+def async():
+    pass
+#: W606
+def await():
+    pass
+#: W606
+class async:
+    pass
+#: W606
+class await:
+    pass
+#: Okay
+async def read_data(db):
+    data = await db.fetch('SELECT ...')
+#: Okay
+if await fut:
+    pass
+if (await fut):
+    pass
+if await fut + 1:
+    pass
+if (await fut) + 1:
+    pass
+pair = await fut, 'spam'
+pair = (await fut), 'spam'
+with await fut, open():
+    pass
+with (await fut), open():
+    pass
+await foo()['spam'].baz()()
+return await coro()
+return (await coro())
+res = await coro() ** 2
+res = (await coro()) ** 2
+func(a1=await coro(), a2=0)
+func(a1=(await coro()), a2=0)
+await foo() + await bar()
+(await foo()) + (await bar())
+-await foo()
+-(await foo())