1 files changed, 54 insertions, 0 deletions
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
 ##############################################################################