Fix format string checking (#80)

format strings (PEP 498) defines a new AST node, JOINEDSTR which
contains a list of string expressions.

Author: jgarst

pyflakes/checker.py

@@ -937,7 +937,7 @@ def ignore(self, node):
         MATMULT = ignore
 
     # additional node types
-    COMPREHENSION = KEYWORD = FORMATTEDVALUE = handleChildren
+    COMPREHENSION = KEYWORD = FORMATTEDVALUE = JOINEDSTR = handleChildren
 
     def DICT(self, node):
         # Complain if there are duplicate keys with different values

pyflakes/test/test_other.py

@@ -1800,3 +1800,11 @@ def test_matmul(self):
         def foo(a, b):
             return a @ b
         ''')
+
+    @skipIf(version_info < (3, 6), 'new in Python 3.6')
+    def test_formatstring(self):
+        self.flakes('''
+        hi = 'hi'
+        mom = 'mom'
+        f'{hi} {mom}'
+        ''')