Fix an analyzer crash with multiple list comprehensions in py3

The crash was caused by selecting the wrong child symbol table for the
list comprehension. Previously we were always selecting the first one in
the list of subtables. This is wrong if there is more than 1 list
comprehension, since each one gets its own subtable. This is works fine
if the list comprehension only uses symbols defined within itself or
symbols that are shared between it and the first list comprehension in
the enclosing scope, since that will resolve to the same value. The
specific error case we are fixing is when there is a list comprehension
that is not the first one that references a symbol from the enclosing
function scope that the first list comprehension does not reference.

Both the symbol tables and the ListComp node are annotated with a line
number attribute, by matching these up we can select the correct
subtable to go with our list comprehension. This is still naive, but is
better than what we had before.

fixes aws#412

chalice/analyzer.py

@@ -565,7 +565,8 @@ def visit_GeneratorExp(self, node):
 
     def _handle_comprehension(self, node, comprehension_type):
         # type: (Union[ast.ListComp, ast.GeneratorExp], str) -> None
-        child_scope = self._get_matching_sub_namespace(comprehension_type)
+        child_scope = self._get_matching_sub_namespace(
+            comprehension_type, node.lineno)
         if child_scope is None:
             # If there's no child scope (listcomps in py2) then we can
             # just analyze the node.elt node in the current scope instead
@@ -577,11 +578,11 @@ def _handle_comprehension(self, node, comprehension_type):
             ParsedCode(node.elt, child_table), self._binder)
         child_infer.bind_types()
 
-    def _get_matching_sub_namespace(self, name):
-        # type: (str) -> symtable.SymbolTable
+    def _get_matching_sub_namespace(self, name, lineno):
+        # type: (str, int) -> symtable.SymbolTable
         namespaces = [
             t for t in self._symbol_table.get_sub_namespaces()
-            if t.get_name() == name]
+            if t.get_name() == name and t.get_lineno() == lineno]
         if not namespaces:
             return
         # We're making a simplification and using the genexpr subnamespace.

tests/unit/test_analyzer.py

@@ -457,6 +457,17 @@ def test_can_handle_list_expr_with_api_calls():
     """) == {'dynamodb': set(['list_tables'])}
 
 
+def test_can_handle_multiple_listcomps():
+    assert aws_calls("""\
+        bar_key = 'bar'
+        baz_key = 'baz'
+        items = [{'foo': 'sun', 'bar': 'moon', 'baz': 'stars'}]
+        foos = [t['foo'] for i in items]
+        bars = [t[bar_key] for t in items]
+        bazs = [t[baz_key] for t in items]
+    """) == {}
+
+
 # def test_can_handle_dict_comp():
 #     assert aws_calls("""\
 #         import boto3