optimise 'for in [...]' into 'for in (...)'

Cython/Compiler/Optimize.py

@@ -3122,6 +3122,15 @@ class ConstantFolding(Visitor.VisitorTransform, SkipDeclarations):
         node.if_clauses = if_clauses
         return node
 
+    def visit_ForInStatNode(self, node):
+        self.visitchildren(node)
+        # iterating over a list literal? => tuples are more efficient
+        sequence = node.iterator.sequence
+        if isinstance(sequence, ExprNodes.ListNode):
+            node.iterator.sequence = ExprNodes.TupleNode(
+                sequence.pos, args=sequence.args, mult_factor=sequence.mult_factor)
+        return node
+
     # in the future, other nodes can have their own handler method here
     # that can replace them with a constant result node
 

tests/run/for_in_iter.py

@@ -2,6 +2,7 @@
 # tag: forin
 
 import sys
+import cython
 
 try:
     from builtins import next
@@ -36,6 +37,18 @@ def for_in_list():
     [1, 2, 3, 4, 5]
     """
 
+@cython.test_assert_path_exists('//TupleNode//IntNode')
+@cython.test_fail_if_path_exists('//ListNode//IntNode')
+def for_in_literal_list():
+    """
+    >>> for_in_literal_list()
+    [1, 2, 3, 4]
+    """
+    l = []
+    for i in [1,2,3,4]:
+        l.append(i)
+    return l
+
 class Iterable(object):
     """
     >>> for_in_pyiter(Iterable(5))