bpo-32856: Optimize the assignment idiom in comprehensions. (GH-16814)

Now `for y in [expr]` in comprehensions is as fast as a simple
assignment `y = expr`.

5 files changed, 79 insertions, 0 deletions

diff --git a/Lib/test/test_dictcomps.py b/Lib/test/test_dictcomps.py
index 927e310..16aa651 100644
--- a/Lib/test/test_dictcomps.py
+++ b/Lib/test/test_dictcomps.py
@@ -111,5 +111,22 @@ class DictComprehensionTest(unittest.TestCase):
         self.assertEqual(actual, expected)
         self.assertEqual(actual_calls, expected_calls)
 
+    def test_assignment_idiom_in_comprehensions(self):
+        expected = {1: 1, 2: 4, 3: 9, 4: 16}
+        actual = {j: j*j for i in range(4) for j in [i+1]}
+        self.assertEqual(actual, expected)
+        expected = {3: 2, 5: 6, 7: 12, 9: 20}
+        actual = {j+k: j*k for i in range(4) for j in [i+1] for k in [j+1]}
+        self.assertEqual(actual, expected)
+        expected = {3: 2, 5: 6, 7: 12, 9: 20}
+        actual = {j+k: j*k for i in range(4)  for j, k in [(i+1, i+2)]}
+        self.assertEqual(actual, expected)
+
+    def test_star_expression(self):
+        expected = {0: 0, 1: 1, 2: 4, 3: 9}
+        self.assertEqual({i: i*i for i in [*range(4)]}, expected)
+        self.assertEqual({i: i*i for i in (*range(4),)}, expected)
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Lib/test/test_genexps.py b/Lib/test/test_genexps.py
index fd712bb..86e4e19 100644
--- a/Lib/test/test_genexps.py
+++ b/Lib/test/test_genexps.py
@@ -15,6 +15,22 @@ Test nesting with the inner expression dependent on the outer
     >>> list((i,j) for i in range(4) for j in range(i) )
     [(1, 0), (2, 0), (2, 1), (3, 0), (3, 1), (3, 2)]
 
+Test the idiom for temporary variable assignment in comprehensions.
+
+    >>> list((j*j for i in range(4) for j in [i+1]))
+    [1, 4, 9, 16]
+    >>> list((j*k for i in range(4) for j in [i+1] for k in [j+1]))
+    [2, 6, 12, 20]
+    >>> list((j*k for i in range(4) for j, k in [(i+1, i+2)]))
+    [2, 6, 12, 20]
+
+Not assignment
+
+    >>> list((i*i for i in [*range(4)]))
+    [0, 1, 4, 9]
+    >>> list((i*i for i in (*range(4),)))
+    [0, 1, 4, 9]
+
 Make sure the induction variable is not exposed
 
     >>> i = 20
diff --git a/Lib/test/test_listcomps.py b/Lib/test/test_listcomps.py
index ddb169f..62b3319 100644
--- a/Lib/test/test_listcomps.py
+++ b/Lib/test/test_listcomps.py
@@ -16,6 +16,22 @@ Test nesting with the inner expression dependent on the outer
     >>> [(i,j) for i in range(4) for j in range(i)]
     [(1, 0), (2, 0), (2, 1), (3, 0), (3, 1), (3, 2)]
 
+Test the idiom for temporary variable assignment in comprehensions.
+
+    >>> [j*j for i in range(4) for j in [i+1]]
+    [1, 4, 9, 16]
+    >>> [j*k for i in range(4) for j in [i+1] for k in [j+1]]
+    [2, 6, 12, 20]
+    >>> [j*k for i in range(4) for j, k in [(i+1, i+2)]]
+    [2, 6, 12, 20]
+
+Not assignment
+
+    >>> [i*i for i in [*range(4)]]
+    [0, 1, 4, 9]
+    >>> [i*i for i in (*range(4),)]
+    [0, 1, 4, 9]
+
 Make sure the induction variable is not exposed
 
     >>> i = 20
diff --git a/Lib/test/test_peepholer.py b/Lib/test/test_peepholer.py
index 567e6a1..7913e91 100644
--- a/Lib/test/test_peepholer.py
+++ b/Lib/test/test_peepholer.py
@@ -495,6 +495,20 @@ class TestTranforms(BytecodeTestCase):
             return 6
         self.check_lnotab(f)
 
+    def test_assignment_idiom_in_comprehensions(self):
+        def listcomp():
+            return [y for x in a for y in [f(x)]]
+        self.assertEqual(count_instr_recursively(listcomp, 'FOR_ITER'), 1)
+        def setcomp():
+            return {y for x in a for y in [f(x)]}
+        self.assertEqual(count_instr_recursively(setcomp, 'FOR_ITER'), 1)
+        def dictcomp():
+            return {y: y for x in a for y in [f(x)]}
+        self.assertEqual(count_instr_recursively(dictcomp, 'FOR_ITER'), 1)
+        def genexpr():
+            return (y for x in a for y in [f(x)])
+        self.assertEqual(count_instr_recursively(genexpr, 'FOR_ITER'), 1)
+
 
 class TestBuglets(unittest.TestCase):
 
diff --git a/Lib/test/test_setcomps.py b/Lib/test/test_setcomps.py
index fb7cde0..ecc4fff 100644
--- a/Lib/test/test_setcomps.py
+++ b/Lib/test/test_setcomps.py
@@ -21,6 +21,22 @@ Test nesting with the inner expression dependent on the outer
     >>> list(sorted({(i,j) for i in range(4) for j in range(i)}))
     [(1, 0), (2, 0), (2, 1), (3, 0), (3, 1), (3, 2)]
 
+Test the idiom for temporary variable assignment in comprehensions.
+
+    >>> sorted({j*j for i in range(4) for j in [i+1]})
+    [1, 4, 9, 16]
+    >>> sorted({j*k for i in range(4) for j in [i+1] for k in [j+1]})
+    [2, 6, 12, 20]
+    >>> sorted({j*k for i in range(4) for j, k in [(i+1, i+2)]})
+    [2, 6, 12, 20]
+
+Not assignment
+
+    >>> sorted({i*i for i in [*range(4)]})
+    [0, 1, 4, 9]
+    >>> sorted({i*i for i in (*range(4),)})
+    [0, 1, 4, 9]
+
 Make sure the induction variable is not exposed
 
     >>> i = 20