bpo-35224: Reverse evaluation order of key: value in dict comprehensions (GH-14139)
… as proposed in PEP 572; key is now evaluated before value. https://bugs.python.org/issue35224
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@ -645,10 +645,12 @@ the original TOS1.
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.. opcode:: MAP_ADD (i)
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Calls ``dict.setitem(TOS1[-i], TOS, TOS1)``. Used to implement dict
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Calls ``dict.__setitem__(TOS1[-i], TOS1, TOS)``. Used to implement dict
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comprehensions.
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.. versionadded:: 3.1
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.. versionchanged:: 3.8
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Map value is TOS and map key is TOS1. Before, those were reversed.
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For all of the :opcode:`SET_ADD`, :opcode:`LIST_APPEND` and :opcode:`MAP_ADD`
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instructions, while the added value or key/value pair is popped off, the
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@ -337,6 +337,12 @@ all mutable objects.) Clashes between duplicate keys are not detected; the last
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datum (textually rightmost in the display) stored for a given key value
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prevails.
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.. versionchanged:: 3.8
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Prior to Python 3.8, in dict comprehensions, the evaluation order of key
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and value was not well-defined. In CPython, the value was evaluated before
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the key. Starting with 3.8, the key is evaluated before the value, as
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proposed by :pep:`572`.
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.. _genexpr:
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@ -81,6 +81,35 @@ class DictComprehensionTest(unittest.TestCase):
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compile("{x: y for y, x in ((1, 2), (3, 4))} += 5", "<test>",
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"exec")
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def test_evaluation_order(self):
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expected = {
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'H': 'W',
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'e': 'o',
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'l': 'l',
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'o': 'd',
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}
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expected_calls = [
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('key', 'H'), ('value', 'W'),
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('key', 'e'), ('value', 'o'),
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('key', 'l'), ('value', 'r'),
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('key', 'l'), ('value', 'l'),
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('key', 'o'), ('value', 'd'),
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]
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actual_calls = []
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def add_call(pos, value):
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actual_calls.append((pos, value))
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return value
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actual = {
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add_call('key', k): add_call('value', v)
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for k, v in zip('Hello', 'World')
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}
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self.assertEqual(actual, expected)
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self.assertEqual(actual_calls, expected_calls)
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if __name__ == "__main__":
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unittest.main()
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@ -212,6 +212,11 @@ class NamedExpressionAssignmentTest(unittest.TestCase):
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self.assertEqual(a, False)
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def test_named_expression_assignment_16(self):
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a, b = 1, 2
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fib = {(c := a): (a := b) + (b := a + c) - b for __ in range(6)}
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self.assertEqual(fib, {1: 2, 2: 3, 3: 5, 5: 8, 8: 13, 13: 21})
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class NamedExpressionScopeTest(unittest.TestCase):
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@ -473,6 +473,8 @@ class RoundtripLegalSyntaxTestCase(unittest.TestCase):
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self.check_suite("foo(b := 2, a=1)")
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self.check_suite("foo((b := 2), a=1)")
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self.check_suite("foo(c=(b := 2), a=1)")
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self.check_suite("{(x := C(i)).q: x for i in y}")
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#
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# Second, we take *invalid* trees and make sure we get ParserError
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@ -0,0 +1,2 @@
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Reverse evaluation order of key: value in dict comprehensions as proposed in PEP 572.
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I.e. in ``{k: v for ...}``, ``k`` will be evaluated before ``v``.
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@ -2940,8 +2940,8 @@ main_loop:
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}
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case TARGET(MAP_ADD): {
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PyObject *key = TOP();
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PyObject *value = SECOND();
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PyObject *value = TOP();
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PyObject *key = SECOND();
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PyObject *map;
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int err;
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STACK_SHRINK(2);
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@ -4238,10 +4238,10 @@ compiler_sync_comprehension_generator(struct compiler *c,
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ADDOP_I(c, SET_ADD, gen_index + 1);
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break;
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case COMP_DICTCOMP:
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/* With 'd[k] = v', v is evaluated before k, so we do
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/* With '{k: v}', k is evaluated before v, so we do
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the same. */
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VISIT(c, expr, val);
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VISIT(c, expr, elt);
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VISIT(c, expr, val);
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ADDOP_I(c, MAP_ADD, gen_index + 1);
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break;
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default:
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@ -4327,10 +4327,10 @@ compiler_async_comprehension_generator(struct compiler *c,
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ADDOP_I(c, SET_ADD, gen_index + 1);
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break;
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case COMP_DICTCOMP:
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/* With 'd[k] = v', v is evaluated before k, so we do
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/* With '{k: v}', k is evaluated before v, so we do
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the same. */
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VISIT(c, expr, val);
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VISIT(c, expr, elt);
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VISIT(c, expr, val);
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ADDOP_I(c, MAP_ADD, gen_index + 1);
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break;
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default:
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