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bpo-35224: Reverse evaluation order of key: value in dict comprehensi…

…ons (GH-14139)

… as proposed in PEP 572; key is now evaluated before value.

https://bugs.python.org/issue35224
(cherry picked from commit c8a3541)

Co-authored-by: Jörn Heissler <joernheissler@users.noreply.github.com>
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miss-islington and joernheissler committed Jun 22, 2019
1 parent fa23bd2 commit 874ff65e0a70ff4fd1a67e85cd61d76adfcc219d
@@ -645,10 +645,12 @@ the original TOS1.

.. opcode:: MAP_ADD (i)

Calls ``dict.setitem(TOS1[-i], TOS, TOS1)``. Used to implement dict
Calls ``dict.__setitem__(TOS1[-i], TOS1, TOS)``. Used to implement dict
comprehensions.

.. versionadded:: 3.1
.. versionchanged:: 3.8
Map value is TOS and map key is TOS1. Before, those were reversed.

For all of the :opcode:`SET_ADD`, :opcode:`LIST_APPEND` and :opcode:`MAP_ADD`
instructions, while the added value or key/value pair is popped off, the
@@ -337,6 +337,12 @@ all mutable objects.) Clashes between duplicate keys are not detected; the last
datum (textually rightmost in the display) stored for a given key value
prevails.

.. versionchanged:: 3.8
Prior to Python 3.8, in dict comprehensions, the evaluation order of key
and value was not well-defined. In CPython, the value was evaluated before
the key. Starting with 3.8, the key is evaluated before the value, as
proposed by :pep:`572`.


.. _genexpr:

@@ -81,6 +81,35 @@ def test_illegal_assignment(self):
compile("{x: y for y, x in ((1, 2), (3, 4))} += 5", "<test>",
"exec")

def test_evaluation_order(self):
expected = {
'H': 'W',
'e': 'o',
'l': 'l',
'o': 'd',
}

expected_calls = [
('key', 'H'), ('value', 'W'),
('key', 'e'), ('value', 'o'),
('key', 'l'), ('value', 'r'),
('key', 'l'), ('value', 'l'),
('key', 'o'), ('value', 'd'),
]

actual_calls = []

def add_call(pos, value):
actual_calls.append((pos, value))
return value

actual = {
add_call('key', k): add_call('value', v)
for k, v in zip('Hello', 'World')
}

self.assertEqual(actual, expected)
self.assertEqual(actual_calls, expected_calls)

if __name__ == "__main__":
unittest.main()
@@ -212,6 +212,11 @@ def test_named_expression_assignment_15(self):

self.assertEqual(a, False)

def test_named_expression_assignment_16(self):
a, b = 1, 2
fib = {(c := a): (a := b) + (b := a + c) - b for __ in range(6)}
self.assertEqual(fib, {1: 2, 2: 3, 3: 5, 5: 8, 8: 13, 13: 21})


class NamedExpressionScopeTest(unittest.TestCase):

@@ -473,6 +473,8 @@ def test_named_expressions(self):
self.check_suite("foo(b := 2, a=1)")
self.check_suite("foo((b := 2), a=1)")
self.check_suite("foo(c=(b := 2), a=1)")
self.check_suite("{(x := C(i)).q: x for i in y}")


#
# Second, we take *invalid* trees and make sure we get ParserError
@@ -0,0 +1,2 @@
Reverse evaluation order of key: value in dict comprehensions as proposed in PEP 572.
I.e. in ``{k: v for ...}``, ``k`` will be evaluated before ``v``.
@@ -2944,8 +2944,8 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
}

case TARGET(MAP_ADD): {
PyObject *key = TOP();
PyObject *value = SECOND();
PyObject *value = TOP();
PyObject *key = SECOND();
PyObject *map;
int err;
STACK_SHRINK(2);
@@ -4238,10 +4238,10 @@ compiler_sync_comprehension_generator(struct compiler *c,
ADDOP_I(c, SET_ADD, gen_index + 1);
break;
case COMP_DICTCOMP:
/* With 'd[k] = v', v is evaluated before k, so we do
/* With '{k: v}', k is evaluated before v, so we do
the same. */
VISIT(c, expr, val);
VISIT(c, expr, elt);
VISIT(c, expr, val);
ADDOP_I(c, MAP_ADD, gen_index + 1);
break;
default:
@@ -4327,10 +4327,10 @@ compiler_async_comprehension_generator(struct compiler *c,
ADDOP_I(c, SET_ADD, gen_index + 1);
break;
case COMP_DICTCOMP:
/* With 'd[k] = v', v is evaluated before k, so we do
/* With '{k: v}', k is evaluated before v, so we do
the same. */
VISIT(c, expr, val);
VISIT(c, expr, elt);
VISIT(c, expr, val);
ADDOP_I(c, MAP_ADD, gen_index + 1);
break;
default:

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