… as proposed in PEP 572; key is now evaluated before value.
https://bugs.python.org/issue35224
.. 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
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:
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()
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):
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
--- /dev/null
+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``.
}
case TARGET(MAP_ADD): {
- PyObject *key = TOP();
- PyObject *value = SECOND();
+ PyObject *value = TOP();
+ PyObject *key = SECOND();
PyObject *map;
int err;
STACK_SHRINK(2);
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:
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: