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
(cherry picked from commit c8a35417db8853a253517a3e5190e174075c6384)
Co-authored-by: Jörn Heissler <joernheissler@users.noreply.github.com>
diff --git a/Doc/library/dis.rst b/Doc/library/dis.rst
index 5b79be6..39a3e13 100644
--- a/Doc/library/dis.rst
+++ b/Doc/library/dis.rst
@@ -645,10 +645,12 @@
.. 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
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index 8b71106..432327a 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -337,6 +337,12 @@
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:
diff --git a/Lib/test/test_dictcomps.py b/Lib/test/test_dictcomps.py
index afe68a8..927e310 100644
--- a/Lib/test/test_dictcomps.py
+++ b/Lib/test/test_dictcomps.py
@@ -81,6 +81,35 @@
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()
diff --git a/Lib/test/test_named_expressions.py b/Lib/test/test_named_expressions.py
index e15111c..f73e6fe 100644
--- a/Lib/test/test_named_expressions.py
+++ b/Lib/test/test_named_expressions.py
@@ -212,6 +212,11 @@
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):
diff --git a/Lib/test/test_parser.py b/Lib/test/test_parser.py
index b830459..e5285c6 100644
--- a/Lib/test/test_parser.py
+++ b/Lib/test/test_parser.py
@@ -473,6 +473,8 @@
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
diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-06-17-06-03-55.bpo-35224.FHWPGv.rst b/Misc/NEWS.d/next/Core and Builtins/2019-06-17-06-03-55.bpo-35224.FHWPGv.rst
new file mode 100644
index 0000000..5a1a79b
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2019-06-17-06-03-55.bpo-35224.FHWPGv.rst
@@ -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``.
diff --git a/Python/ceval.c b/Python/ceval.c
index 7063647..eddcc8d 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -2944,8 +2944,8 @@
}
case TARGET(MAP_ADD): {
- PyObject *key = TOP();
- PyObject *value = SECOND();
+ PyObject *value = TOP();
+ PyObject *key = SECOND();
PyObject *map;
int err;
STACK_SHRINK(2);
diff --git a/Python/compile.c b/Python/compile.c
index 4d3ecfe..7bdf406 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -4238,10 +4238,10 @@
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 @@
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: