New submission from Pierre Quentel <pierre.quen...@gmail.com>:
PEP 634 specifies that
"A mapping pattern may not contain duplicate key values. (If all key patterns
are literal patterns this is considered a syntax error; otherwise this is a
runtime error and will raise ValueError.)"
but this is not what happens with the latest release:
Python 3.10.0b3 (tags/v3.10.0b3:865714a, Jun 17 2021, 20:39:25) [MSC v.1929 64
bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> x = {'a': 1}
>>> match x:
... case {'a': 1, 'a': 2}: # (A)
... print('ok')
...
>>> x = {'a': 3}
>>> match x:
... case {'a': 1, 'a': 2}: # (B)
... print('ok')
...
>>> x = {'a': 1, 'b': 2}
>>> match x:
... case {'a': 1, 'a': 2}: # (C)
... print('ok')
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
ValueError: mapping pattern checks duplicate key ('a')
>>>
If I understand the PEP correctly, all these examples should raise a
SyntaxError for the line
case {'a': 1, 'a': 2}:
since all key patterns are literal patterns, and the key 'a' is duplicated.
Cases (A) where the subject matches one of the key-value patterns, and (B) when
it doesn't, fail without raising SyntaxError.
Case (C) where one of the keys in the subject is not present in the mapping
pattern raises a ValueError at runtime instead of SyntaxError.
This behaviour is tested in test_patma.py:
def test_patma_251(self):
x = {"a": 0, "b": 1}
w = y = z = None
with self.assertRaises(ValueError):
match x:
case {"a": y, "a": z}:
w = 0
self.assertIs(w, None)
self.assertIs(y, None)
self.assertIs(z, None)
but this doesn't seem compliant with the specification.
BTW, it's not clear to me why the SyntaxError should be limited to the case
when all keys are literal patterns; it could be raised whenever a literal
pattern is repeated, even when there are value patterns or a double-star
pattern, like in
case {'a': 1, 'a': 2, c.imag, **rest}:
----------
components: Interpreter Core
messages: 397195
nosy: quentel
priority: normal
severity: normal
status: open
title: Pattern Matching - duplicate keys in mapping patterns
versions: Python 3.10
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue44589>
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