bpo-35634: Raise an error when first passed kwargs contains duplicated keys. by serhiy-storchaka · Pull Request #11438

bpo-35634: Raise an error when first passed kwargs contains duplicated keys. by serhiy-storchaka · Pull Request #11438 · python/cpython

		@@ -0,0 +1,3 @@
		``func(**kwargs)`` will now raise an error when ``kwargs`` is a mapping
		containing multiple entries with the same key. An error was already raised
		when other keyword arguments are passed before ``**kwargs`` since Python 3.6.

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What does "other keyword arguments are passed before **kwargs" mean? Error was already raised when there are multiple **kwargs with overlapping keys since PEP 448 was implemented. The change in Python 3.6 was intended for optimization.

Python 3.5.3 (default, Sep 27 2018, 17:25:39) 
[GCC 6.3.0 20170516] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def foo(**kwargs): pass
... 
>>> foo(**{'a': 1}, **{'a': 1})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() got multiple values for keyword argument 'a'

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The following cases are errors since 3.6:

g(a=3, **MultiDict([('x', 1), ('x', 2)]))
g(**MultiDict([('a', 3)]), **MultiDict([('x', 1), ('x', 2)]))