New submission from Barry A. Warsaw:
A colleague discovered an interesting implementation different between
C-defined functions and Python-defined functions called with **kws arguments.
He tried to do this:
>>> from collections import defaultdict
>>> '{foo}{bar}'.format(**defaultdict(str))
''
He wondered how this could work, especially given:
>>> def what(**kws):
... print(type(kws), repr(kws))
...
>>> what(**defaultdict(str))
<class 'dict'> {}
The Language Reference clearly states that when what() is called, a new
dictionary is create, which is populated by keyword arguments not consumed by
positional args.
http://docs.python.org/3/reference/compound_stmts.html#function-definitions
http://docs.python.org/3/reference/expressions.html#calls
What isn't described is the behavior when the function is defined in C using
METH_KEYWORDS. In that case, the object passed through the ** argument is
passed unscathed to the underlying methodobject defined to take METH_KEYWORDS.
str.format() is of the latter type so it gets the actual defaultdict. what()
of course gets the defined behavior.
It would be nice if the implementation details of the function did not leak
into this behavior, but this is admittedly a corner case of the CPython
implementation. I haven't checked any of the alternative implementations to
see what they do. My guess is that CPython won't change for practical and
backward compatibility reasons, thus I've opened this issue as a documentation
bug. At the very least, an implementation note in the Language Reference
should be added.
----------
assignee: docs@python
components: Documentation
messages: 193559
nosy: barry, docs@python
priority: normal
severity: normal
status: open
title: Undocumented different between METH_KEYWORDS and **kws
versions: Python 2.7, Python 3.3, Python 3.4
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue18531>
_______________________________________
_______________________________________________
Python-bugs-list mailing list
Unsubscribe:
http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
