New submission from Raymond Hettinger <rhettin...@users.sourceforge.net>:
The old % formatting allowed arbitrary mappings:
>>> class Default(dict):
... def __missing__(self, key):
... return key
...
>>> s = '%(name)s was born in %(country)s'
>>> s % Default(name='Guido')
'Guido was born in country'
But the new str.format() demands mappings be first converted to a
regular dict using **kwargs, so something like the above is not possible.
>>> s = '{name} was born in {country}'
>>> s.format(**Default(name='Guido'))
Traceback (most recent call last):
File "<pyshell#27>", line 1, in <module>
s.format(**Default(name='Guido'))
KeyError: 'country'
There is a work-around using string.vformat() but it is obscure and awkward:
>>> import string
>>> string.Formatter().vformat(s, (), Default(name='Guido'))
'Guido was born in country'
Instead, it would be better to offer an alternate method:
>>> s.format_from_mapping(Default(name='Guido'))
'Guido was born in country'
----------
messages: 88173
nosy: rhettinger
severity: normal
status: open
title: str.format_from_mapping()
type: feature request
versions: Python 2.7, Python 3.2
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue6081>
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