str.format_map()

[rhettinger]

BPO	6081
Nosy	@rhettinger, @terryjreedy, @jaraco, @ericvsmith, @ezio-melotti, @merwok, @bitdancer, @florentx
Files	6081_1.patch issue6081.diff 6081_2.diff issue6081-1.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ericvsmith'
closed_at = <Date 2010-11-04.17:07:37.170>
created_at = <Date 2009-05-21.23:23:15.179>
labels = ['interpreter-core', 'easy', 'type-feature']
title = 'str.format_map()'
updated_at = <Date 2011-01-28.19:20:05.673>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2011-01-28.19:20:05.673>
actor = 'jaraco'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2010-11-04.17:07:37.170>
closer = 'eric.smith'
components = ['Interpreter Core']
creation = <Date 2009-05-21.23:23:15.179>
creator = 'rhettinger'
dependencies = []
files = ['16576', '16586', '16632', '19482']
hgrepos = []
issue_num = 6081
keywords = ['patch', 'easy', 'needs review']
message_count = 22.0
messages = ['88173', '88425', '101265', '101269', '101272', '101273', '101283', '101324', '101325', '101342', '101346', '101349', '101603', '101609', '101692', '113443', '113610', '113617', '113622', '120361', '120422', '127325']
nosy_count = 10.0
nosy_names = ['rhettinger', 'terry.reedy', 'jaraco', 'eric.smith', 'ezio.melotti', 'eric.araujo', 'r.david.murray', 'gruszczy', 'ebehar', 'flox']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue6081'
versions = ['Python 3.2']

[rhettinger]

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'

[ericvsmith]

I think this would be useful.

I don't fee terribly strongly about it, but I think I'd like the name
str.format_using_mapping(). When I initially saw this, I thought from
the name it was creating a format object (whatever that would be) from a
mapping object.

[gruszczy]

I have created a small patch, that adds method that formats using a dict. It's the first time I have written anything in python implementation, so I would very appreciate any advice. Change allows the following:

>>> m = Mapping(a='b')
>>> '{a} {c}'.format_using_mapping(m)
'b c'
>>>

[ezio-melotti]

Thanks for the patch. It would be nice if you could include unit tests too.

[bitdancer]

I don't think this patch satisfies Raymond's request. It is explicitly checking for a __missing__ attribute, but Raymond was talking about a more general facility whereby you can pass in an arbitrary object that implements the mapping interface. Using the __missing__ facility was just an example of why this would be useful.

[ericvsmith]

I agree with David.

Although it's not clear to my why the code doesn't just work with the addition of do_string_format_using_mapping and without the other code. It's possible the existing code is too dict-specific and should be calling a more generic PyObject interface, like PyMapping_GetItemString instead of PyDict_GetItem.

[gruszczy]

My first intention was simply to push mapping from args to kwargs, just like Eric suggested, but that didn't help with __missing__, only with accepting a dict instead of pushing keyword arguments.

I didn't like explicitly asking for __missing__ either, but since I have little knowledge of what should be called, I didn't know what to use. I too believe something else the PyDict_GetItem should be called, something that would take care of __missing__ and other possibilities (I don't know what exactly and really would like to know what these are) internally.

I am going to check, whether PyMapping_GetItemString is going to help. But can this really be called on a dict (or a subclass of dict)? What about retrieving getitem method from the given object and simply calling it? Wouldn't that do the trick?

[ericvsmith]

I believe this patch fixes the issue. Tests and documentation are still needed, of course.

[ericvsmith]

Added a comment to explain the change.

[gruszczy]

Could you point me, where to add tests and documentation? I would happily add those.

[ericvsmith]

http://docs.python.org/library/stdtypes.html#str.format, for starters. This is in Doc/library/stdtypes.rst.

For tests, probably in Lib/test/test_unicode.py.

I'm not sure if we should add this to 2.7 (or even 3.2, for that matter), but if so, we should start by patching trunk and then porting to py3k.

[gruszczy]

Ok, unfortunately this code won't work for certain tests. Take those:

    self.assertEqual("My name is {0}".format('Fred'), "My name is Fred")

We pass only one argument, which is a dict and this won't satisfy such test. We need to think about a different way of passing those arguments there. We can do one of two thins:

• the last argument of args tuple would be an object that can be subscripted for format values
• keyword with above object

I believe the second version is more explicit and therefore better.