string.Formatter does not support key/attribute access on unnumbered fields

@ericvsmith

BPO	27307
Nosy	@ericvsmith, @tekknolagi
PRs	gh-71494: string.Formatter unnumbered key/attributes #21767
Files	0001-Issue-27307-Support-index-attribute-access-for-unnum.patch

^{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 = None
closed_at = None
created_at = <Date 2016-06-13.15:32:42.624>
labels = ['3.7', '3.8', 'type-bug', 'library', '3.9']
title = 'string.Formatter does not support key/attribute access on unnumbered fields'
updated_at = <Date 2020-08-07.15:32:47.479>
user = 'https://bugs.python.org/tbeadle'

bugs.python.org fields:

activity = <Date 2020-08-07.15:32:47.479>
actor = 'python-dev'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-06-13.15:32:42.624>
creator = 'tbeadle'
dependencies = []
files = ['43376']
hgrepos = []
issue_num = 27307
keywords = ['patch']
message_count = 5.0
messages = ['268452', '364613', '364623', '364630', '364642']
nosy_count = 4.0
nosy_names = ['eric.smith', 'python-dev', 'tbeadle', 'tekknolagi']
pr_nums = ['21767']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27307'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

Support for unnumbered fields in string.Formatter.format was added in http://bugs.python.org/issue13598, however, it does not support accessing an index or attribute of an unnumbered field like str.format does. Instead, it raises an unhelpful "KeyError: ''":

In [1]: import string

In [2]: fmt = string.Formatter()

In [3]: fmt.format('{[0]}', ['a', 'b'])
\---------------------------------------------------------------------------
```pytb
KeyError                                  Traceback (most recent call last)
<ipython-input-3-a923004fe8e8> in <module>()
----> 1 fmt.format('{[0]}', ['a', 'b'])
```


/usr/lib64/python2.7/string.pyc in format(*args, \*\*kwargs)
    557                 raise TypeError("format() missing 1 required positional "
    558                                 "argument: 'format_string'")
--\> 559         return self.vformat(format_string, args, kwargs)
    560 
    561     def vformat(self, format_string, args, kwargs):

/usr/lib64/python2.7/string.pyc in vformat(self, format_string, args, kwargs)
    561     def vformat(self, format_string, args, kwargs):
    562         used_args = set()
--\> 563         result = self.\_vformat(format_string, args, kwargs, used_args, 2)
    564         self.check_unused_args(used_args, args, kwargs)
    565         return result

/usr/lib64/python2.7/string.pyc in \_vformat(self, format_string, args, kwargs, used_args, recursion_depth)
    583                 # given the field_name, find the object it references
    584                 #  and the argument it came from
--\> 585                 obj, arg_used = self.get_field(field_name, args, kwargs)
    586                 used_args.add(arg_used)
    587 

/usr/lib64/python2.7/string.pyc in get_field(self, field_name, args, kwargs)
    644         first, rest = field_name.\_formatter_field_name_split()
    645 
--\> 646         obj = self.get_value(first, args, kwargs)
    647 
    648         # loop through the rest of the field_name, doing

/usr/lib64/python2.7/string.pyc in get_value(self, key, args, kwargs)
    603             return args[key]
    604         else:
--\> 605             return kwargs[key]
    606 
    607 

KeyError: ''

The attached patch adds this functionality.

aronancher asked in http://bugs.python.org/issue13598#msg218114 if the original patch was going to make it in to python 2.7. Perhaps that could get a look?

It would be good if someone could convert this to a pull request and beef up the tests.

I'll take a look at the patch and see if this solves my problem. If it does, I'll update my PR with tests.

Looks like the patch solves my problem, so I am going to update my PR sometime today.

Okay, well it doesn't provide the desired behavior of raising the error when switching back and forth between manual and auto numbering, so I am looking into that.

gh-71494: string.Formatter: support keys/attributes in unnumbered fie…

Fixed by: #21767

pythongh-71494: string.Formatter: support keys/attributes in unnumber…

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string.Formatter does not support key/attribute access on unnumbered fields #71494

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tbeadle commented on Jun 13, 2016

ericvsmith commented on Mar 19, 2020

tekknolagi commented on Mar 19, 2020

tekknolagi commented on Mar 19, 2020

tekknolagi commented on Mar 19, 2020

encukou commented on Jan 31, 2025

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