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In Python 2.7, I can use an empty format string to indicate automatically numbered positional arguments when formatting the builtin string object.
http://docs.python.org/release/2.7/library/string.html#format-string-syntax
If I subclass string.Formatter, this expected functionality doesn't work.
http://docs.python.org/release/2.7/library/string.html#string-formatting
Python 2.7.2+ (default, Oct 4 2011, 20:06:09) 
[GCC 4.6.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from string import Formatter
>>> class MyFormatter(Formatter):
... pass
... 
>>> fs_a = 'a{0}b{1}c'
>>> fs_b = 'a{}b{}c'
>>> fs_a.format(1, 2)
'a1b2c'
>>> fs_b.format(1, 2)
'a1b2c'
>>> fmt = MyFormatter()
>>> fmt.format(fs_a, 1, 2)
'a1b2c'
>>> fmt.format(fs_b, 1, 2)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/lib/python2.7/string.py", line 545, in format
 return self.vformat(format_string, args, kwargs)
 File "/usr/lib/python2.7/string.py", line 549, in vformat
 result = self._vformat(format_string, args, kwargs, used_args, 2)
 File "/usr/lib/python2.7/string.py", line 571, in _vformat
 obj, arg_used = self.get_field(field_name, args, kwargs)
 File "/usr/lib/python2.7/string.py", line 632, in get_field
 obj = self.get_value(first, args, kwargs)
 File "/usr/lib/python2.7/string.py", line 591, in get_value
 return kwargs[key]
KeyError: ''

We know that string.Formatter does not properly duplicate str.format.
#13598 is specifically about use of {}.
Search "string Formatter" for other related issues.