Issue 14297: Custom string formatter doesn't work like builtin str.format (original) (raw)

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 "", line 1, in 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: ''