[issue5237] Allow auto-numbered replacement fields in str.format() strings

[Eric Smith]

Eric Smith <eric at trueblade.com> added the comment:

I believe this patch is complete. I need to add tests and documentation,
but the code itself should be finished.

Here's the normal case:
>>> '{} {}'.format('test', 0)
'test 0'

It also handles error checking:
>>> '{1} {}'.format('test', 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: cannot switch from manual field specification to automatic
field numbering
>>> '{} {1}'.format('test', 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: cannot switch from automatic field numbering to manual field
specification

You can use named fields along with auto-numbered fields:
>>> '{test} {}'.format(1, test=2)
'2 1'

You can nest either named or auto-numbered fields:
>>> '{:^{}}'.format('x', 10)
'    x     '
>>> 'pi={:{fmt}} {:{fmt}}'.format(3.1415, 2.71828, fmt='1.4f')
'pi=3.1415 2.7183'

Attribute access is supported:
>>> '{.__abs__}'.format(0)
"<method-wrapper '__abs__' of int object at 0x85db8f8>"

As is subscripting:
>>> '{[x]}'.format({'x':4})
'4'
>>> '{[1]}'.format([1, 2])
'2'

I'll work on the tests over the weekend, then commit to trunk and py3k.

We need to decide what to do about string.Formatter (which I just
realized I erroneously called string.Format in the previous message).

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Added file: http://bugs.python.org/file13326/issue5237-0.patch

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