And here is a patch for Greg Ewing's proposal: http://mail.python.org/pipermail/python-dev/2011-June/111934.html

Again, decimal integers rather than any kind of integers are used.

Both patches alter the exceptions expected in various places in test_unicode's test_format:

"{0.}".format() raises a ValueError (because the format string is invalid) rather than an IndexError (because there is no argument)

"{0[}".format(), likewise.

"{0]}".format() raises a ValueError (because the format string is invalid) rather than a KeyError (because "0]" is taken to be the name of a keyword argument---meaning that the test suite was testing the actual behavior of the implementation rather than the documented behavior).

"{c]}".format(), likewise.

In this patch, "{0[{1}]}".format('abcdef', 4) raises a ValueError rather than a TypeError, because "{1}", being neither a decimalinteger nor an identifier, invalidates the replacement field.

Both patches also add tests for constructions like this:

"{[0]}".format([3]) --> '3'
"{.__class__}".format(3) --> "<type 'int'>"

This conforms with the documentation (and current behavior), since in it arg_name is defined to be optional, but it is not currently covered in test_format, that I could tell, anyway.