New submission from David Chambers: I first raised this issue on Stack Overflow: http://stackoverflow.com/questions/19203194 The [replacement field grammar][1] states that an [integer][2] is a valid field_name, but this is inaccurate: >>> '{0}'.format('zero') 'zero' >>> '{0x0}.format('zero') KeyError: '0x0' >>> '{0o0}.format('zero') KeyError: '0o0' >>> '{0b0}.format('zero') KeyError: '0b0' This [comment][3] by Eric Smith suggests that the above is the intended behaviour:

get_integer uses the narrowest possible definition for integer indexes, in order to pass all other strings to mappings.

The documentation should be updated to match the actual behaviour. abarnert on Stack Overflow suggested the following change: -arg_name ::= [identifier | integer] +arg_name ::= [identifier | digit+] [1]: http://docs.python.org/2/library/string.html#format-string-syntax [2]: http://docs.python.org/2/reference/lexical_analysis.html#grammar-token-integ... [3]: http://bugs.python.org/issue8985#msg107705 ---------- assignee: docs@python components: Documentation messages: 199024 nosy: davidchambers, docs@python priority: normal severity: normal status: open title: Erroneous reference to "integer" in format string grammar versions: Python 2.6, Python 2.7, Python 3.1, Python 3.2, Python 3.3, Python 3.4 _______________________________________ Python tracker http://bugs.python.org/issue19175 _______________________________________