Format strings are documented to only allow identifiers or integers as
the field_name, but allow almost anything.

Python 3.0a3

This is a feature explained in PEP 3101:

"""
    Implementation note: The implementation of this proposal is
    not required to enforce the rule about a simple or dotted name
    being a valid Python identifier.  Instead, it will rely on the
    getattr function of the underlying object to throw an exception if
    the identifier is not legal.  The str.format() function will have
    a minimalist parser which only attempts to figure out when it is
    "done" with an identifier (by finding a '.' or a ']', or '}',
    etc.).
""" <http://www.python.org/dev/peps/pep-3101>;


Why is that a problem for you?

I was reading the 3.0 documentation:
http://docs.python.org/dev/3.0/library/string.html#formatstrings which
indicated that an identifier was required.  I was unaware of the
implementation note in the PEP.

They only issue remaining is that for the example in the title of this
issue, "{ +(}".format(**{' +(': 44}), no error is generated.  I agree
that this is a nit and presumably the will of the community reviewing
the PEP, so agree to having the issue rejected.

I would reject it myself, but am unable to do so...

Thank you for pointing out the PEP to me.