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.

After discussing this with Talin, we've decided this is "works as
designed". The problem, if any, is that **{' +(': 44} is allowed as a
parameter. I think that perhaps that should be an error, but I haven't
had time to research if this is already a known issue.

I should have been clearer. By "The problem, if any, is that **{' +(':
44} is allowed as a parameter", I meant in general, for all
function/method calls. It's not an issue that's restricted to str.format().