Currently if you do

  "{:r}".format(None)

you get the error message

  TypeError: non-empty format string passed to object.__format__

or on newer versions

  TypeError: non-empty format string passed to NoneType.__format__

(which is at least *some* improvement).

Similar behaviour occurs for other types that don't implement their own __format__ method.

This seems to me to be an error on two counts:

1. object.__format__ *should* support ":r", and probably ":s" too.

2. It's unclear what the error message actually means (I had to Google it to find out what was going wrong), *especially* in a situation where it isn't obvious that the argument evaluates to None.

Since the error itself would still happen if an unsupported specifier was used, we should improve the message.  A better choice might be

  TypeError: format "{:x}" not supported for type NoneType.

Having looked carefully through the current code, I notice that the error message *has* been improved (though it'd be nice if it included a copy of the format string it's rejecting), and also that the alternative

  "{!r}".format(None)

does work as expected, and has the advantage that other formatting features are supported, e.g.

  "{!r:10.10}".format(None)

will work as expected.

I'm going to close this bug report.