output formatting for user-defined types

[Russ]

I'd like to get output formatting for my own classes that mimics the
built-in output formatting. For example,

>>> x = 4.54
>>> print "%4.2f" % x

4.54

In other words, if I substitute a class instance for "x" above, I'd
like to make the format string apply to an element or elements of the
instance. Is that possible? If so, how? Thanks.