Callable assertion?

[Roy Smith]

In article <h5GcnUOo2b742R2iXTWJhQ at comcast.com>,
 "Terry Reedy" <tjreedy at udel.edu> wrote:

> "Roy Smith" <roy at panix.com> wrote in message
> news:roy-F9A420.08554505102003 at reader2.panix.com...
> > I've got a function which gets passed a call-back function as a
> > parameter.  I want to check to make sure the thing passed in is
> indeed
> > callable.
> 
> The acid test is to try calling it:
> 
> try: param()
> except TypeError, msg: <do whatever>

The problem there is that it really calls the function!  I want to test 
the paramater for correctness in my constructor, but I don't actually 
want it called until it's supposed to be called.  Calling the function 
to prove it's callable is kind of like checking to see if a gun's safety 
is on by pulling the trigger :-)

Not to mention that param might indeed be callable and the TypeError is 
being generated (and not caught) somewhere further down.