howto check does module 'asdf' exist? (is available for import)

[Paul Rubin]

Asun Friere <afriere at yahoo.co.uk> writes:
> > howto check does module 'asdf' exist (is available for import) or no?
> try :
>   import asdf
>   del asdf
> except ImportError :
>   print "module asdf not available"
> else :
>   print "module asdf available for loading"

But this has a side effect: if asdf is already loaded, it deletes it.