del

[Hans Nowak]

Daniel Silva wrote:
> What is the purpose of the del statement?  Why would you want to undefine 
> a variable?  

It's not only used for removing names from a namespace, it's also used for 
deleting elements from lists, dicts, etc.

It's not uncommon to delete a name when you don't need it anymore.  Especially 
in modules.  For example, in site.py:

for m in sys.modules.values():
     if hasattr(m, "__file__") and m.__file__:
         m.__file__ = os.path.abspath(m.__file__)
del m

m is only a temporary variable that isn't necessary anymore after it has done 
its job.  Leaving it around makes it available when site.py is imported:

   import site
   print site.m

which isn't really bad, but it's tidier to remove unnecessary names like that.

More precisely, how many programs would be broken if a python
> compiler did not implement it?

Lots of them.  Let's see if del is used in the standard library...

(C:\Python22\Lib) $ grep " *del " *.py | wc
     216     783    8643

Hmm, 216 times.  I think it's safe to say that it's widely used. :-)

Cheers,

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