Use cases for del

[Grant Edwards]

On 2005-07-07, Ron Adam <rrr at ronadam.com> wrote:
> Grant Edwards wrote:
>
>> On 2005-07-06, Ron Adam <rrr at ronadam.com> wrote:
>> 
>> 
>>>It would be a way to set an argument as being optional without
>>>actually assigning a value to it.  The conflict would be if
>>>there where a global with the name baz as well.  Probably it
>>>would be better to use a valid null value for what ever baz if
>>>for.  If it's a string then "", if its a number then 0, if it's
>>>a list then [], etc...
>> 
>> Except those aren't "null values" for those types.  0 is a
>> perfectly good integer value, and I use it quite often. There's
>> a big difference between an "invalid integer value" and an
>> integer with value 0.
>
> Why would you want to use None as an integer value?

1) So I know whether an parameter was passed in or not. Perhaps
   it's not considered good Pythonic style, but I like to use a
   single method for both get and set operations.  With no
   parameters, it's a get.  With a parameter, it's a set:

   class demo:
      def foo(v=None):
          if v is not None:
              self.v = v
          return self.v              

2) So I can use it as sort of a NaN equivalent.

   if self.fd is None:
      self.fd = os.open('foo.bar','w')
   
   if self.fd is not None:
      os.close(self.fd)
      self.fd = None      

> If a value isn't established yet, then do you need the name
> defined?

I find it more obvious to set the name to None during the
periods that it isn't valid than to delete it and check for a
NameError when I want to know if the value is usable or not.

> Wouldn't it be better to wait until you need the name then
> give it a value?

"Better" is a value judgement.  I prefer setting it None and
than deleting it and then checking for existance.

-- 
Grant Edwards                   grante             Yow!  Hey, LOOK!! A pair of
                                  at               SIZE 9 CAPRI PANTS!! They
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