default arguments newbie question

[Erik Max Francis]

Aahz Maruch wrote:

> Yup.  Technically, you really want to write "if l is None", though; if
> you don't actually care about it being precisely None, the common
> idiom
> is to write "if l".

Eh?  I don't see that.  Any value which is not None will compare unequal
to None, correct?  Certainly this true for the fundamental types like
strings, numbers, tuples, and lists:

max at charmaine:~% python
Python 2.0 (#5, Nov 10 2000, 21:47:15) 
[GCC egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)] on linux2
Type "copyright", "credits" or "license" for more information.
>>> s = ''
>>> i = 0
>>> s == None
0
>>> i == None
0
>>> t = ()
>>> () == None
0
>>> l = []
>>> l == None
0

Only None compares equal to None, and since any reference to None _is_
the unique object of type None, the idiom `x == None' is just as valid
as `x is None'.

Now as a matter of style I would agree with you; I idiomatically use `x
is None' to test if x is None, but as a matter of the language I don't
think this stylistic issue is mandated.  (In other words, the
_technically_ in your response should have been a _according to the
style which I prefer_, which is quite a difference.)

-- 
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