Why return None?

Peter Otten __peter__ at web.de
Wed Aug 25 12:43:04 CEST 2004

Martin DeMello wrote:

> Anthony Baxter <anthonybaxter at gmail.com> wrote:
>> On Wed, 25 Aug 2004 08:26:26 GMT, Martin DeMello
>> <martindemello at yahoo.com> wrote:
>> > It seems to be a fairly common pattern for an object-modifying method
>> > to return None - however, this is often quite inconvenient.
>> list.reverse() modifies the list in place. The python idiom is that
>> these don't return a reference to the modified list. Although note the
> Yes, but why? I mean, is there either an advantage to returning None or
> some inherent danger in returning self?
> martin

I think Guido would rather have newbies stumbling over

>>> a = list("abc")
>>> zip(a, a.reverse())
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: zip argument #2 must support iteration


>>> class List(list):
...     def reverse(self):
...             list.reverse(self)
...             return self
>>> a = List("abc")
>>> zip(a, a.reverse())
[('c', 'c'), ('b', 'b'), ('a', 'a')]

The latter is more likely to remain undetected until some damage is done.
Still, I would prefer it.


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