[Python-ideas] Default arguments in Python - the return - running out of ideas but...

[George Sakkis]

On Fri, May 15, 2009 at 6:52 AM, Arnaud Delobelle
<arnodel at googlemail.com> wrote:

> 2009/5/15 Larry Hastings <larry at hastings.org>:
>>
>> On Thu, May 14, 2009 at 9:31 PM, Curt Hagenlocher <curt at hagenlocher.org>
>> wrote:
>>
>>> I think this takes the discussion in a more practical direction. Imagine
>>> that there were a special method name __immutable__ to be implemented
>>> appropriately by all builtin types.
>>
>> Python already has something *vaguely* like this; __hash__ is only supposed
>> to be implemented on immutable objects.  So if the object supports __hash__
>> one could declare that it is supposed to be immutable.
>>
>> However, it's occasionally useful to define __hash__ on mutable objects, and
>> indeed it's defined by default on user-defined classes.  So __hash__ isn't a
>> viable substitute for __immutable__ (&c).
>
> However immutability is a shallow thing: tuple are immutable but ([])
> still can be changed!

More to the point, immutability is *not* the issue as Steven D'Aprano
showed. There are perfectly legitimate reasons for using a default
value that just happens to be mutable, without mutating it in the
function body though. Dict is the most common example (especially
since there is no frozendict type that could be used in its place).

George