Dynamically determine base classes on instantiation

[Steven D'Aprano]

On Fri, 17 Aug 2012 04:50:43 -0700, Richard Thomas wrote:

> On Thursday, 16 August 2012 19:49:43 UTC+2, Steven D'Aprano  wrote:
>> On Thu, 16 Aug 2012 10:03:51 -0700, Richard Thomas wrote:
>> 
>> > class Foo(object):
>> >     def __new__(cls, arg):
>> >         if isinstance(arg, list):
>> >             cls = FooList
>> >         elif isinstance(arg, dict):
>> >             cls = FooDict
>> >         return object.__new__(cls, arg)
>> > 
>> > class FooList(Foo, list):
>> >     pass
>> > 
>> > class FooDict(Foo, dict):
>> >     pass
>>
>>
>> Did you actually try your code?
>>
>>
> I rarely test code. I'm confident in, however undeserved the confidence.
> :) In this case that's not an error I've ever seen before. Obvious easy
> fix:
> 
> return cls.__new__(cls, arg)

It might be easy, but it's obviously wrong.

py> sys.setrecursionlimit(10)
py> x = Foo([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in __new__
  File "<stdin>", line 7, in __new__
  File "<stdin>", line 7, in __new__
  File "<stdin>", line 7, in __new__
  File "<stdin>", line 7, in __new__
  File "<stdin>", line 7, in __new__
  File "<stdin>", line 7, in __new__
RuntimeError: maximum recursion depth exceeded


> Incidentally when I reply to your posts through the groups.google.com
> interface it inserts a blank quoted line between each pair of lines. My
> first thought was that it was a line endings bug with Google's app but
> in retrospect that seems very likely to have been fixed years ago. Any
> ideas?

Makes you think that Google is interested in fixing the bugs in their 
crappy web apps? They have become as arrogant and as obnoxious as 
Microsoft used to be.


-- 
Steven