Sublassing tuple works, subclassing list does not

Hrvoje Niksic hniksic at xemacs.org
Wed Mar 31 13:07:55 CEST 2010


"Frank Millman" <frank at chagford.com> writes:

>>>> class MyList(list):
> ...   def __new__(cls, names, values):
> ...     for name, value in zip(names, values):
> ...       setattr(cls, name, value)
> ...     return list.__new__(cls, values)

Did you really mean to setattr the class here?  If I'm guessing
your intentions correctly, it should be:

        def __new__(cls, names, values):
          self = list.__new__(cls, values)
          for name, value in zip(names, values):
            setattr(self, name, value)
          return self

> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
> TypeError: list() takes at most 1 argument (2 given)
>>>>
>
> I can find a workaround, but I would be interested to know the reason
> why it does not work.

Because you didn't define __init__, so MyList inherited the one from
list, and it has a different signature to what you're calling it with.
Simply add an __init__ with the proper signature, and the proper upcall
to list.__init__:

   def __init__(self, names, values):
       list.__init__(self, values)

But if you're doing that, you don't need __new__ at all.  Simply
override __init__ and place your setattr loop there:

>>> class MyList(list):
...    def __init__(self, names, values):
...      for name, value in zip(names, values):
...        setattr(self, name, value)
...      list.__init__(self, values)
...
>>> MyList(['a'], [1])
[1]
>>> _.a
1



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