Why do class methods always need 'self' as the first parameter?

Ian Kelly ian.g.kelly at gmail.com
Fri Sep 2 22:30:14 CEST 2011


On Fri, Sep 2, 2011 at 11:51 AM, John Roth <johnroth1 at gmail.com> wrote:
>> I don't see how you could get rid of the wrappers.  Methods would
>> still need to be bound, somehow, so that code like this will work:
>>
>> methods = {}
>> for obj in objs:
>>     if obj.is_flagged:
>>         methods[obj.user_id] = obj.do_work
>>     else:
>>         methods[obj.user_id] = obj.do_other_work
>> # ...
>> methods[some_user_id]()
>>
>> Without method wrappers, how does the interpreter figure out which
>> instance is bound to the method being called?
>>
>> Cheers,
>> Ian
>
> Good question.
>
> Currently the instance wrapper is created during method instantiation,
> so the instance is obviously available at that point. There are two
> rather obvious ways of remembering it. One is to use the invocation
> stack, which has the instance. Another would be for the compiler to
> create a local variable for the instance and possibly the class and
> fill them in at instantiation time. Both of these require fixing the
> names "self" and "cls" so the compiler knows what to do with them. The
> first would require giving these two names their own bytecodes, the
> second makes them simple local variables the same as the ones in the
> method headers. The latter also allows them to be changed by the
> method, which is probably not the world's best programming practice
> although it's possible now.

That's not what I asked.  Both of those options are storing the
instance within a stack frame, once it's been called.  I'm asking how
you would remember the instance during the interval from the time when
the method
is accessed until when it has been called.

In the code above, the method is accessed just before it is stored in
the dictionary.  That is when the method wrapper is currently created,
and the instance is available.  It is not called until much later,
possibly not even within the same function.  How would you remember
the instance over that period without wrapping the function?

Cheers,
Ian



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