Home Original page

Why the 'self' argument?

Bengt Richter bokr at oz.net
Sat Sep 6 12:28:58 EDT 2003 
On Fri, 5 Sep 2003 20:15:00 -0400, "John Roth" <newsgroups at jhrothjr.com> wrote:

>
>"Grant Edwards" <grante at visi.com> wrote in message
>news:3f591d28$0$175$a1866201 at newsreader.visi.com...
>> In article <vli387c9mm140a at news.supernews.com>, John Roth wrote:
>>
>> >> So that there's no difference between a function and a method.
>> >>
>> >> Simplicity and orthogonality are good things -- despite what
>> >> C++ proponents thing.
>> >>
>> >> > Hence my comment that requiring it is more complex than not
>> >> > requiring it.
>> >>
>> >> No, it's more complex the Java/Ruby way, since you have to have
>> >> two sets of rules for what a name means depending on whether
>> >> you're inside a "normal" function or a method.  In Python
>> >> there's just one set of rules -- mostly.
>> >
>> > As I said earlier, it's quite possible to define it so that there
>> > is always an instance of some kind; whether that's an instance
>> > a class or the module itself.
>>
>> I don't follow.  You're talking about defining a keyword that
>> always refers to the first parameter passed to a function?  And
>> the declared parameters would refer to the 2nd through Nth
>> parameters?  What if the keyword isn't used in the function
>> definition, then do the delcared parameters refer to the 1st
>> through Nth?
>
>When Python invokes a function, it binds the operands in the
>function call to the identifiers named in the function / method
>header. If this is a method, it binds the instance (which is not
>in the invocation parameter list, btw.) to the first
>parameter in the method header.
>
>If you make "self" a reserved word, then all it has to do
>is bind the instance to "self," rather than the first identifier
>in the parameter list.
>
>In current python, there are two classes of functions (module
>level and embedded) and three classes of methods (instance,
>class and static.) Instance methods get the current instance,
>class methods get the class object as the instance, and the other
>three categories get nothing.
>
>As a separate suggestion, I'd have Python bind the module
>object to "self" for module functions and static methods. I
>haven't figured out what I want done with embedded methods
>and unbound methods yet. Notice that this eliminates the
>need for the global statement for module functions - all
>identifiers are accessible for rebinding through "self."
>
>> > I think my comments have shown that you can reduce the amount
>> > of scoping / name space rules noticably.
>>
>> Compared to what?  It sure sounds like you're introducing more
>> rules than there are now.  How would you propose reducing the
>> number of rules?
>
>If you don't have to write "self" as the first parameter of a method,
>that reduces the complexity. Everything else remains the same.

Will this still be possible?

 >>> def foo(*args): print args
 ...
 >>> class A(object): pass
 ...
 >>> class B(A): pass
 ...
 >>> a = A()
 >>> b = B()
 >>> A.bar = foo
 >>> b.bar('howdy')
 (<__main__.B object at 0x00906E70>, 'howdy')
 >>> a.bar('howdy')
 (<__main__.A object at 0x00907170>, 'howdy')
 >>> foo('howdy')
 ('howdy',)

 >>> A.bar('hello')
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
 TypeError: unbound method foo() must be called with A instance as first argument (got str instan
 ce instead)
 >>> A.__dict__['bar']('hello')
 ('hello',)

I.e., the method-vs-function distinction is a matter of how you access the function dynamically,
not how you define it. There is no static distinction in functionality, AFAIK (though I guess
there could possibly be some speculative optimizations).

(The above doesn't even get into the multiple name issues I alluded to).

Regards,
Bengt Richter
More information about the Python-list mailing list