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Date: Sat, 16 May 2015 02:16:52 +0100
From: MRAB <python@mrabarnett.plus.com>
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To: python-list@python.org
Subject: Re: Building CPython
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On 2015-05-16 01:43, BartC wrote:
> On 15/05/2015 23:44, Marko Rauhamaa wrote:
>> BartC <bc@freeuk.com>:
>>
>>> What /is/ a method lookup? Is it when you have this:
>>>
>>>   A.B()
>>>
>>> and need to find whether the expression A (or its class or type) has a
>>> name B associated with it? (And it then needs to check whether B is
>>> something that can be called.)
>>>
>>> If so, does that have to be done using Python's Dict mechanism? (Ie.
>>> searching for a key 'B' by name and seeing if the object associated
>>> with it is a method. That does not sound efficient.)
>>
>> That is a general feature among high-level programming languages. In
>> Python, it is even more complicated:
>>
>>   * first the object's dict is looked up for the method name
>>
>>   * if the method is not found (it usually isn't), the dict of the
>>     object's class is consulted
>>
>>   * if the method is found (it usually is), a function object is
>>     instantiated that delegates to the class's method and embeds a "self"
>>     reference to the object to the call
>>
>> IOW, two dict lookups plus an object construction for each method call.
>
> OK, I didn't know that objects have their own set of attributes that are
> distinct from the class they belong to. I really ought to learn more
> Python!.
>
> (Yet, I have this crazy urge now to create my own bytecode interpreter
> for, if not exactly Python itself, then an equivalent language. Just to
> see if I can do any better than CPython, given the same language
> restraints.
>
> Although I'm hampered a little by not knowing Python well enough. Nor
> OOP, but those are minor details... Anyway it sounds more fun than
> trying to decipher the layers of macros and conditional code that appear
> to be the CPython sources.)
>
>   > IOW, two dict lookups plus an object construction for each method call.
>
> I suppose in many cases an object will have no attributes of its own,
> and so it can rapidly bypass the first lookup. I don't understand the
> need for an object creation (to represent A.B so that it can call it?)
> but perhaps such an object can already exist, prepared ready for use.
>
It's possible to do:

     f = A.B
     ...
     f()

so it's necessary to have an object for A.B.

The question is how much you would gain from optimising A.B() as a
special case (increase in speed vs increase in complexity).