Building CPython

[BartC]

On 15/05/2015 23:44, Marko Rauhamaa wrote:
> BartC <bc at 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.

-- 
Bartc