On 10/21/2013 11:53 AM, Saager Mhatre wrote:
[snip]
That's pretty much what always foiled my attempts at understanding Python
MetaClasses, I was looking for power where there was none to find. The best
comparison I could find was to Groovy's Compile time AST transforms, but
even those are even more powerful as they drop down a level of abstraction
and hand you the AST for the an rated element.
I agree with you partially - MOP in python can get ugly, but there's
plenty of power there. I'll try to explain, though this stuff is
notoriously hard to articulate (may be just for me).
So the main role of metaprogramming was to expose the language's object
system as an object model to the programmer, so that it could be queried
and manipulated at runtime. Does python do this? Yes, but not quite in
the way that Kiczales outlined.
Instead we get some rough-and-ready tools with which you can simulate
the effects that a "proper" MOP system (such as in CLOS) might have, and
when having to make a choice between flexibility vs performance and code
clarity, python's designers have almost invariably chosen the latter.
To begin with, there is a level of metacircularity in the new-style type
system:
>>> isinstance(object, object)
True
>>> isinstance(type, type)
True
>>> isinstance(object, type)
True
>>> isinstance(type, object)
True
This is what the metaclass system uses. With old-style classes, you had
a lot of flexibility to change the class of objects at runtime. Nowadays
there are a lot more restrictions, mostly due to performance, but you
can still do some silly stuff:
>>> MyType = type('MyType', (type,), {})
>>> MyCircularType = MyType('MyCircularType', (MyType,), {})
>>> MyCircularType.__class__ = MyCircularType
>>> isinstance(MyCircularType, MyCircularType)
True
And then of course there's things like get/setattribute - combined with
the abc module in the standard library, the type trickery above and
python's basic metaclass system, you could effectively create your own
metacircular MOP protocol on top of python.*
Another major issue is that python's syntax makes it just not a great
language for embedded DSLs and MOP, even to the extent that other
dynamic languages (Ruby!) are. That's not going to change though, and
I'm personally okay with it for the most part. If I wanted to program
CLOS, I'd program CLOS.
-Taj.
* In fact, I spent quite a while doing so a few years ago - implementing
the OMG MOF on python - and it worked pretty well, but it proved
difficult to get it to the point where I could even properly explain it
to people.
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