Metaclasses in Python

http://stackoverflow.com/a/6581949

by fogus • 15 years ago

158 30 15 years ago

emehrkay4• 15 years ago

Python is a very cool language. I wonder if using __metaclass__ in some fashion would allow for javascript-like prototyping.

grifaton• 15 years ago

Jonathan Fine gave a talk on this at Europython this year.

Looks like there's a video available at http://ep2011.europython.eu/conference/talks/javascript-for-....

beza1e1• 15 years ago

What prototyping stuff is missing in Python?

emehrkay4• 15 years ago

Well in JS you could do String.prototype.fnName = function(){}; to add fnName as a method to all strings even if they are already instantiated, is that possible with Python?

grot4• 15 years ago

In javascript you can modify an object

    var foo = Object();
    foo.blah = function(x,y) { ... };

But in python, that doesn't quite work. You can only do

    foo = object()
    foo.blah = lambda x,y: ...

lambdas are a bit more limited as they are restricted to one line, and you can't have print statements, which makes complicated expressions rather ugly.

edit: ah, as someone noted, the second code snippet should be something like foo = Foo() where

    class Foo(object):
        pass

198d• 15 years ago

It's interesting that you bring up Javascript. I've just started working in Python full-time recently and have noticed the OOP stuff "feels" a little like working with objects in Javascript. Specifically around how the built in super works and more generally how methods are defined on classes to begin with. In Javascript you can do interesting things with call and apply on function objects to share functionality between two objects that work the same. Similarly in Python, since the methods defined in a class are also available on the class object itself, you can use them outside of an instance and simply give them a self to operate on. The class object almost acts like the prototype object on the function object you're using as a constructor. I know the two systems are different, but they do "feel" real similar. Most people I mention this to seem a bit surprised and have that "never thought of that" look on their face. Anyone else witness this?

darklajid• 15 years ago

Previously: https://news.ycombinator.com/item?id=2860031

eschulte• 15 years ago

If you find yourself doing things like this in Python you should probably switch to Ruby.

grunar• 15 years ago

tl;dr don't use metaclasses

pjscott• 15 years ago

Slightly longer version: don't use metaclasses unless you really know what you're doing.

DrHankPym• 15 years ago

Slightly shorter longer version: classes are objects, see 'type()'

prolepunk• 15 years ago

Is it me or the first metaclass example doesn't work.

I'm using Python 2.7.2+ on Debian and I can't get __metaclass__ force upper case attributes inside of a 'module'

lang2• 15 years ago

If I'm not mistaken the authors of Smalltalk realized that metaclasses were a mistake long before they appeared in Python. Yet they were added to Python anyway.

lang2• 15 years ago

I find metaclasses philosophically unsound. If one introduces metaclasses, what about meta-metaclasses, meta-meta-metaclasses. In this regard Self's prototypical oop seems more coherent - there are only objects. Even object/class duality looks philosophically more attractive than object/class/metaclass trio.

lloeki• 15 years ago

meta is all relative and means 'beyond'. A metaclass being a class, it can itself have a metaclass, and so on, and they are all only objects, where the author's following excerpt embodies:

    MyClass = MetaClass()
    MyObject = MyClass()

Those two code blocks are effectively equivalent:

    class Quux(type):
        #__metaclass__ = type is implicit
        pass
    
    class Qux(object):
        __metaclass__ = Quux
        pass

and:

    Quux = type('Quux', (type,), {})
    Qux = Quux('Qux', (object,), {})

the __metaclass__ attribute merely puts in words what's the callable being called when the keyword class gets parsed and subsequently handled in the current namespace.

This class/object unification is one of a bunch of unifications I find extremely satisfying when thinking about how Python ticks.

Metaclasses in Python

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