Ivan Illarionov a écrit :
On Mon, 19 May 2008 13:53:31 -0700, [EMAIL PROTECTED] wrote:
On 17 mai, 11:50, Ivan Illarionov <[EMAIL PROTECTED]> wrote:
(snip)
How did I come to this:http://code.djangoproject.com/changeset/7098
I measured this and there was a marginal speed increase when
classmethods wher moved to metaclass.
IIRC (please correct me if I'm wrong), this part of code is only called
when the class is created - in which case it makes sense to move it
where it belongs, ie to the metaclass. This is by no mean a use case for
classmethods.
Yes, this is not the use case for class methods, but they where used
there.
Don't blame the tool for being misused.
The whole point of my post was to say that classmethods are used
in a wrong way too often
I think it's the first time I see such a misuse of classmethods, and I
have read quite a lot of (sometimes pretty hairy) Python code.
and most of Python programmers don't know that
the same thing can be implemented with metaclass methods.
I'd say that most Python programmers using metaclasses know when to use
a metaclass method and when to use a classmethod. One case of (slight)
misuse is certainly not enough to infer a general rule, and that doesn't
make classmethods bad in anyway. And while we're at it, I'd consider
using a custom metaclass only to implement the equivalent of a
classmethod a misuse too - and in this case, kind of a WTF.
As a last point, there's at least one very big difference between a
metaclass method and a classmethod, which is that you cannot call a
metaclass method on an instance:
class MyMeta(type):
def bar(cls):
print "%s.bar()" % cls
class Foo(object):
__metaclass__ = MyMeta
@classmethod
def baaz(cls):
print "%s.baaz()" % cls
Foo.baaz()
Foo.bar()
f = Foo()
f.baaz()
f.bar()
=>
<class '__main__.Foo'>.baaz()
<class '__main__.Foo'>.bar()
<class '__main__.Foo'>.baaz()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/tmp/python-18506gCT.py", line 17, in <module>
f.bar()
AttributeError: 'Foo' object has no attribute 'bar'
I've had use case where I needed to call classmethods on instances,
using a metaclass method would have required an explicit call, ie
type(obj).some_method() instead of obj.some_method(), which would have
uselessly exposed this knowledge to client code.
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