Classmethods are evil

[Bruno Desthuilliers]

Ivan Illarionov a écrit :
> After re-reading "Python is not Java" I finally came to conclusion that 
> classmethods in Python are a very Bad Thing. 
 >
> I can't see any use-case of them that couldn't be re-written more clearly 
> with methods of metaclass or plain functions.

Plain functions don't give you polymorphic dispatch and can't be 
overriden. Using metaclass methods requires a custom metaclass, which 1/ 
adds boilerplate code and cognitive load and 2/may bring problems, 
specially wrt/ MI.

> They have the following issues:
> 1. You mix instance-level and class-level functionality in one place 
> making your code a mess.

May I remind you that every name defined at the top-level of a class 
statement becomes a class attribute ?

Anyway : most of the use case I've had so far for classmethods required 
collaboration between the instance and the class, and I definitively 
prefer to have related functionalities defined in one place.

> 2. They are slower than metaclass methods or plain functions.

Slower than plain functions, indeed - but so are instancemethods. Slower 
that metaclass methods ? I never benchmarked this, but I don't see any 
reason that should be the case. Anyway, you could also argue that 
properties are slower than plain attributes, which are slower than local 
vars etc...

> I really want to hear your opinions on the subject.


Mine is that classmethods are a GoodThing that make simple thing easy.