[Python-Dev] Callable, non-descriptor class attributes. (original) (raw)

Guido van Rossum guido at python.org
Sat Mar 12 04:48:41 CET 2011

On Fri, Mar 11, 2011 at 10:09 PM, Steven D'Aprano <steve at pearwood.info> wrote:

Thomas Wouters wrote:

One of the things brought up at the language summit (and I believe at the VM summit, although I wasn't there) was the unpredictable behaviour of callables turning into methods when they're class attributes. [...]  1. Make staticmethod a callable object directly (it isn't, currently) and apply it to any Python function that replaces a (in CPython) CFunction. The change to staticmethod may be a good idea regardless, but the policy of making other implementations comply to this quirk in CPython seems (to me) like unnecessary descrimination[*]. I've been bitten by the (to me) surprising fact that staticmethod objects are not directly callable. It's hard to write a method/function inside a class which is callable both when the class is being created, and afterwards, even though naively staticmethod seems like it should do the job. A toy example: class Toy(object):  @staticmethod  def spam(n):  return ' '.join(['spam']*n)  lunch = spam(5) + "with a fried egg on top" +1 on making staticmethods callable. I would have found that useful in the past.

IIUC Thomas found that this breaks some current use of staticmethod.

 4. Make it an error to have a callable class attribute that isn't a descriptor (although maybe we only discussed this one in my head.) Do you mean to make one, or both, of these an error? class C:  spam = len

Making this illegal is what #4 means, yes. But I find it unacceptable.

C.dict['ham'] = lambda: None

This is already illegal (except for classic classes in Python 2). Also lambda creates a standard function object so it is (or has?) a descriptor.

-- --Guido van Rossum (python.org/~guido)

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