Behavior of staticmethod in Python 3

Peter Otten __peter__ at web.de
Sat Nov 23 16:23:53 CET 2013


Marco Buttu wrote:

> On 11/23/2013 10:01 AM, Peter Otten wrote:
> 
>>> In Python 3 the following two classes should be equivalent:
>> Says who?
>>
>>> >$ cat foo.py
>>> >class Foo:
>>> >      def foo():
>>> >          pass
>>> >      print(callable(foo))
>>> >
>>> >class Foo:
>>> >      @staticmethod
>>> >      def foo():
>>> >          pass
>>> >      print(callable(foo))
>>> >
>>> >But they do not:
>>> >
>>> >$ python3 foo.py
>>> >True
>>> >False
>>>
>>
>> Your script is saying that a staticmethod instance is not a callable
>> object. It need not be because
>>
>> Foo.foo()
> 
> Yes, you are right about Python 3. But in Python 2, if I am not going
> wrong, there is not solution, and I need to define a function outside
> the class. For instance:
> 
> $ cat config.py
> class Configuration(object):
> 
>      def positiveCheck(value):
>          if not value > 0:
>              raise AttributeError('Must be a positive number')
> 
>      attributes = {
>              # Attribute name: (type, checkrule)
>              'myattr': (int, positiveCheck),
>      }

Just add

       positiveCheck = staticmethod(positiveCheck)

at this point and everything should work in both Python 2 and 3.
Alternatively use Steven's callable_staticmethod as a decorator.

> 
>      def __setattr__(self, name, value):
>          if not name in Configuration.attributes:
>              raise AttributeError("Attribute `%s` non allowed." %name)
> 
>          expected_type, checkrule = Configuration.attributes[name]
>          if not isinstance(value, expected_type):
>              raise TypeError('The value %s is not of type %s' \
>                      %(value, expected_type.__name__))
>          if callable(checkrule):
>              print('calling %s(%s)' %(checkrule.__name__, value))
>              checkrule(value)
> 
>          super(Configuration, self).__setattr__(name, value)
> 
> The positive check works fine:
> 
>  >>> from config import Configuration
>  >>> c = Configuration()
>  >>> c.myattr = -10
> calling positiveCheck(-10)
> Traceback (most recent call last):
>      ...
> AttributeError: Must be a positive number
> 
> But I cannot use the method as a function:
> 
>  >>> Configuration.positiveCheck(-10)
> Traceback (most recent call last):
>      ...
> Configuration instance as first argument (got int instance instead).
> 
> Furthemore, I cannot use the method as a staticmethod, becase otherwise
> it will not be callable inside the class body.


PS: AttributeErrors should be raised when an attribute does not exist or 
cannot be set; I recommend that you raise a ValueError in positiveCheck().




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