Python and initialization of class variables - how to use class method to initialize members?

Question

I want to initialize class member with use class method but not know how to call it.

Can you suggest some solution - maybe it is very trivial but I can not find solution?

This code will not work - I do not why?

class X(object):
  @staticmethod
  def __Y():
    return 1

  CONSTANT = __Y()


x = X()
print x.CONSTANT

This will work but I need use call method to initialize class members.

class X(object):
  CONSTANT = 1

x = X()
print x.CONSTANT

Note, I am not want initialize object variables but class variable.

user4815162342 · Accepted Answer · 2013-05-09 09:19:11Z

7

Drop the @staticmethod decorator and the first approach will work as well. You don't need staticmethod just to call a function inside a class statement.

Since that way the function will not be usable when called from class instances, it is an idiom to also remove it after use. In your example:

class X(object):
    def __y():
      return 1

    CONSTANT = __y()
    # ... other uses of __y, if any
    del __y

To understand why your approach didn't work, consider what staticmethod does. It wraps a normal function into a descriptor object that, when retrieved from the class, produces the original function unchanged, i.e. without the usual "bound method" semantics. (Retrieving a normal def function from an instance or a class would get you a bound method that automagically inserts self as the first argument.)

However, the descriptor returned by staticmethod is itself not callable, its sole function is to produce the callable object when accessed through the class or instance. Proposals to make the staticmethod descriptor callable were rejected because such use of staticmethod is erroneous in the first place.

edited May 9, 2013 at 9:19

answered May 9, 2013 at 7:45

user4815162342

159k22 gold badges350 silver badges418 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Bibhas Debnath Over a year ago

Just curious, Dropping @staticmethod will turn def __Y() to def __Y(self), right? How is it possible to call the method without instantiating the class? In this case, CONSTANT = __Y(X())?

user4815162342 Over a year ago

@Bibhas No, the insertion of self only happens when you access __Y through class or instance. If you call it from the class definition, a def is just a def. As no magic happens in the first place, you don't need @staticmethod to prevent it from happening.

Ade YU Over a year ago

@Bibhas Do call x.__Y() where x is some instantiated object. Instead, just call __Y() in the class definition will not lead __Y(self).

Bibhas Debnath Over a year ago

Oh. It's just another function. It doesn't become a method. Not accessible from the instance.

Bibhas Debnath Over a year ago

Doesn't calling x.__Y() pass x as the first argument always?

|

Collectives™ on Stack Overflow

Python and initialization of class variables - how to use class method to initialize members?

1 Answer 1

6 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related