CODEWITHSUNDEEP
pythonclassinstance
ferryard
ferryard

Reputation: 135

Class variable access in all class method

I want to have a class variable, so that the value can be access in all instances, but I also want to access the variable in methods inside the class. Is that possible? I have tried this, but it didn't work at all.

class myClass:
   myvariable = 1

   def add():
      myvariable+= 1

   def print1():
      print myvariable

I want to make two instances, one only do add method, the other only do print1 method

Upvotes: 6

Views: 10061

Answers (1)

Martijn Pieters
Martijn Pieters

Reputation: 1121534

Yes, just access the variable on the class object:

class myClass(object):
    myvariable = 1

    def add(self):
        myClass.myvariable += 1

    def print1(self):
        print myClass.myvariable

or if you want to set it per sub-class, use type(self):

class myClass(object):
    myvariable = 1

    def add(self):
        type(self).myvariable += 1

    def print1(self):
        print type(self).myvariable

The difference is that the latter will create a separate attribute on any subclass when set, masking the base class attribute. This is just like setting an attribute on an instance would mask the class attribute.

Although you can get the class attribute via self as well (print self.myvariable), explicit is better than implicit here, and avoids accidentally being masked by an instance attribute of the same name. Setting class attributes always has to be done on the class; setting it on self would create or update an instance attribute instead (not shared).

Do inherit your classes from object though; using new-style classes has many advantages, not in the least that type(self) will then actually return the class. In old-style classes (not inheriting from object) you'd have to use self.__class__ instead.

Using object as a base also gives you a third option, class methods with the @classmethod decorator; use these when you only need to access the class object, not the instance. These methods are bound to the current (sub)class, so their effect on class attributes is the same as using type(self):

class myClass(object):
    myvariable = 1

    @classmethod
    def add(cls):
        cls.myvariable += 1

    @classmethod
    def print1(cls):
        print cls.myvariable

Upvotes: 14

Related Questions