Change Python Class attribute dynamically

class attributes can be read on the class or an instance, but you can only set them on the class (trying to set them on an instance will only create an instance attribute that will shadow the class attribute).

If the condition is known at import time, you can just test it in the class body:

xxx = True 
class A(object):
 cls_attr = 'value'
class B(A):
 if xxx:
 cls_attr = 'this_value'
 else
 cls_attr = 'that_value'

Now if you want to change it during the program's execution, you either have to use a classmethod:

class B(A):
 @classmethod
 def set_cls_attr(cls, xxx): 
 if xxx:
 cls.cls_attr = 'this_value'
 else:
 cls.cls_attr = 'that_value'

or if you need to access your instance during the test:

class B(A):
 def set_cls_attr(self, xxx): 
 cls = type(self)
 if xxx:
 cls.cls_attr = 'this_value'
 else:
 cls.cls_attr = 'that_value'

What about using classmethod and polymorphically overriding it in subclass?

class A:
 @classmethod
 def cls_attr(cls):
 return 'value'
class B(A):
 @classmethod
 def cls_attr(cls):
 if cond():
 return 'this'
 else:
 return 'that'
assert A.cls_attr() == 'value' 
cond = lambda: True
assert B.cls_attr() == 'this'
cond = lambda: False
assert B.cls_attr() == 'that'

The easiest solution for me is with property decorator:

class B:
 @property
 def attr_name(self):
 """ do your stuff to define attr_name dynamically """
 return attr_name

This seems to do what you want:

>>> class B(A):
 @classmethod
 def set_cls_val(cls, x):
 if x == 1:
 cls.cls_attr = "new"
>>> c = B()
>>> c.cls_attr
'value'
>>> c.set_cls_val(B, 1)
>>> c.cls_attr
'new'
>>> B.cls_attr
'new'

Just set it within the function.

EDIT: Updated to set the class attribute and not the instance attribute, thanks @bruno-desthuilliers.

EDIT: Updates once again, thanks @bruno-desthuilliers. I should think my answers through more clearly. But what you want is answered below.

• python
• class
• django-admin