Here is a quick example:
class KEY(object):
A = 'a'
A = 'b'
B = 'b'
Is there a way to test that 'A' has been repeatedly defined in class 'KEY' regardless of the assigned value?
2 Answers
Here's an attempt at answer:
Class variables are not going to work
You can't control redefinition for these. Basically, the metaclass is what is called to create your class, and it's passed a dict of attribute => value. We could say it's "already too late".
Using instance variables
You can control what happens when you do a.b = c after class instantiation, so what you could do is:
class RedefinitionMixin(object):
def __setattr__(self, k, v):
if hasattr(self, k):
raise Exception('Attribute redefinition!')
super(RedefinitionMixin, self).__setattr__(k, v)
class KEY(RedefinitionMixin, object):
def __init__(self):
self.A = 'a'
self.A = 'b'
self.B = 'b'
This may cause some unexpected behavior if you're already overriding __setattr__ though!
Note that this is only going to work in case you actually instantiate the class.
To circumvent the issue of using an instance, you could implement a singleton pattern.
Overall, I wouldn't recommend that solution, but that's the best I can come up with now.
2 Comments
Binarycrayon
+1 for giving the reason "Class variables are not going to work", another way is through static analysis by using inspect.getSource to the class and check appearance of each variable name in the class, though you could argue that is no difference compare to pylint.
Binarycrayon
Just tried it, actualy pylint thinks that redefined class variable is a valid case.
If you're using Python 3, it's possible to do this cleanly using a dict subclass that only allows nonexisting keys to be set and the __prepare__ method of a metaclass. More info here. In Python 2 it's harder; I wrote a horrible hack to achieve it, with some limits.
1 Comment
Thomas Orozco
I didn't know about this new method in python 3, that's a great answer!
__new__work? or that happens even beforehand? Sorry I am not quite familier with python 'class loading' process under the hood, and I don't feel I should work on__import__.__prepare__method which could make the class parsing use am ordered dict, or some other specialized mapping.