Access static class variable in instance method

Question

Say I have class Test defined as this:

class Test
 test_var = 2
 def test_func():
 print(test_var)

I can find out what test_var is fine like so:

>>> Test.test_var
2

...But calling Test.test_func() does not work.

>>> Test.test_func()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 4, in test
NameError: name 'test_var' is not defined

If I change Test.test_func() like this (note that this is pseudo-code):

redef test_func():
 print(Test.test_var)

It works fine:

>>> Test.test_func()
2

...and that makes sense. But how can I make the first example work, keeping in mind that I want test_func to be an instance method?

Note that the code posted above is example code, and so typos should be ignored.

• tdelaney: You should post a working example so typos do make a difference. You have this "redef" thing... but it would be more helpful to show what you actually did.
• tdelaney: Your basic problem is that test_func is not a class method or instance method so it doesn't know about the class namespace.
• Quelklef: @tdelaney I just re-wrote the class, changing said function. Not how I said it was pseudo-code.
• Quelklef: @tdelaney is test_func not a class method?
• tdelaney: No, you need to decorate it with @classmethod and add a parameter to the definition. Its a couple of lines of code so I'll post it as an answer. BTW, is this python 2 or 3?

Answer 1

You can always access class-level attributes via the instance, ie self, as long as you have not shadowed them with an instance attribute of the same name. So:

def test_func(self):
 print(self.test_var)

Answer 2

In your example, test_func is just a function and although its defined in the class namespace, the function itself doesn't know about the class namespace. You want either a regular instance method or a class method.

class Test:
 test_var = 2
 def instance_test(self):
 # instance methods will look in self first and class namespace second
 print(self.test_var)
 @classmethod
 def class_test(cls):
 # class methods take the class itself as first argument
 print(cls.test_var)
t = Test()
t.instance_test()
Test.class_test()

Answer 3

You need to either pass self (almost always what you want) to the class method or add a @classmethod or @staticmethod decorator if you don't need self. Then create an instance of the class and call the test_func method.

Examples:
# test_var is an class variable and test_func has a classmethod decorator
>>> class Test:
... test_var = 2
... @classmethod
... def test_func(cls):
... print(cls.test_var)
... 
>>> t = Test()
>>> t.test_func()
2
# test_var is an class variable and test_func has a staticmethod decorator
>>> class Test:
... test_var = 2
... @staticmethod
... def test_func():
... print(Test.test_var)
... 
>>> t = Test()
>>> t.test_func()
2
# test_var is an instance variable here
>>> class Test:
... self.test_var = 2
... def test_func(self):
... print(self.test_var)
... 
>>> t = Test()
>>> t.test_func()
2

• Quelklef: That's not the issue, the code I posted above is just an example. Good eye, though.