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.

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
– tdelaney, Commented Dec 12, 2015 at 19:01
Your basic problem is that test_func is not a class method or instance method so it doesn't know about the class namespace. — tdelaney
– tdelaney, Commented Dec 12, 2015 at 19:02
@tdelaney I just re-wrote the class, changing said function. Not how I said it was pseudo-code. — Quelklef
– Quelklef, Commented Dec 12, 2015 at 19:08
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? — tdelaney
– tdelaney, Commented Dec 12, 2015 at 19:20

Daniel Roseman · Accepted Answer · 2015-12-12 18:57:03Z

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)

answered Dec 12, 2015 at 18:57

Daniel Roseman

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Comments

tdelaney · Accepted Answer · 2015-12-12 19:27:21Z

0

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()

answered Dec 12, 2015 at 19:27

tdelaney

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Comments

Joe · Accepted Answer · 2015-12-12 19:34:02Z

0

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

edited Dec 12, 2015 at 19:34

answered Dec 12, 2015 at 18:55

Joe

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1 Comment

Quelklef Over a year ago

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

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