How can I access "static" class variables within methods in Python?

Question

If I have the following code:

class Foo(object):
    bar = 1

    def bah(self):
        print(bar)
        
f = Foo()
f.bah()

It complains

NameError: global name 'bar' is not defined

How can I access class/static variable bar within method bah?

Solution 1

Instead of bar use self.bar or Foo.bar. Assigning to Foo.bar will create a static variable, and assigning to self.bar will create an instance variable.

Solution 2

Define class method:

class Foo(object):
    bar = 1
    @classmethod
    def bah(cls):    
        print cls.bar

Now if bah() has to be instance method (i.e. have access to self), you can still directly access the class variable.

class Foo(object):
    bar = 1
    def bah(self):    
        print self.bar

Solution 3

As with all good examples, you've simplified what you're actually trying to do. This is good, but it is worth noting that python has a lot of flexibility when it comes to class versus instance variables. The same can be said of methods. For a good list of possibilities, I recommend reading Michael Fötsch' new-style classes introduction, especially sections 2 through 6.

One thing that takes a lot of work to remember when getting started is that python is not java. More than just a cliche. In java, an entire class is compiled, making the namespace resolution real simple: any variables declared outside a method (anywhere) are instance (or, if static, class) variables and are implicitly accessible within methods.

With python, the grand rule of thumb is that there are three namespaces that are searched, in order, for variables:

1. The function/method
2. The current module
3. Builtins

{begin pedagogy}

There are limited exceptions to this. The main one that occurs to me is that, when a class definition is being loaded, the class definition is its own implicit namespace. But this lasts only as long as the module is being loaded, and is entirely bypassed when within a method. Thus:

>>> class A(object):
        foo = 'foo'
        bar = foo


>>> A.foo
'foo'
>>> A.bar
'foo'

but:

>>> class B(object):
        foo = 'foo'
        def get_foo():
            return foo
        bar = get_foo()



Traceback (most recent call last):
  File "<pyshell#11>", line 1, in <module>
    class B(object):
  File "<pyshell#11>", line 5, in B
    bar = get_foo()
  File "<pyshell#11>", line 4, in get_foo
    return foo
NameError: global name 'foo' is not defined

{end pedagogy}

In the end, the thing to remember is that you do have access to any of the variables you want to access, but probably not implicitly. If your goals are simple and straightforward, then going for Foo.bar or self.bar will probably be sufficient. If your example is getting more complicated, or you want to do fancy things like inheritance (you can inherit static/class methods!), or the idea of referring to the name of your class within the class itself seems wrong to you, check out the intro I linked.

Solution 4

class Foo(object):
     bar = 1
     def bah(self):
         print Foo.bar

f = Foo() 
f.bah()

Solution 5

bar is your static variable and you can access it using Foo.bar.

Basically, you need to qualify your static variable with Class name.

View more solutions

Comments

[bedwyr]

More information regarding Python and statics can be found here: stackoverflow.com/questions/68645/python-static-variable

[Alex S]

bedwyr, "print self.bar" will not create any instance variables (although assigning to self.bar will).

[Jeff Shannon]

Also, I'd be careful saying 'main module', as it is the function's containing module that's searched, not the main module... And looking up the attribute from an instance ref is a different thing, but this answer does explain why you need the instance/class ref.

[mk12]

But if you don't intend for there to be an ivar, its clearer to use Foo.classmember.

[mk12]

Why not just Foo.bar, instead of self.__class__.bar?

[Exectron]

@Mk12: When you've got class inheritance, a "class variable"could be in a base class or a subclass. Which do you want to refer to? Depends on what you're trying to do. Foo.bar would always refer to an attribute of the specified class--which might be a base class or a subclass. self.__class__.bar would refer to whichever class the instance is a type of.

[holdenweb]

Now we have reached that unfortunate point when we become so aware of the details of the language that we can discriminate between two finely attuned use cases. It does indeed depend what you want to do, but many people won't attend to such subtleties.

[ToolmakerSteve]

BTW, this is a RARE usage of "class variable". Much more common to have it defined in a specific class, here Foo. This is useful information for some advanced programming situations. But almost certainly not what the original question wanted as an answer (anyone who needs THIS answer will already know how to do Foo.bar). +1 because I learned something from this.

[Bill]

I'm learning about when to use class variables and methods (as opposed to instance variables and methods). When you want to access a class variable in an instance method, is it necessary to use self.__class__.bar? I noticed that self.bar works even though bar is a class variable not an instance variable.

[Bob Stein]

@Bill use self.__class__.bar if you're varying the class variable.

[Trevor Boyd Smith]

when using static variables, it's a good idea to read the gotchas from here: stackoverflow.com/questions/68645/… . @Constantin gives one of the many gotchas.

[Iván C.]

Well, self.__class__.bar is also valid, but it looks bad :P

[User1010]

Consider adding more information to expand on your answer, or consider editing other answers to add this detail.