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

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?

Answer 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.

Answer 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

Answer 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.

Answer 4

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

f = Foo() 
f.bah()

Answer 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.

Answer 6

You can access class variables by object and directly by class name from the outside or inside of class and basically, you should access class variables directly by class name because if there are the same name class and instance variables, the same name instance variable is prioritized while the same name instance variable is ignored when accessed by object. So, using class name is safer than using object to access class variables.

For example, you can access the class variable by object and directly by class name from the outside of the class as shown below:

class Person:
    name = "John" # Class variable

obj = Person()
print(obj.name) # By object
print(Person.name) # By class name

Output:

John
John

But, if you add the same name instance variable as the class variable by object:

class Person:
    name = "John" # Class variable

obj = Person()
obj.name = "Tom" # Adds the same name instance variable as class variable
print(obj.name) # By object
print(Person.name) # By class name

Or, if you add the same name instance variable as the class variable by self in __init__():

class Person:
    name = "John" # Class variable
    
    def __init__(self, name):
        self.name = name # Adds the same name instance variable as class variable

obj = Person("Tom")
print(obj.name) # By object
print(Person.name) # By class name

The same name instance variable is prioritized when accessed by object:

Tom  # By object
John # By class name

And, you can also access the class variable by self and directly by class name from the inside of the instance method as shown below:

class Person:
    name = "John" # Class variable
        
    def test(self): # Instance method
        print(self.name) # By "self"
        print(Person.name) # By class name

obj = Person()
obj.test()

Output:

John
John

But, if you add the same name instance variable as the class variable by object:

class Person:
    name = "John" # Class variable
        
    def test(self): # Instance method
        print(self.name) # By "self"
        print(Person.name) # By class name

obj = Person()
obj.name = "Tom" # Adds the same name instance variable as the class variable
obj.test()

Or, if you add the same name instance variable as the class variable by self in __init__():

class Person:
    name = "John" # Class variable
    
    def __init__(self, name):
        self.name = name # Adds the same name instance variable as the class variable
        
    def test(self): # Instance method
        print(self.name) # By "self"
        print(Person.name) # Directly by class name

obj = Person("Tom")
obj.test()

The same name instance variable is prioritized when accessed by self:

Tom  # By "self"
John # By class name