How to reference a static attribute from within a class in Python?

Question

I have the following python snippet:

class myClass:
    myVar = 'a'

    def __init__(self):
        self.myOtherVar = 'b'
        myVar = 'c'  # Gets assigned but only as a local variable.

    print myVar            # prints 'a' !
    print self.myOtherVar  # says 'self' not found

My question is this; What's the proper way to print the contents of myVar from within myClass and/or re-assign them from init?

Answer 1

The problem you are facing is because you don't understand how the scoping of class declarations work. A class declaration is executed in its own scope. After the execution is completed a new class object is created and the obtained scope is attached to the class as its __dict__.

Note: the class scope is not searched from within the methods scopes! This means that you have to reference class attributes as MyClass.attribute when inside a method definition.

For example:

class MyClass:
    var = 1

    # we are executing this code as a single block
    # so you must reference the variable as is usual
    print(var)

    # default values are *not* inside the definition.
    # they are evaluated in the outer scope, so use plain "var" here
    def method(self, a_default=var):
        print(a_default)

    def other_method(self):

        # inside methods you are in a different scope
        print(MyClass.var)

        # equivalent *if* no "var" instance attributes exists
        print(self.var)

Note: since the class doesn't still exist when executing its declaration you cannot refer to MyClass at the "top level" of MyClass declaration:

class MyClass:
    var = 1
    print(MyClass.var)   # error: MyClass still doesn't exist.

---

A side effect of this, is that the following code:

class MyClass:
    x = 1
    results = list(i+x for i in range(10))

Produces:

NameError                                 Traceback (most recent call last)
<ipython-input-6-f1d4417b2e52> in <module>()
----> 1 class MyClass:
      2     x = 1
      3     results = list(i+x for i in range(10))
      4 

<ipython-input-6-f1d4417b2e52> in MyClass()
      1 class MyClass:
      2     x = 1
----> 3     results = list(i+x for i in range(10))
      4 

<ipython-input-6-f1d4417b2e52> in <genexpr>(.0)
      1 class MyClass:
      2     x = 1
----> 3     results = list(i+x for i in range(10))
      4 

NameError: name 'x' is not defined

Because generator expressions (and list-comprehensions in python3) are, in fact, considered functions with their own scope. Since the class scope isn't searched from inner function scopes the x cannot be found.

You can word around this using a function definition and default values:

class MyClass:
    x = 1
    def _make_results(x=x):
        return list(i+x for i in range(10))
    results = _make_results()
    del _make_results    # otherwise it would be added as a method.
    # or:
    results = (lambda x=x: list(i+x for i in range(10)))()

This isn't usually a problem since class definitions rarely contain anything other than method definitions and a few constants.

---

There are already a few questions on SO about class scopes:

• The scope of names defined in class block doesn't extend to the methods' blocks. Why is that?
• Short Description of the Scoping Rules?
• Nested classes' scope?
• Variable scopes in python classes
• Why static binding works differently for class and function?

Answer 2

self.var will:

1. give var in self.__dict__ if present
2. give var in self.__class__.__dict__ if present
3. AttributeError

So use this or self.__class__.var if you want to access the static variable minding inheritance. If you extend myClass, the children instances will access the static variable in the child class.

If you want to access the static variable in myClass even when called from descendants, use myClass.var.

As for reassigning them, this must be done explicitly on the class object, or the assignment will just target the instance.

Answer 3

class myClass:
    myVar = 'a'

    def __init__(self):
        self.myOtherVar = 'b'

    print myVar  # -> 'a'

    class EmptyClass: pass
    s = EmptyClass()
    __init__(s)

    myVar = s.myOtherVar
    print myVar  # -> 'b'

print myClass.myVar  # -> 'b'