Tagged: static variable

Static variables and methods in Python

How to declare a data member or a method static in Python? Static means, that the member is on a class level rather on the instance level. Static variables exist only in single instance per class and are not instantiated. If a static variable is changed in one instance of the class, the change will affect its value in all other instances.

Static methods don’t refer to any instance of the class and can be called outside of it. They also cannot access any non-static data members of the class for obvious reasons. Let’s have a look how to get some static from Python.

Variables

All variables defined on the class level in Python are considered static. See this example:

class Example:
    staticVariable = 5

# Access through class
print Example.staticVariable  # prints 5

# Access through instance
instance = Example()
print instance.staticVariable # still 5

# Change within instance
instance.staticVariable = 6
print instance.staticVariable # 6
print Example.staticVariable  # 5

# Change through class
Example.staticVariable = 7
print instance.staticVariable # still 6
print Example.staticVariable  # now 7

Seems pretty straight-forward to me. Only confusion might come from the fact, that you can have two different variables in your class under the same name (one static and one ordinary). But I recommend (for your own sakes) to avoid that behavior entirely.

Methods

With static methods it gets a little more complex. In Python, there are two ways of defining static methods within a class.

@staticmethod

Method decorated with this decorator shares with the class only the namespace. Note that, no arguments are mandatory in the method definition. Static method can access classes static variables. See in the following example:

class Example:
    name = "Example"
    @staticmethod
    def static():
        print "%s static() called" % Example.name

class Offspring1(Example):
    name = "Offspring1"

class Offspring2(Example):
    name = "Offspring2"
    @staticmethod
    def static():
        print "%s static() called" % Offspring2.name

Example.static()    # prints Example
Offspring1.static() # prints Example
Offspring2.static() # prints Offspring2

@classmethod

The difference between class method and static method in Python is, that class method recieves one mandatory argument — a class name it was called from. Let’s have a look: 

class Example:
    name = "Example"
    @classmethod
    def static(cls):
        print "%s static() called" % cls.name

class Offspring1(Example):
    name = "Offspring1"
    pass

class Offspring2(Example):
    name = "Offspring2"
    @classmethod
    def static(cls):
        print "%s static() called" % cls.name

Example.static()    # prints Example
Offspring1.static() # prints Offspring1
Offspring2.static() # prints Offspring2

Which one should you use? The first option allows you only to access the static variables in the same class. With the second approach you’ll be able to modify class variables of the subclasses without the neccessity of redefining the method when using inheritance. I prefer the first variant, because I personaly think it’s a cleaner solution, but the second variant might come useful in certain situations as well.

Sources

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