Python 3

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Introduction to Python

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Class Variables / Attributes

class variables / attributes

Class variables are data stored directly in the class; they are actually attributes set in the class.

class MyClass:

    var = 10              # a class variable / attribute


val = MyClass.var         # 10 (retrieve the class attribute)

MyClass.var2 = 'hello'    # set an attribute directly in the class object


obj = MyClass()           # MyClass instance

var is assigned inside the class block; this makes it a class variable
a class variable is actually a class attribute
note how this variable is accessible as a attribute of the class (MyClass.var)
we can also set attributes directly in the class using <class>.<attribute> syntax and they will work in the same way
keep in mind the class variables are neither global nor local; they are assigned using a similar syntax, but they are actually attributes stored in the class itself
this is why when accessing them, we must prepend the class name (MyClass.var)

class variables / attributes are also accessible through its instances

Instances can read class attributes using the same syntax as when reading their own attributes.

class MyClass:
    var = 10              # a class variable / attribute


obj = MyClass()           # a MyClass instance / object


print((obj.var))            # 10

Note that obj.var is the same syntax as the one we use to access instance attributes.

<object>.<attribute> 'cascading' lookup

If an attribute can't be found in an object, it is searched for in the class.

class MyClass:
    classval = 10           # class attribute

    def __init__(self):
        self.instval = 99   # instance attribute

    def get(self):
        return self.instval


a = MyClass()

print(a.instval)            # 99 (found in the instance)
print(a.classval)           # 10 (found in the class

val = a.get()               # access class variable get() through the instance

when we access (or "look up") an attribute from an instance, it may come from one of two places
the attribute may be found in the instance; if not, it may be found in the class
Python uses a 'cascading lookup' to check first in the instance, then the class
this lookup is necessary in part because any method defined in the class is a class variable - so method calls must also find the attribute

class variable is shared data for instances

Here's an example of class data that is common to each instance.

class MyClass:
    instance_count = 0

    def __init__(self, letter):
        self.id = letter
        MyClass.instance_count = MyClass.instance_count + 1


a = MyClass('alpha')
b = MyClass('beta')

print(a.id)                 # 'a'
print(b.id)                 # 'b'

print(a.instance_count)     # 2
print(b.instance_count)     # 2

instance_count is a class variable
it is incremented every time an instance is created
each instance a and b have their own .id value
but each instance is also aware of how many instances have been created
(this is not a practical example but demonstrates a class variable in action)

Ex. 14.1

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