Introduction to Python

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Class Methods and Static Methods

instance / object methods

Instance methods are designed to read or write attributes in the instance.

class Counter:

    # a 'setter' method
    def __init__(self, val):
        self.value = val       # set the value in the instance's attribute

    # a 'getter' method
    def getval(self):
        return self.value      # read the value in the instance's attribute

    def increment(self):
        self.value = self.value + 1


a = Counter(10)

• all of the methods we have seen so far are instance methods
• an instance method is one that works with the instance
• instance methods are designed to read or write attributes in the instance
• the instance method is marked in the definition by the initial argument self
• since self is the instance and thus has access to the instance attributes, it is always present in the method's arguments list

class methods

Class methods are designed to read or write attributes in the class.

class MyClass:
    instance_count = 0

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

    @classmethod
    def reset_instance_count(cls):
        cls.instance_count = 0

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

print(a.instance_count)     # 2

a.reset_instance_count()

print(a.instance_count)     # 0

• the @ ("pie") notation above the function is called a decorator
• note that reset_instance_count(cls) has a @classmethod decorator above its definition
• note also that the argument to reset_instance_count() is cls, not self
• cls refers to the class object itself (here it is MyClass)
• since this class object has access to all of its attributes, we can modify them inside the class method
• again note that we have to name the class in __init__ as we don't have direct access to the class object except by name

Ex. 14.2

static methods

Static methods do not work with instance or class, but still may belong to the class.

import datetime

class MyClass:
    instance_list = []

    def __init__(self, letter):
        self.id = letter
        formatted_time = MyClass.get_now()
        MyClass.instance_list.append((self, formatted_time))

    @staticmethod
    def get_now():
        dt = datetime.datetime.now()   # a datetime object
        ds = dt.strftime('%H:%M:%S')   # '17:06:41' (or current time)
        return ds

a = MyClass('alpha')
time.sleep(1)               # wait 1 second
b = MyClass('beta')

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

print(MyClass.instance_list)     # [(<MyClass>, '17:06'), (<MyClass>, '17:07')]

• the static method 'get_now()' returns the current time as a formatted string
• it does not require the instance or the class to do its job
• so it is not appropriate to take self or cls as an argument
• in order to turn it into a 'plain' function, we must add the decorater @staticmethod
• (we are recording the time each instance was created possibly for logging or debugging purposes, but its practical use doesn't matter for this example)

Ex. 14.3