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Classes

#############################

class Person:
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return "Person(name: " + self.name + ")"

####################

class Man(Person):
    def __init__(self, name, age):   # Note 3, don't forget to call the ancestor's __init__
        Person.__init__(self, name)   # Note 1, include self
        self.age = age

    def __str__(self):
        return "Man(name: " + self.name + ", age: " + str(self.age) + ")"

####################

joe = Person('joe')   # Note 2, don't include self
print joe   # Person(name: joe)

lac = Man('lac', 32)
print lac   # Man(name: lac, age: 32)

All data attributes of a class are initialized in the __init__ method. Tip: always assign an initial value to all data attributes in the __init__ method! It'll save you hours of debugging.

Note 1 & 2: When you call a method of the ancestor class from your class, you MUST include the self argument! However, when you call a class method from outside, you don't need to specify the self argument.

Note 3: __init__ methods are optional, but if you define one, then you must call the ancestor's __init__ method! This is not called automatically for you.

Class Attributes

class Count:
    cnt = 0   # class attributes are here; data attribute go to __init__

    def __init__(self):
        self.__class__.cnt += 1
        # or: Count.cnt += 1

####################

print Count.cnt   # 0, works without instances

cnt = Count()
print cnt.cnt     # 1

cnt2 = Count()
print cnt2.cnt    # 2

print Count.cnt   # 2

Here cnt is a class attribute, i.e. it is common for all the instances of the class Count. There are two ways to modify its value from __init__:

• self.__class__.cnt += 1, or
• Count.cnt += 1