Python Class (1)
Class & Instance
- attributes & methods are inside of the class
- A method is a function that “belongs to” an object
- instance variable contains data that is unique to each instance
__init__: when we create an object, it initializes the object as default.__init__(self): each method in class automatically receives instance as the first argument (self)
Basic Example
a = 10 # local variable
class Test:
b = 20 # class variable
print(a)
print(Test().b)
10 20
# creates a new instance of the class 'Test()' and assigns this object to the local variable t
t = Test()
# class variable is accessible only from class object
print(t.b)
t.b = 30
print(t.b)
20 30
t1 = Test()
t2 = Test()
# t1 and t2 are own unique instances of Test class with different locations in memory
print(t1)
print(t2)
<__main__.Test object at 0x0000000004FDA5F8> <__main__.Test object at 0x0000000004FDA668>
# setting attribute b that are unique to each instance
t1.b = 10
t2.b = 20
print(t1.b, t2.b)
10 20
Person Example
Create an instance called obj from Person class. Then, set the attributes of this instance manually.
class Person:
name = str()
age = int()
salary = float()
obj = Person() # create an instance
obj.name = 'kimmy' # set attributes
obj.age = 30
obj.salary = 1000.0
print('name: ',obj.name, ', age: ', obj.age, ', salary: ', obj.salary)
name: kimmy , age: 30 , salary: 1000.0
In this example, create an instance called obj1 from Person1 class and set the attributes of this instance at once by creating a method called ‘setItem()’. Furthermore, I’ve created another method called printInfo() to print the information.
class Person1:
name = str()
age = int()
salary = float()
#method (not function)
def setItem(self, name, age, salary): # self is instance
self.name = name
self.age = age
self.salary = salary
# instance is the only argument that we'll need in this method
def printInfo(self): # don't forget to put self
print('name: {}, age: {}, salary: {}'.format(self.name, self.age, self.salary))
obj1 = Person1()
obj1.setItem('kimmy', 30, 10000)
obj1.printInfo()
name: kimmy , age: 30 , salary: 1000.0
In this example, create __init__ method to directly initialize the instance variables.
- in class Person1, we did not use init method. Therefore, we had to create an object and use setItem method to pass in the information of attributes.
- in class Person2, we can create an object and pass in information of attributes in one line by creating
__init__method.
class Person2:
name = str()
age = int()
salary = float()
def __init__(self, name, age, salary):
self.name = name #set instance variables
self.age = age
self.salary = salary
def printInfo(self):
print('name: {}, age: {}, salary: {}'.format(self.name, self.age, self.salary))
# init method will run automatically
# obj2 will be passed in as self and it will set all these attributes
obj2 = Person2('Kimmy', 30, 10000)
# these two have the same logic
obj2.printInfo() # automatically pass in self
Person2.printInfo(obj2) # calls method on the class and it doesnt know which instance. so pass in intance
name: Kimmy , age: 30 , salary: 10000 name: Kimmy , age: 30 , salary: 10000
