Explain __init__ and __str__ method with an example Python program

Q: What is __ init __ In Python class?

__init__ is a reseved method in python classes. It is called as a constructor in object oriented terminology. This method is called when an object is created from a class and it allows the class to initialize the attributes of the class.

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Table of Contents Show

The init method
Video Tutorial
The init method for Time class
The __str__ method
Clik here to read Solution to Python Application Programming Question Paper Jan 2019 15CS664
The need for __str__ method:
How to call __str__ method
1. Default implementation
2. Custom __str__ method
3. __repr__ method defined only
What is __ init __ and __ str __ in Python?
What is __ init __ In Python explain with example?
What does __ str __ method do in Python?
What is __ init __ In Python class?

The init method

Like the constructors in Java, the init method is a special method that gets called whenever a new object is created or instantiated. Its full name is __init__ (two underscore characters, followed by keyword init, and followed by two more underscores).

Video Tutorial

Let us take an example to understand the __init__ method. An init method for the Time class might look like this. The time class has three attributes Hour, Minute, and Second. As and when a new object of the Time class is created these three attributes are initialized. The different possible cases are shown below.

The init method for Time class

class Time:
     """Represents the time of day. attributes: hour, minute, second"""
  
     def __init__(self, hour=0, minute=0, second=0):
         self.hour = hour
         self.minute = minute
         self.second = second
  
     def print_time(t):
         print('Hour:', t.hour, '\nMinute: ', t.minute, '\nSeconds: ', t.second)
  
 #The parameters are optional, so if you call Time with no arguments, you get the default
 values.
  
 print('----------------')
 time1 = Time()
 time1.print_time()
  
 #If you provide one argument, it overrides hour:
  
 print('----------------')
 time2 = Time(10)
 time2.print_time()
  
 #If you provide two arguments, they override hour and minute.
  
 print('----------------')
 time3 = Time(10,20)
 time3.print_time()
  
 #If you provide two arguments, they override hour, minute and seconds
  
 print('----------------')
 time4 = Time(10,20,30)
 time4.print_time()

Output:

----------------
Hour: 0 
Minute:  0 
Seconds:  0
----------------
Hour: 10 
Minute:  0 
Seconds:  0
----------------
Hour: 10 
Minute:  20 
Seconds:  0
----------------
Hour: 10 
Minute:  20 
Seconds:  30

It is common for the parameters of __init__ to have the same names as the attributes. The statement self.hour = hour stores the value of the parameter hour as an attribute of the self.

As shown in the output, if you create a new object with no parameters, attributes Hour, Minute and Second are initialized to 0 and so on.

The str method

like __init__ method, __str__ is also a special method, that returns a string representation of an object. For example, here is an str method for Time objects.

 class Time:
     """Represents the time of day. attributes: hour, minute, second"""
  
     def __init__(self, hour=0, minute=0, second=0):
         self.hour = hour
         self.minute = minute
         self.second = second
         
     def __str__(self):
         return '%.2d:%.2d:%.2d' % (self.hour, self.minute, self.second)
  
 print('----------------')
 time1 = Time()
 print(time1)
  
 print('----------------')
 time2 = Time(10, 20)
 print(time2) 
 print('----------------')
 time3 = Time(10, 20, 40)
 print(time3)

Output:

----------------
00:00:00
----------------
10:20:00
----------------
10:20:40

As and when the print function is called with primitive data like string, number, etc the values are printed normally. But when the print is called with object as the parameters, the __str__ function is called. The __str__ function returns the string form of the result. Finally, the print function displays the result.

Clik here to read Solution to Python Application Programming Question Paper Jan 2019 15CS664

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Shahpar Khan

The need for str method:

The __str__ method in Python represents the class objects as a string – it can be used for classes. The __str__ method should be defined in a way that is easy to read and outputs all the members of the class. This method is also used as a debugging tool when the members of a class need to be checked.

The __str__ method is called when the following functions are invoked on the object and return a string:

print()
str()

If we have not defined the __str__, then it will call the __repr__ method. The __repr__ method returns a string that describes the pointer of the object by default (if the programmer does not define it).

How to call str method

1. Default implementation

class MyClass:
    x = 0
    y = ""
    def __init__(self, anyNumber, anyString):
        self.x = anyNumber
        self.y = anyString
myObject = MyClass(12345, "Hello")
print(myObject.__str__())
print(myObject.__repr__())
print(myObject)

The above code shows an example where neither __str__ nor __repr__ are defined. Calling __str__ calls the default __repr__ method, and they all give the same output, the pointer of our object.

2. Custom str method

class MyClass:
    x = 0
    y = ""
    def __init__(self, anyNumber, anyString):
        self.x = anyNumber
        self.y = anyString
    def __str__ (self):
        return 'MyClass(x=' + str(self.x) + ' ,y=' + self.y + ')'
myObject = MyClass(12345, "Hello")
print(myObject.__str__())
print(myObject)
print(str(myObject))
print(myObject.__repr__())

The code above shows the output once you have defined the __str__ method. When __str__, print(), or str() are called you will get your defined output. Make note that the __repr__ output remains the same.

3. repr method defined only

class MyClass:
    x = 0
    y = ""
    def __init__(self, anyNumber, anyString):
        self.x = anyNumber
        self.y = anyString
    def __repr__ (self):
        return 'MyClass(x=' + str(self.x) + ' ,y=' + self.y + ')'
myObject = MyClass(12345, "Hello")
print(myObject.__str__())
print(myObject)
print(str(myObject))
print(myObject.__repr__())

In the first example we saw that when __str__ is not defined it automatically calls the __repr__ method. Therefore, the output of all the functions - __str__, str(), and __repr__ - are the same. Moreover, the __repr__ method does not necessarily need to return a string. In case it does not return a string, the print() statements will throw an error.

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Shahpar Khan

What is init and str in Python?

For instance, when you create a new object, Python automatically calls the __new__ method, which in turn calls the __init__ method. The __str__ method is called when you print() an object. On the other hand, user-defined methods, like stefi. run() , are called explicitly.

What is init In Python explain with example?

In an object-oriented approach, the __init__ method is the Python equivalent of the C++ constructor. Every time an object is created from a class, the __init__function is called. The __init__ method only allows the class to initialize the object's attributes. It is only used within classes.

What does str method do in Python?

Python __str__() This method returns the string representation of the object. This method is called when print() or str() function is invoked on an object. This method must return the String object.

What is init In Python class?

"__init__" is a reseved method in python classes. It is called as a constructor in object oriented terminology. This method is called when an object is created from a class and it allows the class to initialize the attributes of the class.