Special Methods in Classes
SpeciaL Methods
We cannot use general built in functions on the classes.
In the case of Lists
mylist = [1,2,3]
len(mylist)
print(mylist)
Output:
3
[1, 2, 3]
In the case of Classes
class sample():
pass
mysample = sample()
len(mysample)
print(mysample)
Output:
TypeError: object of type 'sample' has no len()
<__main__.sample object at 0x7f84a10f2310>
__str__()
method
Special method str() will be defined in the classes to return string representation of the object.
This method is called when print() or str() function is invoked on an object.
Using __str__()
method
class book():
def __init__(self,title,author):
self.title = title
self.author = author
self.pages = pages
def __str__(self):
return f"{self.title} by {self.author}"
b = book('Python Classes','Bhavya')
print(b)
Output:
Python Classes by Bhavya
__len__()
method
The len() function will attempt to call a method named __len__()
in our class.
Using __len__()
method
class book():
def __init__(self,title,author,pages):
self.title = title
self.author = author
self.pages = pages
def __str__(self):
return f"{self.title} by {self.author}"
def __len__(self):
return self.pages
b = book('Python Classes','Bhavya', 200)
len(b)
Output:
200
__del__()
method
__del__()
is a destructor method which is called as soon as all references of the object are deleted i.e when an object is garbage collected.
By using del
keyword all references of object will be deleted and therefore destructor invoked automatically.
Using __del__()
method
class book():
def __init__(self,title,author,pages):
self.title = title
self.author = author
self.pages = pages
def __str__(self):
return f"{self.title} by {self.author}"
def __len__(self):
return self.pages
def __del__(self):
print("A book object has been deleted")
b = book('Python Classes','Bhavya', 200)
del b
Output:
A book object has been deleted
Note
These special methods are called as dunder methods "Double Under (Underscores)" which python will look for to perform special operations.