When instantiating a class, first, __new__() is called to create the instance of a class, then __init__() is called to initialize the instance.
Called to create a new instance of class cls. ...
If __new__() is invoked during object construction and it returns an
instance of cls, then the new instance’s __init__() method will be
invoked like __init__(self[, ...]), ...
Called after the instance has been created (by __new__()), ...
Because __new__() and __init__() work together in constructing objects
(__new__() to create it, and __init__() to customize it), ...
For example, when instantiating Teacher
class, first, __new__() is called to create the instance of Teacher
class, then __init__() is called to initialize the instance as shown below:
class Teacher:
def __init__(self, name):
self.name = name
class Student:
def __init__(self, name):
self.name = name
obj = Teacher("John") # Instantiation
print(obj.name)
This is the output:
<class '__main__.Teacher'>
John
And, using __new__()
of the instance of Teacher
class, we can create the instance of Student
class as shown below:
# ...
obj = Teacher("John")
print(type(obj))
print(obj.name)
obj = obj.__new__(Student) # Creates the instance of "Student" class
print(type(obj))
Now, the instance of Student
class is created as shown below:
<class '__main__.Teacher'>
<__main__.Teacher object at 0x7f4e3950bf10>
<class '__main__.Student'> # Here
Next, if we try to get the value of name
variable from **the instance of Student
class as shown below:
obj = Teacher("John")
print(type(obj))
print(obj.name)
obj = obj.__new__(Student)
print(type(obj))
print(obj.name) # Tries to get the value of "name" variable
The error below occurs because the instance of Student
class has not been initialized by __init__()
yet:
AttributeError: 'Student' object has no attribute 'name'
So, we initialize the instance of Student
class as shown below:
obj = Teacher("John")
print(type(obj))
print(obj.name)
obj = obj.__new__(Student)
print(type(obj))
obj.__init__("Tom") # Initializes the instance of "Student" class
print(obj.name)
Then, we can get the value of name
variable from the instance of Student
class as shown below:
<class '__main__.Teacher'>
John
<class '__main__.Student'>
Tom # Here