The new () method mainly exists in the new class of Python 2 and in Python 3. It is the static method responsible for creating class instances.
When Python instantiates an object, it first calls the__ new__() Method to construct an instance of a class and allocate the memory space of the corresponding type for it. The memory address of the instance is its unique identifier. And then call__ init__() Method to initialize an instance, usually the properties of the instance.
Here are some examples to illustrate:
Example 1: call first__ new__() Method call again__ init__() method
class Person(object):
def __new__(cls):
print("__new__ called")
return super().__new__(cls)
def __init__(self):
print("__init__ called")
a = Person()
result:
__new__ called
__init__ called
Example 2: the new () method constructs a class instance and passes it to its own__ init__() Method, i.e__ init__() The self parameter of the method.
class Person(object):
def __new__(cls):
print("__new__ called")
instance = super().__new__(cls)
print(type(instance))
print(instance)
print(id(instance))
return instance
def __init__(self):
print("__init__ called")
print(id(self))
b = Person()
result:
__new__ called
<class '__main__.Person'>
<__main__.Person object at 0x1093c1580>
4449899904
__init__ called
4449899904
Example 3: if__ new__() Method does not return any instances, the init () method will not be called.
class Person(object):
def __new__(cls):
print("__new__ called")
def __init__(self):
print("__init__ called")
c = Person()
result:
__new__ called
Example 4: if__ new__() Method returns an instance of another class__ init__() Method will not be called. Moreover, the new () method will initialize an object of another class.
class Animal(object):
def __init__(self):
pass
class Person(object):
def __new__(cls):
print("__new__ called")
return Animal()
def __init__(self):
print("__init__ called")
d = Person()
print(type(d))
print(d)
result:
__new__ called
<class '__main__.Animal'>
<__main__.Animal object at 0x10fea3550>
Example 5: if overridden__ new__() Method, except for the CLS parameter, if no other parameters are set, it cannot be used__ init__() Method to set initialization parameters.
class Person(object):
def __new__(cls):
print("__new__ called")
instance = super().__new__(cls)
return instance
def __init__(self, name):
print("__init__ called")
self.name = name
e = Person("Eric")
print(e.name)
result:
Traceback (most recent call last):
File "example.py", line 102, in <module>
e = Person("Eric")
TypeError: __new__() takes 1 positional argument but 2 were given
Example 6: in rewriting__ new__() Method, you need to add * args, * * kwargs to the parameters, or explicitly add the corresponding parameters to pass__ init__() Method initialization parameters.
class Person(object):
def __new__(cls, *args,**kwargs): # Or def __new__(cls, name)
print("__new__ called")
instance = super().__new__(cls)
return instance
def __init__(self, name):
print("__init__ called")
self.name = name
e = Person("Eric")
print(e.name)
result:
__new__ called
__init__ called
Eric
