Common magic methods in Python

Illustrations by Nikita Pilyukshin

♚

Zarten, a front-line Internet worker.

Blog: zhihu.com/people/zarten

An overview of the

Special python methods (magic methods) are called by the Python interpreter. We don’t need to call them ourselves. We use built-in functions to use them. For example, when the special __len__() method is implemented, we simply use len(); Some special methods are called implicitly, for example: for I in x: the built-in iter(x) function is used behind it.

Some common ad-hoc methods and implementations are described below. By implementing a class

Commonly used special methods and implementation

__len__()

Generally returns the number, called with len(). The len() function \ can also be used inside __len__()

class Zarten(): def __init__(self, age): self.age = age self.brother = ['zarten_1', 'zarten_2'] def __len__(self): Return len(self.brother) # len() # return self.age z = Zarten(18) print(len(z))Copy the code

__str__()

The string representation of an object is basically the same as __repr__(), with minor differences:

__str__() is for end users, and __repr__() is for developers, for debugging, logging, etc.

2. On the command line, after implementing __str_(), directly entering the object name will display the object memory address; ‘repr(), on the other hand, has the same effect as print (object).

3. If both are implemented, __str_() is called, and __repr__() is generally implemented in the class at least.

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class Zarten():
def __repr__(self):
return 'my name is Zarten_1'
\
def __str__(self):
return 'my name is Zarten_2'
\
z = Zarten()
print(z)

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my name is Zarten_2

__iter__()

Returns an iterable, usually used \ with __next__()

Copy the code

class Zarten():
def __init__(self, brother_num):
self.brother_num = brother_num
self.count = 0
\
def __iter__(self):
return self
\
def __next__(self):
if self.count >= self.brother_num:
raise StopIteration
else:
self.count += 1
return 'zarten_' + str(self.count)
\
\
zarten = Zarten(5)
for i in zarten:
print(i)

__getitem__()

This special method returns data and can also replace the __iter_() and __next__() methods, as well as support slicing

Copy the code

class Zarten():
def __init__(self):
self.brother = ['zarten_1','zarten_2','zarten_3','zarten_4','zarten_5',]
\
def __getitem__(self, item):
return self.brother[item]
\
zarten = Zarten()
print(zarten[2])
print(zarten[1:3])
\
for i in zarten:
print(i)

__new__()

__new__() is used to construct an instance of a class. The first argument is CLS, which is not normally used. __init__() is used to initialize the instance, so __new__() is executed before __init___().

If __new__() does not return, no object is created and __init___() is not executed;

If __new__() returns an instance of another class, __init___() will not execute either;

Purpose: Use __new___() to implement the singleton pattern

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class Zarten():
def __new__(cls, *args, **kwargs):
print('__new__')
return super().__new__(cls)
\
def __init__(self, name, age):
print('__init__')
self.name = name
self.age = age
\
def __repr__(self):
return 'name: %s age:%d' % (self.name,self.age)
\
zarten = Zarten('zarten', 18)
print(zarten)

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__new__
__init__
name:zarten age:18

Use __new__() to implement the singleton pattern

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class Zarten():
_singleton = None
\
def __new__(cls, *args, **kwargs):
print('__new__')
if not cls._singleton:
cls._singleton = super().__new__(cls)
return cls._singleton
\
def __init__(self, name, age):
print('__init__')
self.name = name
self.age = age
\
def __repr__(self):
return 'name: %s age:%d' % (self.name,self.age)
\
zarten = Zarten('zarten', 18)
zarten_1 = Zarten('zarten_1', 19)
print(zarten)
print(zarten_1)
print(zarten_1 == zarten)

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__new__
__init__
__new__
__init__
name:zarten_1 age:19
name:zarten_1 age:19
True

__call__()

This object can be called like a function. For example, custom functions, built-in functions, and classes are all callable objects. Callable () can be used to determine whether they are callable objects

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class Zarten():
\
def __init__(self, name, age):
self.name = name
self.age = age
\
def __call__(self):
print('name:%s age:%d' % (self.name, self.age))
\
\
z = Zarten('zarten', 18)
print(callable(z))
z()

__enter__()

A context manager class must implement these two special methods: __enter_() and __exit__(), which are called using the with statement.

Use __enter__() to return the object and __exit__() to close the object

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class Zarten():
\
def __init__(self, file_name, method):
self.file_obj = open(file_name, method)
\
def __enter__(self):
return self.file_obj
\
def __exit__(self, exc_type, exc_val, exc_tb):
self.file_obj.close()
print('closed')
\
\
with Zarten('e:\\test.txt', 'r') as f:
r = f.read()
print(r)

__add__()

Addition operator overloading and __radd__() reverse operator overloading

When object as additive, the first will be “+” on the left side of the object to find __add__ (), if can’t find it in the “+” on the right side of the search __radd__ ()

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class Zarten():
\
def __init__(self, age):
self.age = age
\
def __add__(self, other):
return self.age + other
\
def __radd__(self, other):
return self.age + other
\
\
\
z = Zarten(18)
print(z + 10)
print(20 + z)

__del__()

Called at the end of an object’s life cycle, equivalent to a destructor

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class Zarten():
\
def __init__(self, age):
self.age = age
\
def __del__(self):
print('__del__')
\
\
z = Zarten(18)

Summary of special (magic) methods

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