Python full stack development day24-__new__, __DEL__, item series, exception handling

I. Yesterday's content review

1. Reflection

The variable that operates the namespace with the name of the string type.

Reflection usage Scenario: the obvious simplification of the code, can get the variable name is a string type of time,

User input, file read-in, on-line transmission

2.__call__

The __call__ method is called by the object name ()

3.__len__

4.__hash__

5.__eq__

6.__dict__

Object name. __dict__: A variable that looks at the variable of the object space,

Class name. __dict__ class Space

7.__slots__

#__slots__ = (' name ', ' age ')

8.__module__ and __class__

9.isinstance and Issubclass

Two. Summary of today's content 1.__new__

#创建一个对象, is a constructor method in the Initialize Method __init__ (self,... Executes before execution.

Single-Case mode:

　　　　

classA:__instance=Nonedef __init__(self,name): Self.name=namedef __new__(CLS, *args, * *Kwargs):if  notCls.__instance: CLS.__instance= object.__new__(CLS)returnCls.__instancea= A ('Alex') b= A ('Jack')Print(A.name)Print(B.name)

example of a singleton pattern

2.__del__

#在一个对象在内存中删除前, execute the __del__ method

# contrast delattr use a variable of character type to remove this variable from the namespace

# compare @property deleter masquerading as a property, to perform a delete property

#Python解释器

#能够主动回收不用的变量

#在程序结束的时候所有的数据都会被清除

#如果用户主动删除某个变量

# then this variable will be actively deleted

#无论上述哪种方式

#在删除一个变量之前都会主动的执行析构方法__del__

Application: Returning an operating system resource before an object is deleted

Class B:

def __init__ (Self,path):

SELF.F = open (path)

def __del__ (self):

Self.f.close ()

b = B (' USERINFL ')

3.__getitem__, __setitem__, __delitem__

# There are built-in methods in the built-in modules

#是依赖__getitem__方法的

#或者说是依赖item [' A '] this type of call

# Shuffle, draw example:

Random.choice method Dependency (__len__,__getitem__)

Random.shuffle method dependent objects and built-in Parties (__LEN__,__GETITEM__,__SETITEM__)

　　

ImportRandom fromCollectionsImportNamedtuplecard= Namedtuple ('Card', ['Rank','Suit'])classfranchdeck:ranks= [I forIinchRange (2, one)] + list ('Jqka') Suits= ['Hearts','blocks','Black Peach','Plum Blossom']    def __init__(self): self.__cards= [Card (rank, suit) forRankinchFranchdeck.ranks forSuitinchFranchdeck.suits]def __len__(self):returnLen (self.__cards)    def __getitem__(self, item):returnSelf.__cards[item]def __setitem__(self, Key, value): Self.__cards[Key] =valuedef __delitem__(self, key):delSelf.__cards[Key]deck=Franchdeck ()Print(Deck.__dict__)Print(Deck[0]) random.shuffle (deck)Print(Deck.__dict__)Print(Random.choice (deck))

examples of shuffling and drawing cards

4. Exception Handling

#try except

#try except Else

#try except else finally

#try finally

# Assert 1=2

# raise actively throws an exception

# Custom Exception Types

　　

classwuexception (baseexception):def __init__(self, name): Self.__name=namedef __str__(self):returnSelf.__nameTry:    RaiseWuexception ('Let's try something wrong.')exceptwuexception as E:Print(e)

examples of custom exception types

Three. Preview and expand

1. Understanding of Meta-class Metaclass:

1), the class that we created with the Class keyword is created from the base class type class:

Class Foo (object):

A = 1

def func ():

Print (6666)

Equivalent to: Type (' Foo ', (object,), {' A ': 1, ' func ': func})

def func ():

Print (6666)

2), create the class when there is a priority to find __metaclass__, and did not look for the parent class, the parent class did not find the module, has not been found to use the built-in type for class creation

Class A:

__metaclass__ = mm

3), our own custom meta-class:

Class Metaclasstest (Type):

def __new__ (cls,name,bases,dict):

..........

Return super (). __new__ (Cls,name,bases,dict (changeable))

Python full stack development day24-__new__, __DEL__, item series, exception handling