Code tutorial for Python operator overloading

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Python operator overloading

Provided by Python language It has the operator overloading function and enhances the flexibility of the language. This is somewhat similar to but somewhat different from C++. In view of its special nature, today we will discuss Python operator overloading.

The Python language itself provides many magic methods, and its operator overloading is achieved by rewriting these Python built-in magic methods. These magic methods all start and end with double underscores, similar to the form of X. Python uses this special naming method to intercept the operator to achieve overloading. When Python's built-in operations are applied to class objects , Python will search and call the specified method in the object to complete the operation.

Classes can overload built-in operations such as addition and subtraction, printing, function calls, index, etc. Operator overloading makes the behavior of our objects Same as built-in objects. Python will automatically call such a method when calling an operator. For example, if a class implements the add method, this method will be called when an object of the class appears in the + operator.

Common operator overloading methods

##initObject creation: X = Class(args)delX object is recoveredadd/subAddition and subtraction operations X+Y, X+=Y/X-Y, X-=Y##or_repr／strcallgetattrAttributesetattrdelattrDelete##getattributegetitem],X[i:j]setitemdelitemlenboollt, gt, le, ge, X==Y，X!=Y Right side addition##iaddField (enhanced) addition add)IteratecontainsMembership testValue##with obj as var:deleteDescriptor attribute

##Method name	Overloading description	Operator calling method
	Constructor
	Destructor
Operator|	X|Y, X|= Y
Print/Convert	print (X)、repr(X)／str(X)
Function call	X(*args, **kwargs)
	Quote X.undefined
Attribute assignment	X.any=value
Attribute	del X.any
Attribute get	X.any
Index operation	X[ key
Index assignment	X[key],X[ i:j]=sequence
Index and shard deletion	del X[key],del X[i:j]
length	len(X)
Boolean test	bool(X)
eq, ne Specific comparison	The order is XY,X= Y，	Note ：（lt: less than, gt: greater than, less equal, ge: greater equal, eq: equal, ne: not equal ) ##radd
other+X
X+=Y(or else		iter, next
I=iter(X), next ()
item in X (X is any iterable object)	##index	##Integer
hex(X), bin(X), oct(X)	enter, exit	Environment Manager
	get, set,
X.attr, X.attr=value, del X.attr
new	create	before init Create Object

The following is an introduction to the use of commonly used operator methods.

Constructor and destructor: init and del

Their main function is to create and recycle objects. When an instance is created , the initConstructor Method will be called. When the instance object is reclaimed, the destructor del will be executed automatically.

>>> class Human(): 
...   def init(self, n): 
...     self.name = n 
...       print("init ",self.name) 
...   def del(self): 
...     print("del") 
...  
>>> h = Human('Tim') 
init Tim 
>>> h = 'a' 
del

Addition and subtraction operations: add and sub

Overloading these two methods can add +- operator operations to ordinary objects. The following code demonstrates how to use the +- operator. If you remove the sub method in the code and then call the minus operator, an error will occur.

>>> class Computation(): 
...   def init(self,value): 
...     self.value = value 
...   def add(self,other): 
...     return self.value + other 
...   def sub(self,other): 
...     return self.value - other 
...  
>>> c = Computation(5) 
>>> c + 5 
10 
>>> c - 3 
2

String of the objectExpression form: repr and str

## These two methods are used to represent the characters of the object String expression form: The print() and str() methods will call the str method, and the print(), str() and repr() methods will call the repr method. As can be seen from the example below, when two methods are defined at the same time, Python will give priority to searching and calling the str method.

>>> class Str(object): 
...   def str(self): 
...     return "str called"   
...   def repr(self): 
...     return "repr called" 
...  
>>> s = Str() 
>>> print(s) 
str called 
>>> repr(s) 
'repr called' 
>>> str(s) 
'str called'

Index value acquisition and assignment: getitem, setitem
## By implementing these two methods, objects can be processed in the form such as X[i] Get and assign values, and you can also use slicing operations on objects.

>>> class Indexer: 
  data = [1,2,3,4,5,6] 
  def getitem(self,index): 
    return self.data[index] 
  def setitem(self,k,v): 
    self.data[k] = v 
    print(self.data) 
>>> i = Indexer() 
>>> i[0] 
1 
>>> i[1:4] 
[2, 3, 4] 
>>> i[0]=10 
[10, 2, 3, 4, 5, 6]

Set and access attributes: getattr, setattr

We can intercept access to object members by overloading getattr and setattr. getattr is automatically called when accessing a member that does not exist in the object. The setattr method is used to call when initializing object members, that is, the setattr method will be called when setting the dict item. The specific example is as follows:

class A(): 
  def init(self,ax,bx): 
    self.a = ax 
    self.b = bx 
  def f(self): 
    print (self.dict) 
  def getattr(self,name): 
    print ("getattr") 
  def setattr(self,name,value): 
    print ("setattr") 
    self.dict[name] = value 
 
a = A(1,2) 
a.f() 
a.x 
a.x = 3 
a.f()

The running results of the above code are as follows. From the results, it can be seen that the getattr method will be called when accessing the non-existent

variable

x; when init is called, the value will be assigned The operation also calls the setattr method.

setattr 
setattr 
{'a': 1, 'b': 2} 
getattr 
setattr 
{'a': 1, 'x': 3, 'b': 2}

Iterator object: iter, next

Iteration in Python can be implemented directly by overloading the getitem method. See the example below.

>>> class Indexer: 
...   data = [1,2,3,4,5,6] 
...   def getitem(self,index): 
...       return self.data[index] 
...  
>>> x = Indexer() 
>>> for item in x: 
...   print(item) 
...  
1 
2 
3 
4 
5 
6

Iteration can be achieved through the above method, but it is not the best way. Python's iteration operation will first try to call the iter method, and then try to getitem. The iterative environment is implemented by using iter to try to find the iter method, which returns an iterator object. If this method is provided, Python will repeatedly call the next() method of the iterator object until the S

top

Iteration exception occurs. If iter is not found, Python will try to use the getitem mechanism. Let's look at an example of an iterator.

class Next(object): 
  def init(self, data=1): 
    self.data = data 
  def iter(self): 
    return self 
  def next(self): 
    print("next called") 
    if self.data > 5: 
      raise StopIteration 
    else: 
      self.data += 1 
      return self.data 
for i in Next(3): 
  print(i) 
print("-----------") 
n = Next(3) 
i = iter(n) 
while True: 
  try: 
    print(next(i)) 
  except Exception as e: 
    break

The running results of the program are as follows:

next called 
4 
next called 
5 
next called 
6 
next called 
----------- 
next called 
4 
next called 
5 
next called 
6 
next called

It can be seen that after implementing the iter and next methods, you can iterate through for in

Traverse the object

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