Detailed explanation of the usage and working principle of super() function in python

The definition of object methods in Python is very weird. The first parameter is generally named self (equivalent to this in other languages), which is used to pass the object itself. It does not need to be passed explicitly when calling, the system will automatically transfer.

The protagonist we introduce today is super(). Super() is very commonly used in class inheritance. It solves some problems of subclasses calling parent class methods. When the parent class is called multiple times, it is only executed once. , optimized the execution logic, let’s take a look in detail below.

Give an example:

class Foo:
  def bar(self, message):
    print(message)

>>> Foo().bar("Hello, Python.")
Hello, Python.

When there is an inheritance relationship, sometimes it is necessary to call the method of the parent class in the subclass. At this time, the simplest way is to convert the object call into a class When calling, it should be noted that the self parameter needs to be passed explicitly at this time, for example:

class FooParent:
  def bar(self, message):
    print(message)
class FooChild(FooParent):
  def bar(self, message):
    FooParent.bar(self, message)

>>> FooChild().bar("Hello, Python.")
Hello, Python.

This has some disadvantages. For example, if the name of the parent class is modified, multiple modifications will be involved in the subclass. In addition, Python is a language that allows multiple inheritance. The method shown above needs to be written multiple times when there is multiple inheritance, which is cumbersome. In order to solve these problems, Python introduced the super() mechanism. The example code is as follows:

class FooParent:
  def bar(self, message):
    print(message)
class FooChild(FooParent):
  def bar(self, message):
    super(FooChild, self).bar(message)

>>> FooChild().bar("Hello, Python.")
Hello, Python.

On the surface, the super(FooChild, self).bar(message) method and the FooParent.bar(self, message) method are The results are consistent. In fact, the internal processing mechanisms of the two methods are very different. When multiple inheritance is involved, there will be obvious differences. Let’s give an example directly:

Code 1:

class A:
  def __init__(self):
    print("Enter A")
    print("Leave A")
class B(A):
  def __init__(self):
    print("Enter B")
    A.__init__(self)
    print("Leave B")
class C(A):
  def __init__(self):
    print("Enter C")
    A.__init__(self)
    print("Leave C")
class D(A):
  def __init__(self):
    print("Enter D")
    A.__init__(self)
    print("Leave D")
class E(B, C, D):
  def __init__(self):
    print("Enter E")
    B.__init__(self)
    C.__init__(self)
    D.__init__(self)
    print("Leave E")
E()

Result:

Enter E

Enter B

Enter A

Leave A

Leave B

Enter C

Enter A

Leave A

Leave C

Enter D

Enter A

Leave A

Leave D

Leave E

The execution sequence is easy to understand. The only thing that needs attention is that the public parent class A is executed multiple times.

Code 2:

class A:
  def __init__(self):
    print("Enter A")
    print("Leave A")
class B(A):
  def __init__(self):
    print("Enter B")
    super(B, self).__init__()
    print("Leave B")
class C(A):
  def __init__(self):
    print("Enter C")
    super(C, self).__init__()
    print("Leave C")
class D(A):
  def __init__(self):
    print("Enter D")
    super(D, self).__init__()
    print("Leave D")
class E(B, C, D):
  def __init__(self):
    print("Enter E")
    super(E, self).__init__()
    print("Leave E")
E()

Result:

Enter E

Enter B

Enter C

Enter D

Enter A

Leave A

Leave D

Leave C

Leave B

Leave E

can be guaranteed in the super mechanism The public parent class is only executed once. As for the order of execution, it is in accordance with MRO (Method Resolution Order): method resolution order. This MRO mechanism will be introduced in detail later.

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