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This article focuses on "analyzing the garbage collection mechanism of Python3 objects". Interested friends may wish to have a look at it. The method introduced in this paper is simple, fast and practical. Next, let the editor take you to learn "analyzing the garbage collection mechanism of Python3 objects".

This paper summarizes the automatic memory management mechanism of GC as a modern programming language, focusing on two things: 1. Find useless junk resources in memory 2. Clean up the garbage and give up memory to other objects. In Python, it maintains a counter in each object to record the number of references to that object. Once this counter is 0, the object is immediately recycled, and the memory space occupied by the object is freed.

Reference count

We can use simple variable references and destroy to see the reference counting process.

Increase reference count

There are many ways to increase the reference count, that is, if the object is referenced, then the counter will be + 1.

# create the first reference

A = 3

# reference with other variable names

B = a

# become an object of a container

L = [1, a]

# passed as a parameter

Str (a)

Reduce reference count

Similarly, here are some ways to reduce reference counting.

# A local reference is out of scope. For example, at the end of the `str () `function

Str (a)

# the alias of the object is explicitly destroyed

Del a

# an alias of an object is copied to another object

A = 'Python'

# object removed from a window object

L.remove (a)

# the window object itself is destroyed

Del L

Circular reference problem

What is a circular reference? an and B refer to each other and no longer have external references to either of An and B. although their reference count is 1, they should obviously be recycled.

# A local reference is out of scope. For example, at the end of the `str () `function

Str (a)

# the alias of the object is explicitly destroyed

Del a

# an alias of an object is copied to another object

A = 'Python'

# object removed from a window object

L.remove (a)

# the window object itself is destroyed

Del L

In this example, after the program executes the del statement, the An and B objects do not have any references to the two objects, but the two objects also refer to the object respectively, although the two objects have been del, that is, we can no longer use these two objects, that is, garbage objects, but their reference count has not been reduced to zero. That is, according to the reference counting mechanism, they will not be recycled and will always reside in memory, resulting in memory leaks. In order to solve the problem of circular reference of objects, Python introduces two GC mechanisms, tag-cleanup and generational recycling, to solve the problem of optimization.

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