Hướng dẫn __del__ python

Summary: in this tutorial, you will learn about the Python __del__ special method and understand how it works.

Introduction to the Python __del__ method

In Python, the garbage collector manages memory automatically. The garbage collector will destroy the objects that are not referenced.

If an object implements the __del__ method, Python calls the __del__ method right before the garbage collector destroys the object.

However, the garbage collector determines when to destroy the object. Therefore, it determines when the __del__ method will be called.

The __del__ is sometimes referred to as a class finalizer. Note that __del__ is not the destructor because the garbage collector destroys the object, not the __del__ method.

The Python __del__ pitfalls

Python calls the __del__ method when all object references are gone. And you cannot control it in most cases.

Therefore, you should not use the __del__ method to clean up the resources. It’s recommended to use the context manager.

If the __del__ contains references to objects, the garbage collector will also destroy these objects when the __del__ is called.

If the __del__ references the global objects, it may create unexpected behaviors.

If an exception occurs inside the __del__ method, Python does not raise the exception but keeps it silent.

Also, Python sends the exception message to the stderr. Therefore, the main program will be able to be aware of the exceptions during the finalization.

In practice, you’ll rarely use the __del__ method.

Python __del__ example

The following defines a Person class with the special __del__ method, create a new instance of the Person, and set it to None:

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __del__(self):
        print('__del__ was called')


if __name__ == '__main__':
    person = Person('John Doe', 23)
    person = None
Code language: Python (python)

Output:

__del__ was called
Code language: Python (python)

When we set the person object to None, the garbage collector destroys it because there is no reference. Therefore, the __del__ method was called.

If you use the del keyword to delete the person object, the __del__ method is also called:

person = Person('John Doe', 23)
del person
Code language: Python (python)

Output:

__del__ was called
Code language: Python (python)

However, the del statement doesn’t cause a call to the __del__ method if the object has a reference.

Summary

• Python calls the __del__ method right before the garbage collector destroys the object.
• The garbage collector destroys an object when there is no reference to the object.
• Exception occurs inside the __del__ method is not raised but silent.
• Avoid using __del__ for clean up resources; use the context manager instead.

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