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.