Object Destruction: __del__

We’ve seen how the __init__ constructor is called whenever an instance is generated. Its counterpart, the destructor method __del__, is run automatically when an instance’s space is being reclaimed (i.e., at “garbage collection” time):

>>> class Life:
...     def __init__(self, name='unknown'):
...         print('Hello', name)
...         self.name = name
...     def __del__(self):
...         print('Goodbye', self.name)
...
>>> brian = Life('Brian')
Hello Brian
>>> brian = 'loretta'
Goodbye Brian

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Here, when brian is assigned a string, we lose the last reference to the Life instance and so trigger its destructor method. This works, and it may be useful for implementing some cleanup activities (such as terminating server connections). However, destructors are not as commonly used in Python as in some OOP languages, for a number of reasons.

For one thing, because Python automatically reclaims all space held by an instance when the instance is reclaimed, destructors are not necessary for space management.[68] For another, because you cannot always easily predict when an instance will be reclaimed, it’s often better to code termination activities in an explicitly called method (or try/finally statement, described in the next part of the book); in some cases, there may be lingering references to your objects in system tables that prevent destructors from running.

Note

In fact, __del__ can be tricky to use for even more subtle reasons. Exceptions raised within it, for example, simply print a warning message to sys.stderr (the standard error stream) rather than triggering an exception event, because of the unpredictable context under which it is run by the garbage collector. In addition, cyclic (a.k.a. circular) references among objects may prevent garbage collection from happening when you expect it to; an optional cycle detector, enabled by default, can automatically collect such objects eventually, but only if they do not have __del__ methods. Since this is relatively obscure, we’ll ignore further details here; see Python’s standard manuals’ coverage of both __del__ and the gc garbage collector module for more information.

 

[68] In the current C implementation of Python, you also don’t need to close file objects held by the instance in destructors because they are automatically closed when reclaimed. However, as mentioned in Chapter 9, it’s better to explicitly call file close methods because auto-close-on-reclaim is a feature of the implementation, not of the language itself (this behavior can vary under Jython, for instance).