CodeQL documentation

Overly complex __del__ method

ID: py/overly-complex-delete
Kind: problem
Severity: recommendation
Precision: high
Tags:
   - efficiency
   - maintainability
   - complexity
   - statistical
   - non-attributable
Query suites:
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

The __del__ method exists to release any resources held by an object when that object is deleted. The __del__ is called only by the garbage collector which may call it after an indefinite delay or never.

Consequently, __del__ method should not be relied on to release resources, such as file descriptors. Rather, these resources should be released explicitly.

The existence of a complex __del__ method suggests that this is the main or only way to release resources associated with the object.

Recommendation

In order to ensure correct cleanup of the object add an explicit close(), or similar, method. Possibly make the object a context manager.

The del method should just call close()

Example

The first example below shows a class which relies on __del__ to release resources. The second example shows an improved version of the class where __del__ simply calls close.


#Relies on __del__ being called by the garbage collector.
class CachedPreferencesFile

    ...

    def __del__(self):
        for key, value in self.preferences.items():
            self.write_pair(key, value)
        self.backing.close()


#Better version
class CachedPreferencesFile

    ...

    def close(self):
        for key, value in self.preferences.items():
            self.write_pair(key, value)
        self.backing.close()

    def __del__(self):
        self.close()

References