__del__ 中的 python 导入错误，如何解决？

Question

class foo:
    def __del__(self):
        import os

p = foo()

This caused an import error

Exception ignored in: <function foo.__del__ at 0x000001DF9986DE50>
Traceback (most recent call last):
  File "C:\Users\chen\PycharmProjects\pythonProject\main.py", line 3, in __del__
ImportError: sys.meta_path is None, Python is likely shutting down

I'm only using this example to demostrate what's wrong, I need to import libraries in del ,is that achievable?

Answer 1

Per the __del__ docs :

Warning - Due to the precarious circumstances under which __del__() methods are invoked, exceptions that occur during their execution are ignored, and a warning is printed to sys.stderr instead. In particular:

• __del__() can be invoked when arbitrary code is being executed, including from any arbitrary thread. If __del__() needs to take a lock or invoke any other blocking resource, it may deadlock as the resource may already be taken by the code that gets interrupted to execute __del__() .
• __del__() can be executed during interpreter shutdown. As a consequence, the global variables it needs to access (including other modules) may already have been deleted or set to None . Python guarantees that globals whose name begins with a single underscore are deleted from their module before other globals are deleted; if no other references to such globals exist, this may help in assuring that imported modules are still available at the time when the __del__() method is called.

Point is, __del__ is not reliable (point #1 means you could have issues with the module import lock, point #2 means every damn thing can break during shutdown), and when the interpreter is shutting down, you can't, and shouldn't, expect to be able to import anything. The solution is to avoid __del__ (which is fairly unreliable as you've seen) in favor of something more deterministic, like implementing the context manager protocol for your class with __enter__ and __exit__ and using with statements, so cleanup occurs at a deterministic point in time (prior to interpreter shutdown).

Alternatively, don't defer your imports, and cache them somewhere they won't get cleared, eg with hacks like:

import os  # Deferred imports are usually a bad idea anyway

class foo:
    def __del__(self, os=os):  # Fake argument with default caches os separate from globals
        # do stuff with os cached in location it definitely won't be preemptively cleared