Chris Lattner on garbage collection vs. Automatic Reference Counting (2017)

Re:Chris Lattner on garbage collection vs. Automatic Referen... by shakna on Sunday April 24, 2022 @05:15AM (#31142411)

I don't know the _motivation_ for why it leaks, but it leaks because the method calls of a context manager are __enter__ and __exit__, but not __del__. [0]

The motivation is especially opaque, because though CPython does run __exit__ immediately, it isn't guaranteed to do so by the standard. So __exit__ will happen at some point, like __del__ with the GC, but it won't actually call __del__, which makes GC'ing the value more complicated as __del__ can't safely run until __exit__ does.

I _think_ the motivation is something to do with __exit__ being allowed to throw an exception that can be caught, whereas __del__ isn't allowed to do that, but I don't find it that convincing. And I've never seen an example where the introduced variable is ever referenced afterwards in Python's documentation (including in the PEP itself).

[0] https://peps.python.org/pep-0343/

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Re:Chris Lattner on garbage collection vs. Automatic Referen... by chombier on Sunday April 24, 2022 @07:16AM (#31142977)

Thinking about it, my guess is that properly scoping the variable does not really prevent the user from leaking the value anyway:

    with context() as foo: 
        leak = foo

This is unlike list comprehensions, where it is syntactically impossible to bind the comprehension variable to a new name accessible outside the comprehension.

So maybe the reason is that it is not really worth the trouble.

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