3 ms·

I always wondered why does this variable leak outside the `with` scope, because this is really the one place where it should not. IIRC python3 fixed scope leak

• shakna 4y ago
  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/ https://peps.python.org/pep-0343/
  • chombier 4y ago
    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.