Nick Coghlan added the comment:
Martin raises an interesting point - the cases where ResourceWarning is likely
to be appropriate are going to be those where the frame has been suspended
inside a with statement or a try/finally statement.
However, we don't currently track those at runtime or at compile time in any
way - they're implicit in the state of the frame stack.
That doesn't mean we *couldn't* track them though, and I think co_lnotab
provides a hint as to how we could do it with minimal runtime overhead: add a
new co_cleanup data table to code objects that provides efficiently packed
storage for a sequence of "setup/cleanup" bytecode offset 2-tuples.
All code objects would gain an additional pointer, and those that used with and
try/finally would get fractionally larger than that, but frame objects would
remain the same size they are today.
Glossing over the complexities of large bytecode offsets (ala co_lnotab),
consider:
>>> def g_with():
... with ...:
... pass
...
>>> dis.dis(g_with)
2 0 LOAD_CONST 1 (Ellipsis)
3 SETUP_WITH 5 (to 11)
6 POP_TOP
3 7 POP_BLOCK
8 LOAD_CONST 0 (None)
>> 11 WITH_CLEANUP_START
12 WITH_CLEANUP_FINISH
13 END_FINALLY
14 LOAD_CONST 0 (None)
17 RETURN_VALUE
Here, co_cleanup would include a single 2-tuple with '0x03' as an absolute
offset (for the setup) and '0x08' as a relative offset (for the cleanup). If
'(f_lasti - 0x03) < 0x08' then __del__ on a generator-iterator or coroutine
would raise ResourceWarning.
>>> def g_finally():
... try:
... pass
... finally:
... pass
...
>>> dis.dis(g_finally)
2 0 SETUP_FINALLY 4 (to 7)
3 3 POP_BLOCK
4 LOAD_CONST 0 (None)
5 >> 7 END_FINALLY
8 LOAD_CONST 0 (None)
11 RETURN_VALUE
Here, co_cleanup would include a single 2-tuple with '0x00' as an absolute
offset (for the setup) and '0x07' as a relative offset (for the cleanup). If
'(f_lasti - 0x00) < 0x07' then __del__ on a generator-iterator or coroutine
would raise ResourceWarning.
The key is that *only* __del__ would get slower (not explicit close() calls),
and it would only get slower in cases where the frame was still suspended *and*
co_cleanup was populated.
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<http://bugs.python.org/issue28629>
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