On 13/12/2020 22:09, Paul Sokolovsky wrote:
Thanks for hanging with me so far, we're getting to the crux of the
question:
Do you think there can be difference between the following two
expressions:
obj.meth()
(obj.meth)()
?
No. The value of an expression in parentheses is the value of the
expression inside the parentheses, and in this case does not affect the
order of evaluation.
python3.6 -m dis meth_call.py
python3.7 -m dis meth_call.py
Then, to try to explain the difference at the suitable level of
abstraction. If that doesn't provide enough differentiation, it might
be helpful to add the 3rd line:
t = obj.meth; t()
And run all 3 lines thru CPython3.7, and see if the pattern is now
visible, and a distortion in the pattern too.
What would be the explanation for all that?
The explanation is an optimisation introduced in 3.7 that the use of an
intermediate variable prevents. The compiler applies it when it can see
the only use of the attribute is an immediately following call. Having
burrowed into the implementation, I'm certain it tries hard to be
indistinguishable from the unoptimised implementation (and succeeds I
think), even to the point of failing in the same way when that is the
programmed outcome.
LOAD_METHOD goes far enough down the execution of LOAD_ATTR to be sure
the bound object would be a types.MethodType containing a pair of
pointers that CALL_FUNCTION would have to unpack, and pushes the
pointers on the stack instead of creating the new object. Otherwise it
completes LOAD_ATTR and pushes a bound object and a NULL, which is what
CALL_METHOD uses to decide which case it is dealing with.
The meaning of the code is what it does detectably in Python, not what
it compiles to (for some definition of "detectably" that doesn't include
disassembly).
Jeff Allen
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