On 06.07.2019 0:28, Pablo Galindo Salgado wrote:
Hi,
Recently, we moved the optimization for the removal of dead code of the form
if 0:
....
to the ast so we use JUMP bytecodes instead (being completed in PR14116). The
reason is that
currently, any syntax error in the block will never be reported. For example:
if 0:
return
if 1:
pass
else:
return
while 0:
return
at module level do not raise any syntax error (just some examples), In
https://bugs.python.org/issue37500 it was reported
that after that, code coverage will decrease as coverage.py sees these new
bytecodes (even if they are not executed). In general,
the code object is a bit bigger and the optimization now it requires an JUMP
instruction to be executed, but syntax errors are reported.
The discussion on issue 37500 is about if we should prioritize the optimization
or the correctness of reporting syntax errors. In my opinion,
SyntaxErrors should be reported with independence of the value of variables
(__debug__) or constant as is a property of the code being written
not of the code being executed. Also, as CPython is the reference implementation of Python, the danger here is that it could be
interpreted that
this optimization is part of the language and its behavior should be mirrored
in every other Python implementation. Elsewhere we have always
prioritize correctness over speed or optimizations.
I am writing this email to know what other people think. Should we revert the
change and not report Syntax Errors on optimized blocks? Someone
sees a viable way of reporting the errors and not emitting the bytecode for
these block?
"Correctness over speed" is Python's core value, so any syntax errors must be
reported.
Is this optimization so important anyway? `if 0:' seems a niche use case (yes, it's in site.py which is in every installation but the gain
there is pretty small)
If it is, why not kill two birds with one stone and discard the subtree, but
_after_ it has passed scrunity?
AFAIK the standard approach to optimization is to construct a raw tree first
and only then apply any equivalence transformations to it.
--
Regards,
Ivan
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