Although we were originally not sympathetic with it, we may need to offer an opt-out mechanism for those users that care about the impact of the overhead of the new data in pyc files and in in-memory code objectsas was suggested by some folks (Thomas, Yury, and others). For this, we could propose that the functionality will be deactivated along with the extra information when Python is executed in optimized mode (``python -O``) and therefore pyo files will not have the overhead associated with the extra required data. Notice that Python already strips docstrings in this mode so it would be "aligned" with the current mechanism of optimized mode.
Although this complicates the implementation, it certainly is still much easier than dealing with compression (and more useful for those that don't want the feature). Notice that we also expect pessimistic results from compression as offsets would be quite random (although predominantly in the range 10 - 120). On Sat, 8 May 2021 at 01:56, Pablo Galindo Salgado <pablog...@gmail.com> wrote: > One last note for clarity: that's the increase of size in the stdlib, the > increase of size > for pyc files goes from 28.471296MB to 34.750464MB, which is an increase > of 22%. > > On Sat, 8 May 2021 at 01:43, Pablo Galindo Salgado <pablog...@gmail.com> > wrote: > >> Some update on the numbers. We have made some draft implementation to >> corroborate the >> numbers with some more realistic tests and seems that our original >> calculations were wrong. >> The actual increase in size is quite bigger than previously advertised: >> >> Using bytes object to encode the final object and marshalling that to >> disk (so using uint8_t) as the underlying >> type: >> >> BEFORE: >> >> ❯ ./python -m compileall -r 1000 Lib > /dev/null >> ❯ du -h Lib -c --max-depth=0 >> 70M Lib >> 70M total >> >> AFTER: >> ❯ ./python -m compileall -r 1000 Lib > /dev/null >> ❯ du -h Lib -c --max-depth=0 >> 76M Lib >> 76M total >> >> So that's an increase of 8.56 % over the original value. This is storing >> the start offset and end offset with no compression >> whatsoever. >> >> On Fri, 7 May 2021 at 22:45, Pablo Galindo Salgado <pablog...@gmail.com> >> wrote: >> >>> Hi there, >>> >>> We are preparing a PEP and we would like to start some early discussion >>> about one of the main aspects of the PEP. >>> >>> The work we are preparing is to allow the interpreter to produce more >>> fine-grained error messages, pointing to >>> the source associated to the instructions that are failing. For example: >>> >>> Traceback (most recent call last): >>> >>> File "test.py", line 14, in <module> >>> >>> lel3(x) >>> >>> ^^^^^^^ >>> >>> File "test.py", line 12, in lel3 >>> >>> return lel2(x) / 23 >>> >>> ^^^^^^^ >>> >>> File "test.py", line 9, in lel2 >>> >>> return 25 + lel(x) + lel(x) >>> >>> ^^^^^^ >>> >>> File "test.py", line 6, in lel >>> >>> return 1 + foo(a,b,c=x['z']['x']['y']['z']['y'], d=e) >>> >>> ^^^^^^^^^^^^^^^^^^^^^ >>> >>> TypeError: 'NoneType' object is not subscriptable >>> >>> The cost of this is having the start column number and end column number >>> information for every bytecode instruction >>> and this is what we want to discuss (there is also some stack cost to >>> re-raise exceptions but that's not a big problem in >>> any case). Given that column numbers are not very big compared with line >>> numbers, we plan to store these as unsigned chars >>> or unsigned shorts. We ran some experiments over the standard library >>> and we found that the overhead of all pyc files is: >>> >>> * If we use shorts, the total overhead is ~3% (total size 28MB and the >>> extra size is 0.88 MB). >>> * If we use chars. the total overhead is ~1.5% (total size 28 MB and the >>> extra size is 0.44MB). >>> >>> One of the disadvantages of using chars is that we can only report >>> columns from 1 to 255 so if an error happens in a column >>> bigger than that then we would have to exclude it (and not show the >>> highlighting) for that frame. Unsigned short will allow >>> the values to go from 0 to 65535. >>> >>> Unfortunately these numbers are not easily compressible, as every >>> instruction would have very different offsets. >>> >>> There is also the possibility of not doing this based on some build flag >>> on when using -O to allow users to opt out, but given the fact >>> that these numbers can be quite useful to other tools like coverage >>> measuring tools, tracers, profilers and the such adding conditional >>> logic to many places would complicate the implementation considerably >>> and will potentially reduce the usability of those tools so we prefer >>> not to have the conditional logic. We believe this is extra cost is very >>> much worth the better error reporting but we understand and respect >>> other points of view. >>> >>> Does anyone see a better way to encode this information **without >>> complicating a lot the implementation**? What are people thoughts on the >>> feature? >>> >>> Thanks in advance, >>> >>> Regards from cloudy London, >>> Pablo Galindo Salgado >>> >>>
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