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