Include column offsets for bytecode instructions

[pablogsal]

BPO	43950
Nosy	@terryjreedy, @nedbat, @aroberge, @markshannon, @serhiy-storchaka, @ammaraskar, @pablogsal, @miss-islington, @brandtbucher, @isidentical, @sweeneyde, @sobolevn
PRs	bpo-43950: Initial base implementation for PEP 657 #26955 bpo-43950: Print columns in tracebacks (PEP 657) #26958 bpo-43950: optimize column table assembling with pre-sizing the column table #26997 bpo-43950: use 0-indexed column offsets for bytecode positions #27011 bpo-43950: include position in dis.Instruction #27015 bpo-43950: Add option to opt-out of PEP-657 #27023 bpo-43950: Add option to opt-out of PEP-657 #27023 bpo-43950: Add option to opt-out of PEP-657 #27023 bpo-43950: Specialize tracebacks for subscripts/binary ops #27037 bpo-43950: Add documentation for PEP-657 #27047 bpo-43950: implement on-the-fly source tracking for interactive mode #27117 bpo-43950: make BinOp specializations more reliable #27126 bpo-43950: distinguish errors happening on character offset decoding #27217 bpo-43950: ensure source_line is present when specializing the traceback #27313 bpo-43950: support long lines in traceback.py #27336 bpo-43950: check against the raw string, not the pyobject #27337 bpo-43950: support some multi-line expressions for PEP 657 #27339 bpo-43950: support positions for dis.Instructions created through dis.Bytecode #28142 bpo-43950: handle wide unicode characters in tracebacks #28150

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-04-27.10:58:38.225>
labels = []
title = 'Include column offsets for bytecode instructions'
updated_at = <Date 2022-01-18.12:31:06.245>
user = 'https://github.com/pablogsal'

bugs.python.org fields:

activity = <Date 2022-01-18.12:31:06.245>
actor = 'sobolevn'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-04-27.10:58:38.225>
creator = 'pablogsal'
dependencies = []
files = []
hgrepos = []
issue_num = 43950
keywords = ['patch']
message_count = 39.0
messages = ['392054', '392062', '392081', '392082', '392520', '392540', '392541', '392555', '392557', '396866', '396874', '396950', '396957', '396962', '397109', '397352', '397368', '397589', '397666', '397667', '397669', '397673', '397829', '397837', '397844', '397882', '397909', '397910', '397911', '397914', '397915', '398089', '398144', '398169', '398170', '398172', '398197', '400998', '410856']
nosy_count = 12.0
nosy_names = ['terry.reedy', 'nedbat', 'aroberge', 'Mark.Shannon', 'serhiy.storchaka', 'ammar2', 'pablogsal', 'miss-islington', 'brandtbucher', 'BTaskaya', 'Dennis Sweeney', 'sobolevn']
pr_nums = ['26955', '26958', '26997', '27011', '27015', '27023', '27023', '27023', '27037', '27047', '27117', '27126', '27217', '27313', '27336', '27337', '27339', '28142', '28150']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue43950'
versions = []

[pablogsal]

If we could include column offsets from the AST nodes for every bytecode instructions we could leverage these to offer much better tracebacks and a lot more information to debuggers and similar tools. For instance, in an expression such as:

z['aaaaaa']['bbbbbb']['cccccc']['dddddd].sddfsdf.sdfsdf

where one of these elements is None, we could tell exactly what of these pieces is the actual None and we could do some cool highlighting on tracebacks.

Similarly, coverage tools and debuggers could also make this distinction and therefore could offer reports with more granularity.

The cost is not 0: it would be two integers per bytecode instruction, but I think it may be worth the effort.

[pablogsal]

I'm going to prepare a PEP since the discussion regarding if the two integers per bytecode are worth enough is going to be eternal.

[markshannon]

The additional cost will not only be the line number table, but we need to store the line for exceptions that are reraised after cleanup.
Adding a column will mean more stack consumption.

[pablogsal]

Yup, but I still think is worth the cost, giving that debugging improvements are usually extremely popular among users.

[terryjreedy]

Specific examples of current messages and proposed improvements would help focus discussion.

If you are willing to only handle code lines up to 256 chars, only 2 bytes should be needed. (0,0) or (255,255) could mean 'somewhere beyond the 256th char'.

[pablogsal]

Specific examples of current messages and proposed improvements would help focus discussion.

Yeah, I am proposing going from:

>>> x['aaaaaa']['bbbbbb']['cccccc']['dddddd'].sddfsdf.sdfsdf
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not subscriptable

to

>>> x['aaaaaa']['bbbbbb']['cccccc']['dddddd'].sddfsdf.sdfsdf
 ^^^^^^^^^^
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not subscriptable

[pablogsal]

Basically, to highlight in all exceptions the range in the displayed line where the error ocurred. For instance:

>>> foo(a, b/z+2, c, 132432 /x, d /y)
 ^^^^^^^^^
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

[terryjreedy]

Marking what expression evaluated to None would be extremely helpful. So would marking the 0 denominator when there is more than one candidate: "e = a/b + c/d". It should be easy to revise IDLE Shell's print_exception to tag the span. In some cases, the code that goes from a traceback line to a line in a file and marks it could do the same.

What would you do when the expression is not the last line?

try:
x/y
...
except Exception as e:
...
raise e

The except and raise might even be in a separate module.
I look forward to the PEP and discussion.

[pablogsal]

So would marking the 0 denominator when there is more than one candidate: "e = a/b + c/d".

No, it will mark the offset of the bytecode that was getting executed when the exception was raised. Is just a way to mark what is raising the particular exception

What would you do when the expression is not the last line?

There is some logic needed to re-raise exceptions. But basically it boils down to comparate the line numbers to decide if you want to propagate or not the offsets.

[pablogsal]

New changeset 98eee94 by Pablo Galindo in branch 'main':
bpo-43950: Add code.co_positions (PEP-657) (GH-26955)
98eee94

[miss-islington]

New changeset ec8759b by Batuhan Taskaya in branch 'main':
bpo-43950: optimize column table assembling with pre-sizing object (GH-26997)
ec8759b

[miss-islington]

New changeset 44f91fc by Batuhan Taskaya in branch 'main':
bpo-43950: use 0-indexed column offsets for bytecode positions (GH-27011)
44f91fc

[miss-islington]

New changeset 693cec0 by Batuhan Taskaya in branch 'main':
bpo-43950: include position in dis.Instruction (GH-27015)
693cec0

[pablogsal]

New changeset 5644c7b by Ammar Askar in branch 'main':
bpo-43950: Print columns in tracebacks (PEP-657) (GH-26958)
5644c7b

[pablogsal]

New changeset 4823d9a by Ammar Askar in branch 'main':
bpo-43950: Add option to opt-out of PEP-657 (GH-27023)
4823d9a