[New-bugs-announce] [issue39823] Disassembly - improve documentation for bytecode instruction class and set source line no. attribute for every instruction

[S Murthy]

New submission from S Murthy <smurthy at pm.me>:

I note that on disassembling a piece of source code (via source strings or code objects) the corresponding sequence of bytecode instruction objects (https://docs.python.org/3/library/dis.html#dis.Instruction) do not always have the `starts_line` attribute set - the storage and display of this line no. seems to be based on whether a given instruction is the first in a block of instructions which implement a given source line.

I think it would be better, for mapping source and logical lines of code to bytecode instruction blocks, to set `starts_line` for every instruction, and amend the bytecode printing method (`dis._disassemble_bytes`) to keep the existing behaviour by detecting whether an instruction is the first line of an instruction block.

ATM `Instruction` objects are created and generated within this loop in `dis._get_bytecode_instructions`:

def _get_instructions_bytes(code, varnames=None, names=None, constants=None,
                      cells=None, linestarts=None, line_offset=0):
    """Iterate over the instructions in a bytecode string.

    Generates a sequence of Instruction namedtuples giving the details of each
    opcode.  Additional information about the code's runtime environment
    (e.g. variable names, constants) can be specified using optional
    arguments.

    """
    labels = findlabels(code)
    starts_line = None
    for offset, op, arg in _unpack_opargs(code):
        if linestarts is not None:
            starts_line = linestarts.get(offset, None)
         ...
         ...

So it's this line

            starts_line = linestarts.get(offset, None)

which currently causes `starts_line` to be to set to `None` for every instruction which isn't the first in an instruction block - linestarts is a dict of source line numbers and offsets of the first instructions starting the corresponding instruction blocks.

My idea is to (1) change that line above to
 
            starts_line = linestarts.get(offset, starts_line)

which ensures every instruction will have the corresponding source line no. set, (2) amend `Instruction._disassemble` to add a new optional argument `print_start_line` with default of `True` to determine whether to print the source line no., and (3) amend `dis._disassemble_bytes` to accept a new optional argument `start_line_by_block` with a default of `True` which can be used to preserve existing behaviour of printing source line numbers by instruction block.

I was wondering whether this sounds OK, if so, I am happy to submit a PR.

----------
components: Library (Lib)
messages: 363140
nosy: smurthy
priority: normal
severity: normal
status: open
title: Disassembly - improve documentation for bytecode instruction class and set source line no. attribute for every instruction
type: enhancement
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39823>
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