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.
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