Use tb_lineno to point to correct line in traceback

[alexmojaki]

Tracebacks should generally use tb_lineno/lasti instead of f_lineno/lasti. From https://docs.python.org/3/reference/datamodel.html#traceback-objects:

tb_lineno gives the line number where the exception occurred; tb_lasti indicates the precise instruction. The line number and last instruction in the traceback may differ from the line number of its frame object if the exception occurred in a try statement with no matching except clause or with a finally clause.

[alexmojaki]

This is the line where the traceback would point without the change, as it's the last line the frame executed. tb_lineno points to where the exception happened, two lines up.

[samuelcolvin]

Otherwise LGTM.

[alexmojaki]

This is caused by changing to f_code.co_firstlineno, which seems to help pytest include more context from the original markdown file.

[alexmojaki]

I understand this properly now. Previously, setting co_firstlineno to frame.f_lineno + example.start_line was saying that the code started at the problematic line (ignoring the nuance about f_lineno vs tb_lineno) so pytest would show the failing line (so things appeared to work) but it thought that was the start of the code so it didn't show any context before that.

Leaving co_firstlineno unchanged meant it included more context, but too much.

f_code.co_firstlineno + example.start_line works correctly as you'd expect. That's the only way that this expanded test passes, with pytest showing the exact correct context.

[alexmojaki]

#19 doesn't pass this test because the code in line_adjusted_source still starts at the first line, even if it's just padding comments, so the full example file gets included as context for the top level frame.

[sydney-runkle]

LGTM 👍 thanks for the contribution!

[samuelcolvin]

great, thanks so much. We urgently need to fix #20 and #18 and do a new release.