Skip to content

improve exception handling notes around lasti #100467

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Closed
P403n1x87 wants to merge 2 commits into from
Closed

improve exception handling notes around lasti #100467

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
2834cb9
chore: improve exception handling notes around lasti
P403n1x87 Dec 23, 2022
4c6e3ce
Merge branch 'main' into chore/exception-table-lasti
encukou Apr 10, 2024
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
10 changes: 10 additions & 0 deletions Objects/exception_handling_notes.txt
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -92,6 +92,16 @@ If there is an entry, then:
3. push the exception to the stack.
4. jump to the target offset and resume execution.

Currently, the only opcode that requires the lasti flag to be set is RERAISE.
That is, if the jump described by an exception table points to an offset where
a RERAISE (with a non-zero oparg) occurs, the lasti is expected to be on the
stack, and therefore the lasti flag of the exception table entry must be set.
The last example gives an illustration of this. The first table entry describes
a jump where no RERAISE is executed. The second entry describes a jump where a
RERAISE with oparg 1 is executed. This means that the RERAISE will consume the
TOS and assume it is the lasti that was pushed when the exception occurred, as
instructed by the lasti flag.
Comment on lines +95 to +103

@gvanrossum gvanrossum Dec 23, 2022

Copy link
Copy Markdown
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

While it's true that the lasti pushed on the stack is intended to be consumed by RESUME, this description lacks clarity on the Python constructs that generate RERAISE. It would be good to show some examples (maybe an exhaustive list) of constructs that do or don't use RERAISE. E.g.

try:
    xxx
except Exception:
    yyy

does not use RERAISE, but if we replace the last two lines with

finally:
    zzz

then we do. IIUC the other construct that uses RERAISE is the with statement, which after __exit__ returns may use RERAISE.

Maybe @iritkatriel or @markshannon can help review this.



Format of the exception table
-----------------------------
Expand Down