`StopIteration` error raised on starred assignment

[dhirschfeld]

Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

Typeguard version

4.5.0

Python version

3.12

What happened?

On the latest 4.5.0 the below code raised a StopIteration error:

typegurad_bug.py

from typeguard import typechecked


@typechecked
def write_table(table_name: str):
    # This line causes StopIteration in typeguard 4.5.0
    *schema, table_name = table_name.rsplit(".", maxsplit=1)
    return schema, table_name


# This will fail:
write_table("schema.table_name")

❯ python typeguard_bug.py
Traceback (most recent call last):
  File "~/code/sandbox/typeguard_bug.py", line 12, in <module>
    write_table("schema.table_name")
  File "~/code/sandbox/typeguard_bug.py", line 7, in write_table
    *schema, table_name = table_name.rsplit(".", maxsplit=1)
  File "~/code/sandbox/.pixi/envs/py312/lib/python3.12/site-packages/typeguard/_functions.py", line 336, in check_variable_assignment
    next_value = next(iterator)
                 ^^^^^^^^^^^^^^
StopIteration

This doesn't happen on 4.4.4.

I've pinned the version for now.

[agronholm]

I worked on that particular code for 3 days not too long ago and apparently missed an edge case. Bummer.

[dhirschfeld]

I worked on that particular code for 3 days not too long ago and apparently missed an edge case. Bummer.

Hopefully it can at least make a good test case 😔

[agronholm]

I found the problem. You know how there are basically three really hard things in computing: cache invalidation and off-by-one errors?

[agronholm]

Oh, never mind that. It looks like I was calculating the cutoff offset from the entirely wrong variable!

[agronholm]

v4.5.1 is out now.