Fix broken append and var_names paths in SU2.io.data

[pragyaangaur]

Proposed Changes

Fixes two defects in SU2_PY/SU2/io/data.py. Both make documented public API paths of SU2.io raise immediately on any input.

1. save_data(..., append=True) raises NameError.

The append branch calls load(...), which does not exist in the module. The function is load_data(). The stale end-of-function markers (#: def load(), #: def save()) suggest the functions were renamed at some point and this internal call site was missed. git log -S dates it to the v2.0.2 import.

Simply calling load_data() here is not a correct fix. filelock is not reentrant (it is an O_CREAT | O_EXCL lock on a .lock sidecar file), and save_data makes this call from inside its own with filelock(file_name) block. The naive change therefore swaps an instant NameError for a 10 second stall followed by FileLockException, which is strictly worse.

Instead the lock-free read body is extracted into a private _read_data() helper. load_data() wraps it in the filelock, and save_data() calls it directly under the lock it already holds. This keeps the read-modify-write atomic, rather than hoisting the read outside the lock and introducing a TOCTOU race.

2. load_data(var_names=...) raises RuntimeError.

The filter deletes keys while iterating input_data.keys(). This was safe on Python 2, where .keys() returned a list copy, and has been broken since the Python 3 migration. It only triggers when at least one key is actually dropped, which is why a single-key call appears to work. Fixed by iterating a copy of the keys.

Both fixes sit in the same pair of functions and share one root cause, namely leftovers from a rename and from the Python 2 to 3 migration, so I have kept them in a single PR. Happy to split them if reviewers prefer.

Also removes a scipy import probe in load_data() that becomes dead once the read moves into the helper, and corrects the two stale marker comments.

Related Work

No open issue.

No in-tree caller currently passes append=True or var_names, so there is no regression risk to the optimization drivers. These are broken paths in a documented public API rather than a live crash in the design loop.

Out of scope, flagged for a possible follow-up: the matlab (.mat) path of save_data fails with TypeError: 'method' object does not support item assignment inside mat_bunch. I verified this behaves identically before and after this change, and did not touch it, in order to keep this PR to one thing.

Could a maintainer please add the changelog:fix label.]

Verification

SU2_PY has no Python test harness (UnitTests/ is C++/Catch2 only), so no test file is added. The following reproduces both failures on develop and passes on this branch:

import sys, os, tempfile
sys.path.insert(0, "SU2_PY")
os.environ.setdefault("SU2_RUN", tempfile.mkdtemp())
from SU2.io.data import save_data, load_data
 
os.chdir(tempfile.mkdtemp())
save_data("z.pkl", {"DRAG": 0.1, "LIFT": 0.9, "MOMENT_Z": 0.02})
save_data("z.pkl", {"CD": 1.0}, append=True)   # was NameError
print(sorted(load_data("z.pkl")))               # ['CD', 'DRAG', 'LIFT', 'MOMENT_Z']
print(load_data("z.pkl", var_names="DRAG"))     # was RuntimeError

Before and after on develop:

Call	Before	After
save_data(append=True)	NameError	OK
load_data(var_names="DRAG")	RuntimeError	OK
load_data(var_names=["DRAG", "LIFT"])	RuntimeError	OK
load_data() plain (regression check)	OK	OK

pre-commit run --files SU2_PY/SU2/io/data.py passes.

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson). (N/A, Python only)
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary. (No Python test harness exists in the repo; a runnable reproduction is included above.)
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary. (N/A, internal bug fix with no user-facing API change.)