BUG: sample StochasticFlight inputs once per simulation (#1090)

[thatrandomasiandev]

Pull request type

• Code changes (bug fix, features)

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (black rocketpy/ tests/ / make lint) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally
• CHANGELOG.md — no action needed; an LLM workflow auto-updates it after merge

Current behavior

MonteCarlo.__run_single_simulation called _randomize_rail_length, _randomize_inclination, and _randomize_heading, each of which invoked next(self.dict_generator()). One simulation therefore drew three times; the Flight mixed values across draws while last_rnd_dict (and .inputs.txt) kept only the last draw. StochasticFlight.create_object had the same pattern for rail_length.

Fixes #1090

New behavior

StochasticFlight._sample_flight_inputs() performs a single draw. create_object and MonteCarlo.__run_single_simulation both use that helper so flown rail/inclination/heading match last_rnd_dict. The _randomize_* helpers remain for single-field callers and still each draw once.

Breaking change

• No

Additional information

Focused unit tests assert create_object and __run_single_simulation Flight inputs equal last_rnd_dict. Full slow suite not run in this contribution pass.

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.53%. Comparing base (e0ff281) to head (f6bc82c).
⚠️ Report is 43 commits behind head on develop.

Files with missing lines	Patch %	Lines
rocketpy/stochastic/stochastic_flight.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1126      +/-   ##
===========================================
+ Coverage    82.18%   82.53%   +0.34%     
===========================================
  Files          122      128       +6     
  Lines        16355    16619     +264     
===========================================
+ Hits         13441    13716     +275     
+ Misses        2914     2903      -11     

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[ting-hong-shieh]

I compared the flight inputs at current develop (cb6106a717207dd8fc2dfe1446d80ff75022f21b) and head 02fcccb39fa58278ca055f22d0fe80183b9bddd8 with the same stochastic seed (4242).

// develop
{
  "flown":  {"heading": 53.55816157603624, "inclination": 81.41474397857291, "rail_length": 5.2},
  "logged": {"heading": 53.55816157603624, "inclination": 83.62213583873555, "rail_length": 5.2}
}

// PR head
{
  "flown":  {"heading": 52.78714695725011, "inclination": 85.16622077057734, "rail_length": 5.2},
  "logged": {"heading": 52.78714695725011, "inclination": 85.16622077057734, "rail_length": 5.2}
}

The absolute sampled values differ between implementations because the head intentionally makes one draw instead of three. The relevant invariant is that the values flown and recorded are identical within the same run; that fails on develop and holds on this head.

pytest tests/unit/stochastic/test_stochastic_flight.py -q → 5 passed. No branch changes were made.

Environment: Python 3.12.6; NumPy 2.5.2; pytest 9.1.1; macOS 26.5.2 arm64.