[petercorke]

PyPlot.step(0) and PyPlot2.step(0) hung with no message: step ends with plt.pause(dt), and Matplotlib's pause(0) does not return, because a timeout of zero means "wait until the event loop is stopped". Found while writing tests for #873.

Both step methods now raise ValueError("dt is 0, it must be greater than zero: ...") for a dt of zero or less in the window render mode, which is the only mode that calls plt.pause; in a notebook there is no pause and dt=0 is fine (the notebook animation tests pass dt=0.0, which the first version of this PR broke on CI). Documented in the docstring. robot.plot(q, dt=0) raises the same way (after the scene is made). Swift is not affected and its step(0) is unchanged; the one example that calls env.step(0) (examples/mexican-wave.py) uses Swift.

Not changed: other plt.pause calls that take a caller's value; they are listed in #876 (PoseGraph.plot_scan(pause=0) and the Vehicle animation).

Changes public behaviour: a zero or negative dt raised nothing and never returned; it now raises ValueError. No change to anything that worked.

Ran: the full test suite (all pass except the 20 known test_servo_half_turn failures from a stale compiled extension in my worktree) and mypy (clean, 165 files); the real-kernel notebook tests (test_notebook_animation.py, test_swift_notebook.py) against this branch's code, by a kernelspec that imports the worktree, since the default kernel imports the editable install. New tests/test_step_dt.py (14 tests, each with a timeout so that a regression fails instead of hanging).

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[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 10 complexity · 0 duplication

Metric	Results
Complexity	10
Duplication	0

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (649b8fb) to head (8c0a85f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/roboticstoolbox/backends/PyPlot/PyPlot.py	0.00%	2 Missing ⚠️
src/roboticstoolbox/backends/PyPlot/PyPlot2.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #875   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        146     146           
  Lines      17096   17100    +4     
=====================================
- Misses     17096   17100    +4     

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