[j-brendel]

Tests crash with _tkinter.TclError: Can't find a usable tk.tcl

Problem

Running pytest tests can fail with:

_tkinter.TclError: Can't find a usable tk.tcl in the following directories: ...
... ttk/classicTheme.tcl: couldn't read file ... no such file or directory

The traceback points into tkinter's Tk.__init__ (_tkinter.create(...)), triggered from
simba/report.py::generate_plots():

with plt.ion():  # make plotting temporarily interactive, so plt.show does not block
    plt.clf()
    plot(scenario)
    ...

plt.ion() switches matplotlib into interactive mode regardless of args.show_plots (that flag
only guards the later explicit plt.show() call). When matplotlib has no backend configured
explicitly and runs on a machine where Tk is available, it auto-selects an interactive GUI
backend (TkAgg on Windows). Creating the figure then instantiates real Tk/ttk widgets
(e.g. for the toolbar), which is what pulls in ttk::LoadThemes and tries to source the ttk
theme files. The underlying Tcl/Tk installation can be completely intact (confirmed via
python -c "import tkinter; tkinter.Tk()" and by checking the referenced file on disk) — the
real issue is that the test suite ends up creating a real GUI window at all.

This is fragile (depends on the local machine's Tk/ttk setup, antivirus/EDR file scanning, etc.)
and unnecessary: automated tests should not open interactive windows or depend on a display being
available.

Solution

Force matplotlib onto the non-interactive Agg backend for the whole test session, in
tests/conftest.py:

import matplotlib
matplotlib.use("Agg")

from pathlib import Path
...

• conftest.py is always loaded by pytest before any test module, so this is the one place that
  reliably runs before matplotlib.pyplot gets imported anywhere (directly or via
  simba.report). Matplotlib's backend can only be set before the first pyplot import, so
  setting it per test file would be unreliable once any earlier test has already imported
  simba.report.
• Agg still produces the .png/.pdf files via plt.savefig(...), so plot-generation tests
  keep working; only the interactive window is skipped.
• Production/CLI usage of SimBA (generate_plots with args.show_plots = True outside of
  pytest) is unaffected, since conftest.py only applies to the test session.

Implementation (branch fix/plot_tests)

• tests/conftest.py: added matplotlib.use("Agg") before other imports.