Update while_loop audit test for restored idiom; force matplotlib Agg

- tests/audit/test_object_transform_fixes.py: #830 added M-06 asserting a
  None-returning while_loop body raises, but that broke the canonical brainpy
  idiom (body mutates Variable state, returns None) used by real models like
  SpikeTimeGroup. Now that the wrapper threads operands through unchanged,
  rewrite the test to assert that idiom works (state-driven termination) and
  update the module docstring.
- conftest.py (new, repo root): force matplotlib onto the non-interactive Agg
  backend for both test roots (tests/ and brainpy/) so analysis tests that call
  pyplot.show() never open GUI windows, locally or in CI.
- CI-models.yml: set MPLBACKEND=Agg on the pytest steps to match CI.yml.

Author: chaoming0625

.github/workflows/CI-models.yml

@@ -36,6 +36,8 @@ jobs:
           if [ -f requirements-dev.txt ]; then pip install -r requirements-dev.txt; fi
           pip install -e .
       - name: Test with pytest
+        env:
+          MPLBACKEND: Agg  # Use non-interactive backend for matplotlib
         run: |
           pytest tests/
 
@@ -58,6 +60,8 @@ jobs:
           if [ -f requirements-dev.txt ]; then pip install -r requirements-dev.txt; fi
           pip install -e .
       - name: Test with pytest
+        env:
+          MPLBACKEND: Agg  # Use non-interactive backend for matplotlib
         run: |
           pytest tests/
 
@@ -80,5 +84,7 @@ jobs:
           python -m pip install -r requirements-dev.txt
           pip install -e .
       - name: Test with pytest
+        env:
+          MPLBACKEND: Agg  # Use non-interactive backend for matplotlib
         run: |
           python -m pytest tests/

conftest.py

@@ -0,0 +1,13 @@
+# -*- coding: utf-8 -*-
+"""Pytest configuration shared by both test roots (``tests/`` and ``brainpy/``).
+
+Force matplotlib onto the non-interactive ``Agg`` backend so that tests which
+exercise the analysis/plotting code paths (e.g. phase-plane and bifurcation
+analyses that call ``pyplot.show()``) never try to open a GUI window. This keeps
+the suite headless and non-blocking locally and in CI regardless of the
+``MPLBACKEND`` environment variable.
+"""
+
+import matplotlib
+
+matplotlib.use('Agg', force=True)

tests/audit/test_object_transform_fixes.py

@@ -15,7 +15,8 @@
   accept a ``Variable``/``Array`` in ``operands``), H-03 (``for_loop(jit=False)``
   with a zero-length pytree operand returns ``[]`` instead of crashing),
   M-03 (``scan`` returns ``(carry, ys)``), M-05 (``ifelse`` builds mutually
-  exclusive conditions), M-06 (``while_loop`` body returning ``None`` raises).
+  exclusive conditions), M-06 (``while_loop`` body returning ``None`` threads the
+  operands through unchanged so the canonical state-mutation idiom keeps working).
 * ``function.py``   — ``Partial``/``to_object`` behaviour and L-04 (``function``
   emits a ``DeprecationWarning``).
 * ``_utils.py``     — ``warp_to_no_state_input_output`` strips/restores states.
@@ -352,16 +353,18 @@ def body_f(x, y):
     assert len(res) == 2
 
 
-def test_while_loop_body_returning_none_raises():
-    """M-06: a ``while_loop`` body that returns ``None`` would freeze the carry
-    and loop forever -- it must raise a clear ``ValueError`` instead."""
+def test_while_loop_body_returning_none_threads_operands():
+    """M-06: a ``while_loop`` body that returns ``None`` mutates ``Variable`` state
+    in place (the canonical brainpy idiom, e.g. ``SpikeTimeGroup.update`` with empty
+    ``operands``) and threads the operands through unchanged. brainstate tracks the
+    mutated state, which drives the loop condition, so it must NOT raise."""
+    a = bm.Variable(bm.zeros(1))
 
-    def body(x):
-        # returns None -> illegal
-        pass
+    def body():
+        a.value += 1.
 
-    with pytest.raises(ValueError):
-        bm.while_loop(body, lambda x: x < 3., 0.)
+    bm.while_loop(body, lambda: bm.all(a.value < 3.), ())
+    assert float(np.asarray(a.value[0])) == 3.0
 
 
 def test_ifelse_callable_branches_mutually_exclusive():