Add a new TestCase that asserts that examples exit successfully.

Examples changes that were necessary:
- loft.py: failed on macos (only) because of (seemingly) over-precise
  floating-point accuracy assertion. Loosened the tolerance, and
  expressed it as a multiple of the expected value.
  > AssertionError: delta=0.002982314711971412 is greater than tolerance=0.001; got=1306.3375467197516, want=1306.3405290344635
- packed_boxes.py: only emit output files when GEN_DOCS is
  True (mimicking lego.py).

examples/loft.py

@@ -41,7 +41,11 @@ with BuildPart() as art:
     top_bottom = art.faces().filter_by(GeomType.PLANE)
     offset(openings=top_bottom, amount=0.5)
 
-assert abs(art.part.volume - 1306.3405290344635) < 1e-3
+want = 1306.3405290344635
+got = art.part.volume
+delta = abs(got - want)
+tolerance = want * 1e-5
+assert delta < tolerance, f"{delta=} is greater than {tolerance=}; {got=}, {want=}"
 
 show(art, names=["art"])
 # [End]

examples/packed_boxes.py

@@ -12,6 +12,8 @@ import operator
 import random
 import build123d as bd
 
+GEN_DOCS = False
+
 random.seed(123456)
 test_boxes = [bd.Box(random.randint(1, 20), random.randint(1, 20), random.randint(1, 5))
               for _ in range(50)]
@@ -28,7 +30,8 @@ def export_svg(parts, name):
     exporter.add_layer("Hidden", line_color=(99, 99, 99), line_type=bd.LineType.ISO_DOT)
     exporter.add_shape(visible, layer="Visible")
     exporter.add_shape(hidden, layer="Hidden")
-    exporter.write(f"../docs/assets/{name}.svg")
+    if GEN_DOCS:
+        exporter.write(f"../docs/assets/{name}.svg")
 
 export_svg(test_boxes, "packed_boxes_input")
 export_svg(packed, "packed_boxes_output")

tests/test_examples.py

@@ -0,0 +1,86 @@
+"""
+build123d Example tests
+
+name: test_examples.py
+by:   fischman
+date: February 21 2025
+
+desc: Unit tests for the build123d examples, ensuring they don't raise.
+"""
+
+from pathlib import Path
+
+import os
+import subprocess
+import sys
+import tempfile
+import unittest
+
+
+_examples_dir = Path(os.path.abspath(os.path.dirname(__file__))).parent / "examples"
+
+_MOCK_OCP_VSCODE_CONTENTS = """
+from pathlib import Path
+
+import re
+import sys
+from unittest.mock import Mock
+mock_module = Mock()
+mock_module.show = Mock()
+mock_module.show_object = Mock()
+mock_module.show_all = Mock()
+sys.modules["ocp_vscode"] = mock_module
+"""
+
+
+def generate_example_test(path: Path):
+    """Generate and return a function to test the example at `path`."""
+    name = path.name
+
+    def assert_example_does_not_raise(self):
+        with tempfile.TemporaryDirectory(
+            prefix=f"build123d_test_examples_{name}"
+        ) as tmpdir:
+            # More examples emit output files than read input files,
+            # so default to running with a temporary directory to
+            # avoid cluttering the git working directory.  For
+            # examples that want to read assets from the examples
+            # directory, use that.  If an example is added in the
+            # future that wants to both read assets from the examples
+            # directory and write output files, deal with it then.
+            cwd = tmpdir if 'benchy' not in path.name else _examples_dir
+            mock_ocp_vscode = Path(tmpdir) / "_mock_ocp_vscode.py"
+            with open(mock_ocp_vscode, "w", encoding="utf-8") as f:
+                f.write(_MOCK_OCP_VSCODE_CONTENTS)
+            got = subprocess.run(
+                [
+                    sys.executable,
+                    "-c",
+                    f"exec(open(r'{mock_ocp_vscode}').read()); exec(open(r'{path}').read())",
+                ],
+                capture_output=True,
+                cwd=cwd,
+                check=False,
+            )
+            self.assertEqual(
+                0, got.returncode, f"stdout/stderr: {got.stdout} / {got.stderr}"
+            )
+
+    return assert_example_does_not_raise
+
+
+class TestExamples(unittest.TestCase):
+    """Tests build123d examples."""
+
+
+for example in sorted(_examples_dir.iterdir()):
+    if example.name.startswith("_") or not example.name.endswith(".py"):
+        continue
+    setattr(
+        TestExamples,
+        f"test_{example.name.replace('.', '_')}",
+        generate_example_test(example),
+    )
+
+if __name__ == "__main__":
+    unittest.main()