python-graphblas · eriknw · Aug 3, 2026
diff --git a/graphblas/__init__.py b/graphblas/__init__.py
@@ -25,7 +25,45 @@ def get_config():
     import donfig
     import yaml
 
-    config = donfig.Config("graphblas")
+    class Config(donfig.Config):
+        """donfig Config that rejects silent attribute writes.
+
+        Options are set with ``config.set(name=value)`` (optionally as a
+        ``with config.set(name=value):`` block) and read with ``config[name]``.
+        Plain attribute assignment (``config.name = value``) would otherwise
+        create a dead instance attribute and leave the real option unchanged,
+        so we raise instead.
+        """
+
+        def __init__(self, *args, **kwargs):
+            # Writes are unrestricted until donfig's own __init__ finishes, and
+            # the attributes it set there become the allowlist. Deriving that
+            # list beats hardcoding donfig's internals: a donfig release that
+            # adds or renames a field can't then break attribute access here.
+            object.__setattr__(self, "_initializing", True)
+            super().__init__(*args, **kwargs)
+            object.__setattr__(self, "_donfig_attrs", frozenset(self.__dict__) - {"_initializing"})
+            object.__setattr__(self, "_initializing", False)
+
+        def __setattr__(self, key, value):
+            # An instance built without running __init__ has no allowlist yet,
+            # so stay permissive rather than raising a confusing AttributeError
+            # from the guard.
+            if self.__dict__.get("_initializing", True) or key in self._donfig_attrs:
+                object.__setattr__(self, key, value)
+                return
+            if key in self.config:
+                raise AttributeError(
+                    f"Cannot set config option {key!r} by attribute assignment. "
+                    f"Use `graphblas.config.set({key}=...)` to change it (optionally "
+                    f"in a `with` block for a scoped change) and "
+                    f"`graphblas.config[{key!r}]` to read it."
+                )
+            raise AttributeError(
+                f"Unknown config option {key!r}; known options are {sorted(self.config)}."
+            )
+
+    config = Config("graphblas")
     path = Path(__file__).parent / "graphblas.yaml"
     with path.open() as f:
         defaults = yaml.safe_load(f)

diff --git a/graphblas/tests/test_core.py b/graphblas/tests/test_core.py
@@ -94,3 +94,48 @@ def test_packages():
 
 def test_index_max():
     assert gb.MAX_SIZE == 2**60  # True for all current backends
+
+
+def test_config_attribute_assignment_raises():
+    # Setting an option by plain attribute assignment used to silently no-op:
+    # donfig has no __setattr__ for options, so it created a dead instance
+    # attribute and left the real option alone. It should raise and name the
+    # canonical API instead.
+    orig = gb.config["autocompute"]
+    try:
+        with pytest.raises(AttributeError, match=r"config\.set\(autocompute"):
+            gb.config.autocompute = not orig
+        assert gb.config["autocompute"] == orig  # the failed write changed nothing
+        # An unknown name is a mistaken write too, not a new attribute
+        with pytest.raises(AttributeError, match="not_a_real_option"):
+            gb.config.not_a_real_option = 5
+        assert "not_a_real_option" not in vars(gb.config)
+        # Canonical idioms still work: read by item, write by set()
+        gb.config.set(autocompute=not orig)
+        assert gb.config["autocompute"] == (not orig)
+        with gb.config.set(autocompute=orig):
+            assert gb.config["autocompute"] == orig
+        assert gb.config["autocompute"] == (not orig)  # restored on exit
+    finally:
+        gb.config.set(autocompute=orig)
+
+
+def test_config_donfig_attrs_are_derived(monkeypatch):
+    # The allowlist of writable attributes must come from what donfig's
+    # __init__ actually set, not a hardcoded list. Simulate a future donfig
+    # that grows a field: the derived allowlist absorbs it, where a
+    # hardcoded list would start rejecting donfig's own writes.
+    import donfig
+
+    real_init = donfig.Config.__init__
+
+    def init_with_extra_field(self, *args, **kwargs):
+        real_init(self, *args, **kwargs)
+        self.hypothetical_future_field = 1
+
+    monkeypatch.setattr(donfig.Config, "__init__", init_with_extra_field)
+    probe = type(gb.config)("test_derived_allowlist")
+    assert "hypothetical_future_field" in probe._donfig_attrs
+    probe.hypothetical_future_field = 2  # writable, not rejected
+    with pytest.raises(AttributeError, match="autocompute"):
+        probe.autocompute = False