Allow opting out of model nesting

pymc/model/core.py

@@ -25,6 +25,7 @@
     Literal,
     Optional,
     TypeVar,
+    Union,
     cast,
     overload,
 )
@@ -468,6 +469,11 @@ class Model(WithMemoization, metaclass=ContextMeta):
                 # Variable will belong to root and second
                 z = pm.Normal("z", mu=y)  # Variable wil be named "root::second::z"
 
+            # Set None for standalone model
+            with pm.Model(name="third", model=None) as third:
+                # Variable will belong to third only
+                w = pm.Normal("w")  # Variable wil be named "third::w"
+
 
     Set `check_bounds` to False for models with only continuous variables and default transformers
     PyMC will remove the bounds check from the model logp which can speed up sampling
@@ -488,13 +494,13 @@ def __enter__(self: Self) -> Self: ...
 
         def __exit__(self, exc_type: None, exc_val: None, exc_tb: None) -> None: ...
 
-    def __new__(cls, *args, **kwargs):
+    def __new__(cls, *args, model: Union[UNSET, None, "Model"] = UNSET, **kwargs):
         # resolves the parent instance
         instance = super().__new__(cls)
-        if kwargs.get("model") is not None:
-            instance._parent = kwargs.get("model")
-        else:
+        if model is UNSET:
             instance._parent = cls.get_context(error_if_none=False)
+        else:
+            instance._parent = model
         return instance
 
     @staticmethod
@@ -510,9 +516,9 @@ def __init__(
         check_bounds=True,
         *,
         coords_mutable=None,
-        model=None,
+        model: Union[UNSET, None, "Model"] = UNSET,
     ):
-        del model  # used in __new__
+        del model  # used in __new__ to define the parent of this model
         self.name = self._validate_name(name)
         self.check_bounds = check_bounds
 

pymc/model/fgraph.py

@@ -299,9 +299,7 @@ def first_non_model_var(var):
         else:
             return var
 
-    model = Model()
-    if model.parent is not None:
-        raise RuntimeError("model_to_fgraph cannot be called inside a PyMC model context")
+    model = Model(model=None)  # Do not inherit from any model in the context manager
 
     _coords = getattr(fgraph, "_coords", {})
     _dim_lengths = getattr(fgraph, "_dim_lengths", {})

tests/model/test_core.py

@@ -143,13 +143,20 @@ def test_docstring_example(self):
                 # Variable will belong to root and second
                 z = pm.Normal("z", mu=y)  # Variable wil be named "root::second::z"
 
+            # Set None for standalone model
+            with pm.Model(name="third", model=None) as third:
+                # Variable will belong to third only
+                w = pm.Normal("w")  # Variable wil be named "third::w"
+
         assert x.name == "root::x"
         assert y.name == "root::first::y"
         assert z.name == "root::second::z"
+        assert w.name == "third::w"
 
         assert set(root.basic_RVs) == {x, y, z}
         assert set(first.basic_RVs) == {y}
         assert set(second.basic_RVs) == {z}
+        assert set(third.basic_RVs) == {w}
 
 
 class TestNested:
@@ -1106,11 +1113,16 @@ def test_model_parent_set_programmatically():
         y = pm.Normal("y")
 
     with model:
+        # Explict None opts out of model context
+        with pm.Model():
+            z_in = pm.Normal("z_in")
+
         with pm.Model(model=None):
-            z = pm.Normal("z")
+            z_out = pm.Normal("z_out")
 
     assert "y" in model.named_vars
-    assert "z" in model.named_vars
+    assert "z_in" not in model.named_vars
+    assert "z_out" not in model.named_vars
 
 
 class TestModelContext:

tests/model/test_fgraph.py

@@ -267,10 +267,15 @@ def test_context_error():
     with pm.Model() as m:
         x = pm.Normal("x")
 
-        fg = fgraph_from_model(m)
+        fg, _ = fgraph_from_model(m)
 
-        with pytest.raises(RuntimeError, match="cannot be called inside a PyMC model context"):
-            model_from_fgraph(fg)
+        new_m = model_from_fgraph(fg)
+        new_x = new_m["x"]
+
+    assert new_m.parent is None
+    assert x != new_x
+    assert m.named_vars == {"x": x}
+    assert new_m.named_vars == {"x": new_x}
 
 
 def test_sub_model_error():