Allow caller control of get_context() errors.

Some code expects `Model.get_context()` to error out if the return is
None.  Some code expects `_DrawValuesHandler.get_context()` to return
None.  To harmonize this, added optional argument to control error
signaling.  Defaults to signaling error, because this seems to be the
general case.

Author: rpgoldman

pymc3/distributions/distribution.py

@@ -456,7 +456,7 @@ class _DrawValuesContext(Context, metaclass=InitContextMeta):
     def __new__(cls, *args, **kwargs):
         # resolves the parent instance
         instance = super().__new__(cls)
-        instance._parent = cls.get_context()
+        instance._parent = cls.get_context(error_if_none=False)
         return instance
 
     def __init__(self):

pymc3/model.py

@@ -192,14 +192,16 @@ def get_contexts(cls: Type[T]) -> List[T]:
         return cls.contexts.stack
 
     @classmethod
-    def get_context(cls: Type[T]) -> Optional[T]:
+    def get_context(cls: Type[T], error_if_none=True) -> Optional[T]:
         """Return the most recently pushed context object of type ``cls``
         on the stack, or ``None``."""
         idx = -1
         while True:
             try:
                 candidate = cls.get_contexts()[idx]
-            except IndexError:
+            except IndexError as e:
+                if error_if_none:
+                    raise e
                 return None
             if isinstance(candidate, cls):
                 return candidate
@@ -210,7 +212,7 @@ def modelcontext(model: Optional['Model']) -> Optional['Model']:
     none supplied.
     """
     if model is None:
-        found: Optional['Model'] = Model.get_context()
+        found: Optional['Model'] = Model.get_context(error_if_none=False)
         if found is None:
             raise ValueError("No pymc3 model object on context stack.")
         return found