feat: type_nonparametric (#145)

* feat: type_nonparametric

Signed-off-by: nstarman <nstarman@users.noreply.github.com>

* fix: doctest

Signed-off-by: nstarman <nstarman@users.noreply.github.com>

---------

Signed-off-by: nstarman <nstarman@users.noreply.github.com>

plum/parametric.py

@@ -13,6 +13,7 @@
     "CovariantMeta",
     "parametric",
     "type_parameter",
+    "type_nonparametric",
     "type_unparametrized",
     "kind",
     "Kind",
@@ -281,7 +282,7 @@ def __class_nonparametric__(cls):
         """Return the non-parametric type of an object.
 
         :mod:`plum.parametric` produces parametric subtypes of classes. This
-        method can be used to get the non-parametric type of an object.
+        method can be used to get the original non-parametric type of an object.
 
         See Also
         --------
@@ -468,6 +469,72 @@ def type_parameter(x):
     )
 
 
+def type_nonparametric(q: T) -> Type[T]:
+    """Return the non-parametric type of an object.
+
+    :mod:`plum.parametric` produces parametric subtypes of classes. This method
+    can be used to get the original non-parametric type of an object.
+
+    See Also
+    --------
+    :func:`plum.type_unparametrized`
+        A function that returns the non-concrete, but still parametric, type of
+        an object.
+
+    Examples
+    --------
+    In this example we will demonstrate how to retrieve the original
+    non-parametric class from a :func:`plum.parametric` decorated class.
+
+    :func:`plum.parametric` defines a parametric class of the same name as the
+    original class, and then creates a subclass of the original class with the
+    type parameter inferred from the arguments of the constructor.
+
+    >>> from plum import parametric
+    >>> class Obj:
+    ...     @classmethod
+    ...     def __infer_type_parameter__(cls, *arg):
+    ...         return type(arg[0])
+    ...     def __init__(self, x):
+    ...         self.x = x
+    ...     def __repr__(self):
+    ...         return f"Obj({self.x})"
+    >>> PObj = parametric(Obj)
+    >>> pobj = PObj(1)
+
+    >>> type(pobj).mro()
+    [<class 'plum.parametric.Obj[int]'>, <class 'plum.parametric.Obj'>,
+     <class 'plum.parametric.Obj'>, <class 'object'>]
+
+    Note that the class `Obj` appears twice in the MRO. The first one is the
+    parametric class, and the second one is the non-parametric class. The
+    non-parametric class is the original class that was passed to the
+    ``parametric`` decorator.
+
+    Rather than navigating the MRO, we can get the non-parametric class of an
+    object by calling ``type_nonparametric`` function.
+
+    >>> type(pobj) is PObj[int]
+    True
+    >>> type(pobj) is PObj
+    False
+    >>> type(pobj) is Obj
+    False
+
+    >>> type_nonparametric(pobj) is PObj[int]
+    False
+    >>> type_nonparametric(pobj) is PObj
+    False
+    >>> type_nonparametric(pobj) is Obj
+    True
+    """
+    return (
+        q.__class_nonparametric__()
+        if isinstance(type(q), ParametricTypeMeta)
+        else type(q)
+    )
+
+
 def type_unparametrized(q: T) -> Type[T]:
     """Return the unparametrized type of an object.
 

tests/test_parametric.py

@@ -15,7 +15,13 @@
     parametric,
     type_parameter,
 )
-from plum.parametric import CovariantMeta, is_concrete, is_type, type_unparametrized
+from plum.parametric import (
+    CovariantMeta,
+    is_concrete,
+    is_type,
+    type_nonparametric,
+    type_unparametrized,
+)
 
 
 def test_covariantmeta():
@@ -606,3 +612,27 @@ def __repr__(self):
     assert type(pobj) is Obj[int]
     assert type_unparametrized(pobj) is not Obj[int]
     assert type_unparametrized(pobj) is Obj
+
+
+def test_type_nonparametric():
+    """Test the `type_nonparametric` function."""
+
+    class NonParametricObj:
+        @classmethod
+        def __infer_type_parameter__(cls, *arg):
+            return type(arg[0])
+
+        def __init__(self, x):
+            self.x = x
+
+        def __repr__(self):
+            return f"Obj({self.x})"
+
+    Obj = parametric(NonParametricObj)
+
+    pobj = Obj(1)
+
+    assert type(pobj) is Obj[int]
+    assert type_nonparametric(pobj) is not Obj[int]
+    assert type_nonparametric(pobj) is not Obj
+    assert type_nonparametric(pobj) is NonParametricObj