Treat NewTypes like normal subclasses

NewTypes are assumed not to inherit any members from their base classes.
This results in incorrect inference results. Avoid this by changing the
transformation for NewTypes to treat them like any other subclass.

pylint-dev/pylint#3162
pylint-dev/pylint#2296

Author: colatkinson

ChangeLog

@@ -12,6 +12,11 @@ What's New in astroid 2.9.1?
 ============================
 Release date: TBA
 
+* Treat ``typing.NewType()`` values as normal subclasses.
+
+  Closes PyCQA/pylint#2296
+  Closes PyCQA/pylint#3162
+
 * Prefer the module loader get_source() method in AstroidBuilder's
   module_build() when possible to avoid assumptions about source
   code being available on a filesystem.  Otherwise the source cannot

astroid/brain/brain_typing.py

@@ -38,8 +38,6 @@
 from astroid.util import Uninferable
 
 TYPING_NAMEDTUPLE_BASENAMES = {"NamedTuple", "typing.NamedTuple"}
-TYPING_TYPEVARS = {"TypeVar", "NewType"}
-TYPING_TYPEVARS_QUALIFIED = {"typing.TypeVar", "typing.NewType"}
 TYPING_TYPE_TEMPLATE = """
 class Meta(type):
     def __getitem__(self, item):
@@ -52,6 +50,11 @@ def __args__(self):
 class {0}(metaclass=Meta):
     pass
 """
+# PEP484 suggests NewType is equivalent to this for typing purposes
+TYPING_NEWTYPE_TEMPLATE = """
+class {derived}({base}):
+    pass
+"""
 TYPING_MEMBERS = set(getattr(typing, "__all__", []))
 
 TYPING_ALIAS = frozenset(
@@ -106,23 +109,32 @@ def __class_getitem__(cls, item):
 """
 
 
-def looks_like_typing_typevar_or_newtype(node):
+def looks_like_typing_typevar(node):
+    func = node.func
+    if isinstance(func, Attribute):
+        return func.attrname == "TypeVar"
+    if isinstance(func, Name):
+        return func.name == "TypeVar"
+    return False
+
+
+def looks_like_typing_newtype(node):
     func = node.func
     if isinstance(func, Attribute):
-        return func.attrname in TYPING_TYPEVARS
+        return func.attrname == "NewType"
     if isinstance(func, Name):
-        return func.name in TYPING_TYPEVARS
+        return func.name == "NewType"
     return False
 
 
-def infer_typing_typevar_or_newtype(node, context_itton=None):
+def infer_typing_typevar(node, context_itton=None):
     """Infer a typing.TypeVar(...) or typing.NewType(...) call"""
     try:
         func = next(node.func.infer(context=context_itton))
     except (InferenceError, StopIteration) as exc:
         raise UseInferenceDefault from exc
 
-    if func.qname() not in TYPING_TYPEVARS_QUALIFIED:
+    if func.qname() != "typing.TypeVar":
         raise UseInferenceDefault
     if not node.args:
         raise UseInferenceDefault
@@ -132,6 +144,24 @@ def infer_typing_typevar_or_newtype(node, context_itton=None):
     return node.infer(context=context_itton)
 
 
+def infer_typing_newtype(node, context_itton=None):
+    """Infer a typing.TypeVar(...) or typing.NewType(...) call"""
+    try:
+        func = next(node.func.infer(context=context_itton))
+    except (InferenceError, StopIteration) as exc:
+        raise UseInferenceDefault from exc
+
+    if func.qname() != "typing.NewType":
+        raise UseInferenceDefault
+    if not node.args:
+        raise UseInferenceDefault
+
+    derived = node.args[0].as_string().strip("'")
+    base = node.args[1].as_string().strip("'")
+    node = extract_node(TYPING_NEWTYPE_TEMPLATE.format(derived=derived, base=base))
+    return node.infer(context=context_itton)
+
+
 def _looks_like_typing_subscript(node):
     """Try to figure out if a Subscript node *might* be a typing-related subscript"""
     if isinstance(node, Name):
@@ -409,8 +439,13 @@ def infer_typing_cast(
 
 AstroidManager().register_transform(
     Call,
-    inference_tip(infer_typing_typevar_or_newtype),
-    looks_like_typing_typevar_or_newtype,
+    inference_tip(infer_typing_typevar),
+    looks_like_typing_typevar,
+)
+AstroidManager().register_transform(
+    Call,
+    inference_tip(infer_typing_newtype),
+    looks_like_typing_newtype,
 )
 AstroidManager().register_transform(
     Subscript, inference_tip(infer_typing_attr), _looks_like_typing_subscript

tests/unittest_brain.py

@@ -1659,6 +1659,34 @@ def test_typing_types(self) -> None:
             inferred = next(node.infer())
             self.assertIsInstance(inferred, nodes.ClassDef, node.as_string())
 
+    def test_typing_newtype_attrs(self) -> None:
+        ast_nodes = builder.extract_node(
+            """
+        from typing import List, NewType
+        import typing
+        NewType("Foo", str) #@
+        NewType("Bar", "int") #@
+        """
+        )
+        assert isinstance(ast_nodes, list)
+
+        # Base type given by reference
+        foo_node = ast_nodes[0]
+        foo_inferred = next(foo_node.infer())
+        self.assertIsInstance(foo_inferred, astroid.ClassDef)
+
+        # Check base type method is inferred
+        foo_base_class_method = foo_inferred.getattr("endswith")[0]
+        self.assertIsInstance(foo_base_class_method, astroid.FunctionDef)
+
+        # Base type given by string
+        bar_node = ast_nodes[1]
+        bar_inferred = next(bar_node.infer())
+        self.assertIsInstance(bar_inferred, astroid.ClassDef)
+
+        bar_base_class_method = bar_inferred.getattr("bit_count")[0]
+        self.assertIsInstance(bar_base_class_method, astroid.FunctionDef)
+
     def test_namedtuple_nested_class(self):
         result = builder.extract_node(
             """