Add support for inferring exception instances in all contexts

We were able to infer exception instances as ``ExceptionInstance``
only for a handful of cases, but not all. ``ExceptionInstance`` has
support for better inference of `.args` and other exception related
attributes that normal instances do not have.
This additional support should remove certain false positives related
to ``.args`` and other exception attributes in ``pylint``.

Close pylint-dev/pylint#2333

ChangeLog

@@ -12,6 +12,17 @@ Release Date: TBA
 
 * Allow inferring positional only arguments.
 
+* Add support for inferring exception instances in all contexts
+
+  We were able to infer exception instances as ``ExceptionInstance``
+  only for a handful of cases, but not all. ``ExceptionInstance`` has
+  support for better inference of `.args` and other exception related
+  attributes that normal instances do not have.
+  This additional support should remove certain false positives related
+  to ``.args`` and other exception attributes in ``pylint``.
+
+  Close PyCQA/pylint#2333
+
 * Add more supported parameters to ``subprocess.check_output``
 
   Close #722

astroid/arguments.py

@@ -209,7 +209,7 @@ def infer_argument(self, funcnode, name, context):
 
                 if funcnode.type == "method":
                     if not isinstance(boundnode, bases.Instance):
-                        boundnode = bases.Instance(boundnode)
+                        boundnode = boundnode.instantiate_class()
                     return iter((boundnode,))
                 if funcnode.type == "classmethod":
                     return iter((boundnode,))

astroid/protocols.py

@@ -328,7 +328,7 @@ def _arguments_infer_argname(self, name, context):
             yield cls
             return
         if functype == "method":
-            yield bases.Instance(cls)
+            yield cls.instantiate_class()
             return
 
     if context and context.callcontext:
@@ -341,7 +341,7 @@ def _arguments_infer_argname(self, name, context):
         vararg.parent = self
         if not self.arguments and self.parent.name == "__init__":
             cls = self.parent.parent.scope()
-            vararg.elts = [bases.Instance(cls)]
+            vararg.elts = [cls.instantiate_class()]
         yield vararg
         return
     if name == self.kwarg:

astroid/scoped_nodes.py

@@ -2133,11 +2133,7 @@ def infer_call_result(self, caller, context=None):
             context = contextmod.bind_context_to_node(context, self)
             yield from dunder_call.infer_call_result(caller, context)
         else:
-            if any(cls.name in EXCEPTION_BASE_CLASSES for cls in self.mro()):
-                # Subclasses of exceptions can be exception instances
-                yield objects.ExceptionInstance(self)
-            else:
-                yield bases.Instance(self)
+            yield self.instantiate_class()
 
     def scope_lookup(self, node, name, offset=0):
         """Lookup where the given name is assigned.
@@ -2347,6 +2343,12 @@ def instantiate_class(self):
             or self if this is not possible.
         :rtype: Instance or ClassDef
         """
+        try:
+            if any(cls.name in EXCEPTION_BASE_CLASSES for cls in self.mro()):
+                # Subclasses of exceptions can be exception instances
+                return objects.ExceptionInstance(self)
+        except exceptions.MroError:
+            pass
         return bases.Instance(self)
 
     def getattr(self, name, context=None, class_context=True):

tests/unittest_inference.py

@@ -5236,14 +5236,24 @@ def __exit__(self, exc_type, exc_value, traceback):
     assert isinstance(next(node.infer()), nodes.Const)
 
 
-def test_subclass_of_exception():
-    code = """
-    class Error(Exception):
-        pass
+@pytest.mark.parametrize(
+    "code",
+    [
+        """
+        class Error(Exception):
+            pass
 
-    a = Error()
-    a
-    """
+        a = Error()
+        a #@
+        """,
+        """
+        class Error(Exception):
+            def method(self):
+                 self #@
+        """,
+    ],
+)
+def test_subclass_of_exception(code):
     inferred = next(extract_node(code).infer())
     assert isinstance(inferred, Instance)
     args = next(inferred.igetattr("args"))