Allow attribute assignment for exception instances

astroid/builder.py

@@ -32,6 +32,8 @@
 from astroid import nodes
 from astroid import util
 
+objects = util.lazy_import("objects")
+
 # The name of the transient function that is used to
 # wrap expressions to be extracted when calling
 # extract_node.
@@ -224,14 +226,15 @@ def delayed_assattr(self, node):
                 if inferred is util.Uninferable:
                     continue
                 try:
-                    if inferred.__class__ is bases.Instance:
+                    cls = inferred.__class__
+                    if cls is bases.Instance or cls is objects.ExceptionInstance:
                         inferred = inferred._proxied
                         iattrs = inferred.instance_attrs
                         if not _can_assign_attr(inferred, node.attrname):
                             continue
                     elif isinstance(inferred, bases.Instance):
-                        # Const, Tuple, ... we may be wrong, may be not, but
-                        # anyway we don't want to pollute builtin's namespace
+                        # Const, Tuple or other containers that inherit from
+                        # `Instance`
                         continue
                     elif inferred.is_function:
                         iattrs = inferred.instance_attrs

tests/unittest_inference.py

@@ -42,6 +42,8 @@
 from astroid import objects
 from astroid import test_utils
 from astroid import util
+from astroid.objects import ExceptionInstance
+
 from . import resources
 
 
@@ -5348,5 +5350,25 @@ def __init__(*args, **kwargs):
     assert inferred_data.as_string() == "{1: 2}"
 
 
+def test_infer_exception_instance_attributes():
+    code = """
+    class UnsupportedFormatCharacter(Exception):
+        def __init__(self, index):
+            Exception.__init__(self, index)
+            self.index = index
+
+    try:
+       1/0
+    except UnsupportedFormatCharacter as exc:
+       exc #@
+    """
+    node = extract_node(code)
+    inferred = next(node.infer())
+    assert isinstance(inferred, ExceptionInstance)
+    index = inferred.getattr("index")
+    assert len(index) == 1
+    assert isinstance(index[0], nodes.AssignAttr)
+
+
 if __name__ == "__main__":
     unittest.main()