Fix StopIteration raising for python3.7 (#534)

Because we don't support Python 2 any longer in the master branch, we can
return values from generators to signal that we want to throw a StopIteration,
without actually raising the StopIteration itself.

5 files changed, 44 insertions, 30 deletions

diff --git a/astroid/decorators.py b/astroid/decorators.py
index baec43b9..a0625f2c 100644
--- a/astroid/decorators.py
+++ b/astroid/decorators.py
@@ -99,9 +99,8 @@ def path_wrapper(func):
             # Explicit StopIteration to return error information, see
             # comment in raise_if_nothing_inferred.
             if error.args:
-                raise StopIteration(error.args[0])
-            else:
-                raise StopIteration
+                return error.args[0]
+            return
 
     return wrapped
 
diff --git a/astroid/inference.py b/astroid/inference.py
index a0a933eb..5467736e 100644
--- a/astroid/inference.py
+++ b/astroid/inference.py
@@ -202,7 +202,7 @@ def infer_call(self, context=None):
             continue
     # Explicit StopIteration to return error information, see comment
     # in raise_if_nothing_inferred.
-    raise StopIteration(dict(node=self, context=context))
+    return dict(node=self, context=context)
 nodes.Call._infer = infer_call
 
 
@@ -292,7 +292,7 @@ def infer_attribute(self, context=None):
             context.boundnode = None
     # Explicit StopIteration to return error information, see comment
     # in raise_if_nothing_inferred.
-    raise StopIteration(dict(node=self, context=context))
+    return dict(node=self, context=context)
 nodes.Attribute._infer = decorators.path_wrapper(infer_attribute)
 nodes.AssignAttr.infer_lhs = infer_attribute # # won't work with a path wrapper
 
@@ -324,12 +324,18 @@ def infer_subscript(self, context=None):
     handle each supported index type accordingly.
     """
 
-    value = next(self.value.infer(context))
+    try:
+        value = next(self.value.infer(context))
+    except StopIteration:
+        return
     if value is util.Uninferable:
         yield util.Uninferable
         return
 
-    index = next(self.slice.infer(context))
+    try:
+        index = next(self.slice.infer(context))
+    except StopIteration:
+        return
     if index is util.Uninferable:
         yield util.Uninferable
         return
@@ -367,7 +373,7 @@ def infer_subscript(self, context=None):
 
     # Explicit StopIteration to return error information, see comment
     # in raise_if_nothing_inferred.
-    raise StopIteration(dict(node=self, context=context))
+    return dict(node=self, context=context)
 
 nodes.Subscript._infer = decorators.path_wrapper(infer_subscript)
 nodes.Subscript.infer_lhs = infer_subscript
@@ -425,7 +431,7 @@ def _infer_boolop(self, context=None):
 
     # Explicit StopIteration to return error information, see comment
     # in raise_if_nothing_inferred.
-    raise StopIteration(dict(node=self, context=context))
+    return dict(node=self, context=context)
 
 nodes.BoolOp._infer = _infer_boolop
 
@@ -506,7 +512,7 @@ def infer_unaryop(self, context=None):
         yield inferred
     # Explicit StopIteration to return error information, see comment
     # in raise_if_nothing_inferred.
-    raise StopIteration(dict(node=self, context=context))
+    return dict(node=self, context=context)
 
 nodes.UnaryOp._infer_unaryop = _infer_unaryop
 nodes.UnaryOp._infer = infer_unaryop
@@ -822,7 +828,7 @@ def instance_getitem(self, index, context=None):
     new_context.callcontext = contextmod.CallContext(args=[index])
     new_context.boundnode = self
 
-    method = next(self.igetattr('__getitem__', context=context))
+    method = next(self.igetattr('__getitem__', context=context), None)
     if not isinstance(method, bases.BoundMethod):
         raise exceptions.InferenceError(
             'Could not find __getitem__ for {node!r}.',
diff --git a/astroid/node_classes.py b/astroid/node_classes.py
index c9b70102..4f2f439d 100644
--- a/astroid/node_classes.py
+++ b/astroid/node_classes.py
@@ -46,14 +46,14 @@ def unpack_infer(stmt, context=None):
                 yield inferred_elt
         # Explicit StopIteration to return error information, see comment
         # in raise_if_nothing_inferred.
-        raise StopIteration(dict(node=stmt, context=context))
+        return dict(node=stmt, context=context)
     # if inferred is a final node, return it and stop
     inferred = next(stmt.infer(context))
     if inferred is stmt:
         yield inferred
         # Explicit StopIteration to return error information, see comment
         # in raise_if_nothing_inferred.
-        raise StopIteration(dict(node=stmt, context=context))
+        return dict(node=stmt, context=context)
     # else, infer recursively, except Uninferable object that should be returned as is
     for inferred in stmt.infer(context):
         if inferred is util.Uninferable:
@@ -61,7 +61,7 @@ def unpack_infer(stmt, context=None):
         else:
             for inf_inf in unpack_infer(inferred, context):
                 yield inf_inf
-    raise StopIteration(dict(node=stmt, context=context))
+    return dict(node=stmt, context=context)
 
 
 def are_exclusive(stmt1, stmt2, exceptions=None): # pylint: disable=redefined-outer-name
diff --git a/astroid/protocols.py b/astroid/protocols.py
index bd1d594f..7ae93976 100644
--- a/astroid/protocols.py
+++ b/astroid/protocols.py
@@ -246,8 +246,8 @@ def _resolve_looppart(parts, asspath, context):
 def for_assigned_stmts(self, node=None, context=None, asspath=None):
     if isinstance(self, nodes.AsyncFor) or getattr(self, 'is_async', False):
         # Skip inferring of async code for now
-        raise StopIteration(dict(node=self, unknown=node,
-                                 assign_path=asspath, context=context))
+        return dict(node=self, unknown=node,
+                    assign_path=asspath, context=context)
     if asspath is None:
         for lst in self.iter.infer(context):
             if isinstance(lst, (nodes.Tuple, nodes.List)):
@@ -259,8 +259,8 @@ def for_assigned_stmts(self, node=None, context=None, asspath=None):
             yield inferred
     # Explicit StopIteration to return error information, see comment
     # in raise_if_nothing_inferred.
-    raise StopIteration(dict(node=self, unknown=node,
-                             assign_path=asspath, context=context))
+    return dict(node=self, unknown=node,
+                assign_path=asspath, context=context)
 
 nodes.For.assigned_stmts = for_assigned_stmts
 nodes.Comprehension.assigned_stmts = for_assigned_stmts
@@ -359,8 +359,8 @@ def assign_assigned_stmts(self, node=None, context=None, asspath=None):
         yield inferred
     # Explicit StopIteration to return error information, see comment
     # in raise_if_nothing_inferred.
-    raise StopIteration(dict(node=self, unknown=node,
-                             assign_path=asspath, context=context))
+    return dict(node=self, unknown=node,
+                assign_path=asspath, context=context)
 
 
 def assign_annassigned_stmts(self, node=None, context=None, asspath=None):
@@ -414,8 +414,8 @@ def excepthandler_assigned_stmts(self, node=None, context=None, asspath=None):
         yield assigned
     # Explicit StopIteration to return error information, see comment
     # in raise_if_nothing_inferred.
-    raise StopIteration(dict(node=self, unknown=node,
-                             assign_path=asspath, context=context))
+    return dict(node=self, unknown=node,
+                assign_path=asspath, context=context)
 
 
 nodes.ExceptHandler.assigned_stmts = excepthandler_assigned_stmts
@@ -424,7 +424,7 @@ nodes.ExceptHandler.assigned_stmts = excepthandler_assigned_stmts
 def _infer_context_manager(self, mgr, context):
     try:
         inferred = next(mgr.infer(context=context))
-    except exceptions.InferenceError:
+    except (StopIteration, exceptions.InferenceError):
         return
     if isinstance(inferred, bases.Generator):
         # Check if it is decorated with contextlib.contextmanager.
@@ -432,7 +432,10 @@ def _infer_context_manager(self, mgr, context):
         if not func.decorators:
             return
         for decorator_node in func.decorators.nodes:
-            decorator = next(decorator_node.infer(context))
+            try:
+                decorator = next(decorator_node.infer(context))
+            except StopIteration:
+                return
             if isinstance(decorator, nodes.FunctionDef):
                 if decorator.qname() == _CONTEXTLIB_MGR:
                     break
@@ -446,7 +449,8 @@ def _infer_context_manager(self, mgr, context):
         possible_yield_points = func.nodes_of_class(nodes.Yield)
         # Ignore yields in nested functions
         yield_point = next((node for node in possible_yield_points
-                            if node.scope() == func), None)
+                            if node.scope() == func),
+                           None)
         if yield_point:
             if not yield_point.value:
                 # TODO(cpopa): an empty yield. Should be wrapped to Const.
@@ -460,7 +464,7 @@ def _infer_context_manager(self, mgr, context):
     elif isinstance(inferred, bases.Instance):
         try:
             enter = next(inferred.igetattr('__enter__', context=context))
-        except (exceptions.InferenceError, exceptions.AttributeInferenceError):
+        except (StopIteration, exceptions.InferenceError, exceptions.AttributeInferenceError):
             return
         if not isinstance(enter, bases.BoundMethod):
             return
@@ -496,7 +500,10 @@ def with_assigned_stmts(self, node=None, context=None, asspath=None):
         context: TODO
         asspath: TODO
     """
-    mgr = next(mgr for (mgr, vars) in self.items if vars == node)
+    try:
+        mgr = next(mgr for (mgr, vars) in self.items if vars == node)
+    except StopIteration:
+        return
     if asspath is None:
         for result in _infer_context_manager(self, mgr, context):
             yield result
@@ -525,8 +532,8 @@ def with_assigned_stmts(self, node=None, context=None, asspath=None):
             yield obj
     # Explicit StopIteration to return error information, see comment
     # in raise_if_nothing_inferred.
-    raise StopIteration(dict(node=self, unknown=node,
-                             assign_path=asspath, context=context))
+    return dict(node=self, unknown=node,
+                assign_path=asspath, context=context)
 
 nodes.With.assigned_stmts = with_assigned_stmts
 
@@ -561,6 +568,8 @@ def starred_assigned_stmts(self, node=None, context=None, asspath=None):
             context = contextmod.InferenceContext()
         try:
             rhs = next(value.infer(context))
+        except StopIteration:
+            return
         except exceptions.InferenceError:
             yield util.Uninferable
             return
diff --git a/astroid/scoped_nodes.py b/astroid/scoped_nodes.py
index 31494adc..f4975d5e 100644
--- a/astroid/scoped_nodes.py
+++ b/astroid/scoped_nodes.py
@@ -2542,7 +2542,7 @@ class ClassDef(mixins.FilterStmtsMixin, LocalsDictNodeNG,
             if not values:
                 # Stop the iteration, because the class
                 # has an empty list of slots.
-                raise StopIteration(values)
+                return values
 
             for elt in values:
                 try: