Add node_ancestors method (#1169)

I've been thinking about ways to get rid of unbounded `while` loops in
the Pylint codebase. A common use is to loop over a node's ancestors.
The `node_ancestors` method centralizes this logic in one place so
that those `while` loops can be rewritten as `for`.

A few uses are made of this new method. It only comes up a few places
in Astroid, but there are many more in Pylint.

ChangeLog

@@ -19,6 +19,8 @@ Release date: TBA
 
   Closes PyCQA/pylint#3887
 
+* Add ``node_ancestors`` method to ``NodeNG`` for obtaining the ancestors of nodes.
+
 What's New in astroid 2.7.4?
 ============================
 Release date: TBA

astroid/nodes/node_classes.py

@@ -109,17 +109,14 @@ def are_exclusive(stmt1, stmt2, exceptions: Optional[typing.List[str]] = None) -
     # index stmt1's parents
     stmt1_parents = {}
     children = {}
-    node = stmt1.parent
     previous = stmt1
-    while node:
+    for node in stmt1.node_ancestors():
         stmt1_parents[node] = 1
         children[node] = previous
         previous = node
-        node = node.parent
     # climb among stmt2's parents until we find a common parent
-    node = stmt2.parent
     previous = stmt2
-    while node:
+    for node in stmt2.node_ancestors():
         if node in stmt1_parents:
             # if the common parent is a If or TryExcept statement, look if
             # nodes are in exclusive branches
@@ -162,7 +159,6 @@ def are_exclusive(stmt1, stmt2, exceptions: Optional[typing.List[str]] = None) -
                     return previous is not children[node]
             return False
         previous = node
-        node = node.parent
     return False
 
 
@@ -4719,9 +4715,7 @@ def const_factory(value):
 
 def is_from_decorator(node):
     """Return True if the given node is the child of a decorator"""
-    parent = node.parent
-    while parent is not None:
+    for parent in node.node_ancestors():
         if isinstance(parent, Decorators):
             return True
-        parent = parent.parent
     return False

astroid/nodes/node_ng.py

@@ -208,6 +208,13 @@ def last_child(self) -> Optional["NodeNG"]:
             return attr
         return None
 
+    def node_ancestors(self) -> Iterator["NodeNG"]:
+        """Yield parent, grandparent, etc until there are no more."""
+        parent = self.parent
+        while parent is not None:
+            yield parent
+            parent = parent.parent
+
     def parent_of(self, node):
         """Check if this node is the parent of the given node.
 
@@ -218,11 +225,9 @@ def parent_of(self, node):
             False otherwise.
         :rtype: bool
         """
-        parent = node.parent
-        while parent is not None:
+        for parent in node.node_ancestors():
             if self is parent:
                 return True
-            parent = parent.parent
         return False
 
     def statement(self):