Update del handling in if and while statements.

2 files changed, 105 insertions, 0 deletions

diff --git a/pyflakes/checker.py b/pyflakes/checker.py
index 6d99310..9cbe790 100644
--- a/pyflakes/checker.py
+++ b/pyflakes/checker.py
@@ -531,9 +531,47 @@ class Checker(object):
         self.addBinding(node, binding)
 
     def handleNodeDelete(self, node):
+
+        def on_conditional_branch():
+            """
+            Return `True` if node is part of a conditional branch.
+            """
+            current = getattr(node, 'parent', None)
+            while current:
+                if isinstance(current, ast.If):
+                    return True
+                current = getattr(current, 'parent', None)
+            return False
+
+        def is_part_of_while_conditional():
+            """
+            Return `True` if node is part of left or right side of a while
+            test.
+            """
+            current = getattr(node, 'parent', None)
+            while current:
+                if isinstance(current, ast.While):
+                    for descendant in ast.walk(current.test):
+                        if not isinstance(descendant, ast.Name):
+                            continue
+                        if not node.id == descendant.id:
+                            continue
+                        return True
+                current = getattr(current, 'parent', None)
+            return False
+
         name = getNodeName(node)
         if not name:
             return
+
+        if on_conditional_branch():
+            # We can not predict if this conditional branch is going to
+            # be executed.
+            return
+
+        if is_part_of_while_conditional():
+            return
+
         if isinstance(self.scope, FunctionScope) and name in self.scope.globals:
             self.scope.globals.remove(name)
         else:
diff --git a/pyflakes/test/test_undefined_names.py b/pyflakes/test/test_undefined_names.py
index 4b230cc..4dc0a77 100644
--- a/pyflakes/test/test_undefined_names.py
+++ b/pyflakes/test/test_undefined_names.py
@@ -122,6 +122,73 @@ class Test(TestCase):
         """Del an undefined name."""
         self.flakes('del a', m.UndefinedName)
 
+    def test_delConditional(self):
+        """
+        Ignores conditional bindings deletion.
+        """
+        self.flakes('''
+        context = None
+        test = True
+        if False:
+            del(test)
+        assert(test)
+        ''')
+
+    def test_delConditionalNested(self):
+        """
+        Ignored conditional bindings deletion even if they are nested in other
+        blocks.
+        """
+        self.flakes('''
+        context = None
+        test = True
+        if False:
+            with context():
+                del(test)
+        assert(test)
+        ''')
+
+    def test_delWhile(self):
+        """
+        Ignore bindings deletion if node is part of while test's left side.
+        """
+        self.flakes('''
+        def _worker():
+            o = True
+            while o is not True:
+                del o
+                o = False
+        ''')
+
+    def test_delWhileRightSide(self):
+        """
+        Ignore bindings deletion if node is part of while test's right side.
+        """
+        self.flakes('''
+        def _worker():
+            a = False
+            o = True
+            while a == o:
+                del o
+                o = False
+        ''')
+
+    def test_delWhileNested(self):
+        """
+        Ignore bindings deletions if node is part of while's test, even when
+        del is in a nested block.
+        """
+        self.flakes('''
+        context = None
+        def _worker():
+            o = True
+            while o is not True:
+                while True:
+                    with context():
+                        del o
+                o = False
+        ''')
+
     def test_globalFromNestedScope(self):
         """Global names are available from nested scopes."""
         self.flakes('''