clear instead of deleting temporary assertion variables

_pytest/assertion/rewrite.py

@@ -374,7 +374,7 @@ class AssertionRewriter(ast.NodeVisitor):
         """Get a new variable."""
         # Use a character invalid in python identifiers to avoid clashing.
         name = "@py_assert" + str(next(self.variable_counter))
-        self.variables[self.cond_chain].add(name)
+        self.variables.append(name)
         return name
 
     def assign(self, expr):
@@ -437,7 +437,7 @@ class AssertionRewriter(ast.NodeVisitor):
             return [assert_]
         self.statements = []
         self.cond_chain = ()
-        self.variables = collections.defaultdict(set)
+        self.variables = []
         self.variable_counter = itertools.count()
         self.stack = []
         self.on_failure = []
@@ -459,22 +459,11 @@ class AssertionRewriter(ast.NodeVisitor):
         else:
             raise_ = ast.Raise(exc, None, None)
         body.append(raise_)
-        # Delete temporary variables. This requires a bit cleverness about the
-        # order, so we don't delete variables that are themselves conditions for
-        # later variables.
-        for chain in sorted(self.variables, key=len, reverse=True):
-            if chain:
-                where = []
-                if len(chain) > 1:
-                    cond = ast.BoolOp(ast.And(), list(chain))
-                else:
-                    cond = chain[0]
-                self.statements.append(ast.If(cond, where, []))
-            else:
-                where = self.statements
-            v = self.variables[chain]
-            names = [ast.Name(name, ast.Del()) for name in v]
-            where.append(ast.Delete(names))
+        # Clear temporary variables by setting them to None.
+        if self.variables:
+            variables = [ast.Name(name, ast.Store()) for name in self.variables]
+            clear = ast.Assign(variables, ast.Name("None", ast.Load()))
+            self.statements.append(clear)
         # Fix line numbers.
         for stmt in self.statements:
             set_location(stmt, assert_.lineno, assert_.col_offset)