Move assignment of format_variables with enable_assertion_pass_hook

src/_pytest/assertion/rewrite.py

@@ -789,9 +789,6 @@ def visit_Assert(self, assert_):
         self.variables = []  # type: List[str]
         self.variable_counter = itertools.count()
 
-        if self.enable_assertion_pass_hook:
-            self.format_variables = []  # type: List[str]
-
         self.stack = []  # type: List[Dict[str, ast.expr]]
         self.expl_stmts = []  # type: List[ast.stmt]
         self.push_format_context()
@@ -808,6 +805,7 @@ def visit_Assert(self, assert_):
         negation = ast.UnaryOp(ast.Not(), top_condition)
 
         if self.enable_assertion_pass_hook:  # Experimental pytest_assertion_pass hook
+            self.format_variables = []  # type: List[str]
             msg = self.pop_format_context(ast.Str(explanation))
 
             # Failed