Explanation variables only defined if failed or passed with plugins implementing the hook.

src/_pytest/assertion/rewrite.py

@@ -803,10 +803,12 @@ class AssertionRewriter(ast.NodeVisitor):
                     top_condition, module_path=self.module_path, lineno=assert_.lineno
                 )
             )
-        if self.enable_assertion_pass_hook:
-            ### Experimental pytest_assertion_pass hook
+
+        if self.enable_assertion_pass_hook:  # Experimental pytest_assertion_pass hook
             negation = ast.UnaryOp(ast.Not(), top_condition)
             msg = self.pop_format_context(ast.Str(explanation))
+
+            # Failed
             if assert_.msg:
                 assertmsg = self.helper("_format_assertmsg", assert_.msg)
                 gluestr = "\n>assert "
@@ -817,10 +819,14 @@ class AssertionRewriter(ast.NodeVisitor):
             err_msg = ast.BinOp(assertmsg, ast.Add(), err_explanation)
             err_name = ast.Name("AssertionError", ast.Load())
             fmt = self.helper("_format_explanation", err_msg)
-            fmt_pass = self.helper("_format_explanation", msg)
             exc = ast.Call(err_name, [fmt], [])
             raise_ = ast.Raise(exc, None)
-            # Call to hook when passes
+            statements_fail = []
+            statements_fail.extend(self.expl_stmts)
+            statements_fail.append(raise_)
+
+            # Passed
+            fmt_pass = self.helper("_format_explanation", msg)
             orig = astor.to_source(assert_.test).rstrip("\n").lstrip("(").rstrip(")")
             hook_call_pass = ast.Expr(
                 self.helper(
@@ -830,14 +836,16 @@ class AssertionRewriter(ast.NodeVisitor):
                     fmt_pass,
                 )
             )
-
             # If any hooks implement assert_pass hook
             hook_impl_test = ast.If(
                 self.helper("_check_if_assertionpass_impl"), [hook_call_pass], []
             )
-            main_test = ast.If(negation, [raise_], [hook_impl_test])
+            statements_pass = []
+            statements_pass.extend(self.expl_stmts)
+            statements_pass.append(hook_impl_test)
 
-            self.statements.extend(self.expl_stmts)
+            # Test for assertion condition
+            main_test = ast.If(negation, statements_fail, statements_pass)
             self.statements.append(main_test)
             if self.format_variables:
                 variables = [
@@ -845,8 +853,8 @@ class AssertionRewriter(ast.NodeVisitor):
                 ]
                 clear_format = ast.Assign(variables, _NameConstant(None))
                 self.statements.append(clear_format)
-        else:
-            ### Original assertion rewriting
+
+        else:  # Original assertion rewriting
             # Create failure message.
             body = self.expl_stmts
             negation = ast.UnaryOp(ast.Not(), top_condition)
@@ -865,6 +873,7 @@ class AssertionRewriter(ast.NodeVisitor):
             raise_ = ast.Raise(exc, None)
 
             body.append(raise_)
+
         # Clear temporary variables by setting them to None.
         if self.variables:
             variables = [ast.Name(name, ast.Store()) for name in self.variables]