Now dependent on command line option.

src/_pytest/assertion/__init__.py

@@ -24,6 +24,14 @@ def pytest_addoption(parser):
                             expression information.""",
     )
 
+    group = parser.getgroup("experimental")
+    group.addoption(
+        "--enable-assertion-pass-hook",
+        action="store_true",
+        help="Enables the pytest_assertion_pass hook."
+        "Make sure to delete any previously generated pyc cache files.",
+    )
+
 
 def register_assert_rewrite(*names):
     """Register one or more module names to be rewritten on import.

src/_pytest/assertion/rewrite.py

@@ -745,7 +745,8 @@ class AssertionRewriter(ast.NodeVisitor):
         format_dict = ast.Dict(keys, list(current.values()))
         form = ast.BinOp(expl_expr, ast.Mod(), format_dict)
         name = "@py_format" + str(next(self.variable_counter))
-        self.format_variables.append(name)
+        if getattr(self.config._ns, "enable_assertion_pass_hook", False):
+            self.format_variables.append(name)
         self.expl_stmts.append(ast.Assign([ast.Name(name, ast.Store())], form))
         return ast.Name(name, ast.Load())
 
@@ -780,7 +781,10 @@ class AssertionRewriter(ast.NodeVisitor):
         self.statements = []
         self.variables = []
         self.variable_counter = itertools.count()
-        self.format_variables = []
+
+        if getattr(self.config._ns, "enable_assertion_pass_hook", False):
+            self.format_variables = []
+
         self.stack = []
         self.expl_stmts = []
         self.push_format_context()
@@ -793,41 +797,68 @@ class AssertionRewriter(ast.NodeVisitor):
                     top_condition, module_path=self.module_path, lineno=assert_.lineno
                 )
             )
-        negation = ast.UnaryOp(ast.Not(), top_condition)
-        msg = self.pop_format_context(ast.Str(explanation))
-        if assert_.msg:
-            assertmsg = self.helper("_format_assertmsg", assert_.msg)
-            gluestr = "\n>assert "
-        else:
-            assertmsg = ast.Str("")
-            gluestr = "assert "
-        err_explanation = ast.BinOp(ast.Str(gluestr), ast.Add(), msg)
-        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
-        orig = astor.to_source(assert_.test).rstrip("\n").lstrip("(").rstrip(")")
-        hook_call_pass = ast.Expr(
-            self.helper(
-                "_call_assertion_pass", ast.Num(assert_.lineno), ast.Str(orig), fmt_pass
+        if getattr(self.config._ns, "enable_assertion_pass_hook", False):
+            ### Experimental pytest_assertion_pass hook
+            negation = ast.UnaryOp(ast.Not(), top_condition)
+            msg = self.pop_format_context(ast.Str(explanation))
+            if assert_.msg:
+                assertmsg = self.helper("_format_assertmsg", assert_.msg)
+                gluestr = "\n>assert "
+            else:
+                assertmsg = ast.Str("")
+                gluestr = "assert "
+            err_explanation = ast.BinOp(ast.Str(gluestr), ast.Add(), msg)
+            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
+            orig = astor.to_source(assert_.test).rstrip("\n").lstrip("(").rstrip(")")
+            hook_call_pass = ast.Expr(
+                self.helper(
+                    "_call_assertion_pass",
+                    ast.Num(assert_.lineno),
+                    ast.Str(orig),
+                    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])
+            # 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])
 
-        self.statements.extend(self.expl_stmts)
-        self.statements.append(main_test)
-        if self.format_variables:
-            variables = [ast.Name(name, ast.Store()) for name in self.format_variables]
-            clear_format = ast.Assign(variables, _NameConstant(None))
-            self.statements.append(clear_format)
+            self.statements.extend(self.expl_stmts)
+            self.statements.append(main_test)
+            if self.format_variables:
+                variables = [
+                    ast.Name(name, ast.Store()) for name in self.format_variables
+                ]
+                clear_format = ast.Assign(variables, _NameConstant(None))
+                self.statements.append(clear_format)
+        else:
+            ### Original assertion rewriting
+            # Create failure message.
+            body = self.expl_stmts
+            negation = ast.UnaryOp(ast.Not(), top_condition)
+            self.statements.append(ast.If(negation, body, []))
+            if assert_.msg:
+                assertmsg = self.helper("_format_assertmsg", assert_.msg)
+                explanation = "\n>assert " + explanation
+            else:
+                assertmsg = ast.Str("")
+                explanation = "assert " + explanation
+            template = ast.BinOp(assertmsg, ast.Add(), ast.Str(explanation))
+            msg = self.pop_format_context(template)
+            fmt = self.helper("_format_explanation", msg)
+            err_name = ast.Name("AssertionError", ast.Load())
+            exc = ast.Call(err_name, [fmt], [])
+            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]