Use standard terminology for class pattern error message

sharkdp · sharkdp · commit e4d5f3f5b24e · 2026-02-11T10:58:41.000+01:00
When trying to use a class pattern to match on a non-class object, we currently get the following error message: Traceback (most recent call last): File "/tmp/test.py", line 2, in <module> case len(): ~~~^^ TypeError: called match pattern must be a class The terminology "called match pattern" here is a bit confusing. It's not used anywhere else as far as I can tell, and for a beginner, it might read like something is actually being called here. The documentation consistently refers to these patterns as "class patterns": * https://docs.python.org/3/reference/compound_stmts.html#class-patterns * https://peps.python.org/pep-0635/#class-patterns * https://docs.python.org/3/library/ast.html#ast.MatchClass * https://docs.python.org/3/reference/datamodel.html#customizing-positional-arguments-in-class-pattern-matching I am therefore suggesting to change the message to "class pattern must refer to a class". See python#103576 for a PR that previously changed this message (from "… must be a type" to "… must be a class").
diff --git a/Python/ceval.c b/Python/ceval.c
@@ -531,7 +531,7 @@ _PyEval_MatchClass(PyThreadState *tstate, PyObject *subject, PyObject *type,
                    Py_ssize_t nargs, PyObject *kwargs)
 {
     if (!PyType_Check(type)) {
-        const char *e = "called match pattern must be a class";
+        const char *e = "class pattern must refer to a class";
         _PyErr_Format(tstate, PyExc_TypeError, e);
         return NULL;
     }

Original file line number	Diff line number	Diff line change
`@@ -531,7 +531,7 @@ _PyEval_MatchClass(PyThreadState tstate, PyObject subject, PyObject *type,`
`531`	`531`	`Py_ssize_t nargs, PyObject *kwargs)`
`532`	`532`	`{`
`533`	`533`	`if (!PyType_Check(type)) {`
`534`		`- const char *e = "called match pattern must be a class";`
	`534`	`+ const char *e = "class pattern must refer to a class";`
`535`	`535`	`_PyErr_Format(tstate, PyExc_TypeError, e);`
`536`	`536`	`return NULL;`
`537`	`537`	`}`