gh-152315: Add a suggestion for self by SurenNihalani · Pull Request #152326 · python/cpython

SurenNihalani · 2026-06-26T18:36:12Z

Issue: Beginner friendly error messages for missing self #152315

python-cla-bot · 2026-06-26T18:36:17Z

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lul-cas

My two cents, if they're valid <3

lul-cas · 2026-06-26T19:20:55Z

    }
+    if (suggest_missing_self) {
+        self_hint = PyUnicode_FromString(
+            ". Did you forget to declare 'self' as the first parameter?");


I agree with @edvilme: self is only a naming convention. What matters is that the first parameter receives the instance. Consider something like:

". Did you forget the instance parameter (e.g. self) in the function definition?"

This avoids misleading beginners into adding a parameter named self in the wrong position.

This avoids misleading beginners into adding a parameter named self in the wrong position.

I'm not so sure that "instance parameter" means anything more to a beginner than a "self" parameter. "Self" has the advantage of being a very, very common pattern in Python, and I think that's more easily searchable than "instance parameter". For reference, our docs use "self" hundreds of times, whereas "instance parameter" does not appear once.

I would lean more towards this:

Did you forget the 'self' parameter in the function definition?

"self parameter python" on Google will yield millions of results describing exactly what the user wants, whereas "instance parameter python" returns some unrelated results about class methods vs. instance methods and similar.

I must agree with
'"self parameter python"' on Google will yield millions of results'.

lul-cas · 2026-06-26T19:22:28Z

+    if (suggest_missing_self) {
+        self_hint = PyUnicode_FromString(
+            ". Did you forget to declare 'self' as the first parameter?");
+        if (self_hint == NULL) {


If PyUnicode_FromString fails, the original TypeError is never raised and the user gets no error message. Better to skip the hint and still emit the normal _PyErr_Format.

We should actually propagate the exception in this case, and not suggest anything at all. This is already the case when creating kwonly_sig

lul-cas · 2026-06-26T19:24:20Z

    def positional_only(arg, /):
        pass

+    def missing_self(another_arg):


Good setup for the happy path. If possible, add a def method_with_self(self, x) on the same class A to cover the false positive case.

lul-cas · 2026-06-26T19:25:19Z


    /* Copy all positional arguments into local variables */
    Py_ssize_t j, n;
+    int missing_self_hint = suggest_missing_self(func, co, args, argcount);


The heuristic of matching tp_name against the class segment in qualname is solid. A negative test for a @classmethod missing cls would be useful, it probably shouldnt trigger the hint, and that would document the expected behavior.

lul-cas · 2026-06-26T19:27:35Z

+suggest_missing_self(PyFunctionObject *func, PyCodeObject *co,
+                     _PyStackRef const *args, Py_ssize_t argcount)
+{
+    if (co->co_argcount >= argcount) {


Hmm, this fires whenever argcount > co_argcount and the first argument's type matches the class in qualname. That's broader than the beginner mistake this PR targets.

Ex:

This should hint

def lol(a): ... A().lol(1)

but his should not:

def method(self, x): ... A().method(1, 2, 3)

I'd suggest combining argcount == co_argcount + 1 with a second check, like, the first declared parameter is not named "self", or verifying via _PyType_Lookup that this code object is the function bound on the instance's type.

ZeroIntensity

This needs a news entry and a note in "What's New in Python 3.16".

ZeroIntensity · 2026-06-26T21:53:25Z

+
+    PyObject *self = PyStackRef_AsPyObjectBorrow(args[0]);
+    if (self == NULL) {
+        // When first arg is NULL, its not really about self


Suggested change

// When first arg is NULL, its not really about self

// When first arg is NULL, it's not really about self

ZeroIntensity · 2026-06-26T22:08:08Z

+    Py_ssize_t qualname_len;
+    const char *qualname = PyUnicode_AsUTF8AndSize(
+        func->func_qualname, &qualname_len);
+    if (qualname == NULL) {
+        PyErr_Clear();
+        return 0;
+    }
+
+    const char *method_dot = strrchr(qualname, '.');
+    if (method_dot == NULL) {
+        return 0;
+    }
+
+    const char *class_start = qualname;
+    for (const char *p = qualname; p < method_dot; p++) {
+        if (*p == '.') {
+            class_start = p + 1;
+        }
+    }
+    Py_ssize_t class_len = method_dot - class_start;
+    const char *type_name = Py_TYPE(self)->tp_name;
+
+    return (strlen(type_name) == (size_t)class_len
+            && strncmp(type_name, class_start, (size_t)class_len) == 0);
+}


This feels like a nasty hack. Relying on the __qualname__ feels very icky. I need to look into this more, but it's probably possible to determine whether this is an instance method earlier in the eval loop. That would be much cleaner.

…al argumen

…the error message

sobolevn

(not a full review)

I think the important part of this issue is to find cases where self (as a concept, not as a name) is really missing from the func definition.

So, we can clearly tell apart cases where we just forgot a parameter vs where we forgot to add self to the function definition.

sobolevn · 2026-06-27T07:09:24Z

        self.assertEqual(str(cm.exception), message)

+    def test_happy_path(self):
+        self.assertIs(None, A().method_with_self(1, kwarg=2))


There's no real need for this test. We test method calling in many other test cases in CPython. It does not add any real value.

sobolevn · 2026-06-27T07:12:17Z

+    def missing_self(another_arg):
+        pass
+
+    def missing_self_no_args():


Please, add a test case for def method_with_self_arg(x, self): ...

and then call it like inst.method_with_self().

Also: you can add def zero_args_self(self): ... and call it like: A.zero_args_self() and check that error message is also correct.

picnixz

You should add tests with classmethods, staticmethods (whether on regular classes or metaclasses)

picnixz · 2026-06-27T09:08:58Z

        assert(kwonly_sig != NULL);
    }
+    if (should_suggest_missing_self) {
+        self_hint = PyUnicode_FromString(


Your self_hint is non NULL (before assignment). While empty strings are immortal, you still need to decref it if this ever changes. For that use Py_SETREF.

picnixz · 2026-06-27T09:11:37Z

+    if (suggest_missing_self) {
+        self_hint = PyUnicode_FromString(
+            ". Did you forget to declare 'self' as the first parameter?");
+        if (self_hint == NULL) {


We should actually propagate the exception in this case, and not suggest anything at all. This is already the case when creating kwonly_sig

picnixz · 2026-06-27T09:12:21Z

    }
+    if (should_suggest_missing_self) {
+        self_hint = PyUnicode_FromString(
+            ". Did you forget the 'self' parameter in the function definition?");


Would this suggestion be also there if we are using classmethods? Or what about methodws on metaclasses? they can have cls as the first parameter name.

bedevere-app · 2026-06-27T09:16:15Z

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

SurenNihalani requested a review from markshannon as a code owner June 26, 2026 18:36

bedevere-app Bot added the awaiting review label Jun 26, 2026

bedevere-app Bot mentioned this pull request Jun 26, 2026

Beginner friendly error messages for missing self #152315

Open

SurenNihalani force-pushed the missing_self branch 2 times, most recently from 9ef597f to 5c6f5c1 Compare June 26, 2026 19:03

lul-cas reviewed Jun 26, 2026

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terryjreedy requested review from ZeroIntensity and cfbolz June 26, 2026 20:22

ZeroIntensity reviewed Jun 26, 2026

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SurenNihalani added 8 commits June 26, 2026 21:25

Suggest missing 'self' when a method is called with too many position…

7b9384f

…al argumen

Respond to comments about giving typeerror without hint and changing …

a4c0ffe

…the error message

Add test for happy path

38093f5

Missing semicolon

37e1612

Fix tests

5cb6906

Be simpler

d6ef62e

Change methodology

723d15a

Add misc

febd510

SurenNihalani force-pushed the missing_self branch from b2cb0bd to febd510 Compare June 27, 2026 04:25

sobolevn reviewed Jun 27, 2026

View reviewed changes

picnixz requested changes Jun 27, 2026

View reviewed changes

bedevere-app Bot removed the awaiting review label Jun 27, 2026

bedevere-app Bot added the awaiting changes label Jun 27, 2026

	// When first arg is NULL, its not really about self
	// When first arg is NULL, it's not really about self

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bedevere-app Bot commented Jun 27, 2026

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SurenNihalani commented Jun 26, 2026 •

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