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

All commit authors signed the Contributor License Agreement.

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'.

I've updated the error message to include the self parameter:

self_hint = ". Did you forget the 'self' parameter " "in the function definition?";

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

I've switched it to a char*. Does that suffice?

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.

Can you take another look: https://github.com/python/cpython/pull/152326/changes#diff-f27c08f13ff54ee9a69325f9291a3a2510a05b82ff8d813a8ab2e0072af632bb

I've added coverage for common cases.

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.

I've switched it to checking function name matching the self dictionary. Please take a fresh look at the PR and lmk if it suffices.

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.

ok, I've included your runtime arg count == defined arg count + 1 check. It also does both of your suggested checks on the first parameter. Please review it again

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

Added it in the HEAD

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.

I asked AI on other options. Please review this approach

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.

This was my response to #152326 (comment) . I think I interpreted the happy path incorrectly

Removed it.

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.

added bark_with_self_arg and test_unbound_method_with_self_keeps_missing_argument_error for this.

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.

I don't feel super strongly about this, but I disagree with refcounting immortal objects. I've expressed before that too many users rely on certain things being immortal (such as empty strings), so changing that would be very difficult anyway. We might as well enjoy the performance boost rather than trying to maintain forward compatibility with a seemingly impossible case.

I'm ok with not changing this path though I would prefer to have it initialized to NULL instead in this case and before constructing the message we would put it to something. But I won't be strongly opposed to keep things as is.

I've switched it to char*. Does that suffice?

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.

I've added exemption for those cases.

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.

read-the-docs-community · 2026-07-03T04:09:10Z

Documentation build overview

📚 cpython-previews | 🛠️ Build #33422329 | 📁 Comparing 2b80cb2 against main (0a13efc)

🔍 Preview build

2 files changed

± whatsnew/3.16.html
± whatsnew/changelog.html

…al argumen

…the error message

SurenNihalani · 2026-07-03T05:54:36Z

@picnixz , I've added tests with classmethods, staticmethods and instancemethods with classes/metaclasses.

@sobolevn, I've added tests.

@ZeroIntensity , I've added news.

At a high level, I feel like I've taken on too much complexity in one PR. If people can chime on what feels good enough, that'd be great.

This is ready for next round of review.

SurenNihalani · 2026-07-03T19:34:04Z

I have made the requested changes; please review again

bedevere-app · 2026-07-03T19:34:09Z

Thanks for making the requested changes!

@picnixz: please review the changes made to 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 force-pushed the missing_self branch from b2cb0bd to febd510 Compare June 27, 2026 04:25

sobolevn reviewed Jun 27, 2026

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picnixz requested changes Jun 27, 2026

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

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

SurenNihalani requested a review from AA-Turner as a code owner July 3, 2026 04:04

SurenNihalani added 12 commits July 2, 2026 21:25

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

6fb818b

…al argumen

Respond to comments about giving typeerror without hint and changing …

d43c008

…the error message

Add test for happy path

8cec83f

Missing semicolon

6446c5a

Fix tests

eda21d6

Be simpler

b8bf56e

Change methodology

592f419

Add misc

ef5da0f

Dont give hints when first arg is already named self

715b6fa

pythongh-152315: Refine missing self TypeError hint

5ad3b07

pythongh-152315: Use descriptive TypeError test fixtures

3299fe9

pythongh-152315: Document missing self heuristic

ae7720a

SurenNihalani added 4 commits July 2, 2026 21:25

pythongh-152315: Cover metaclass descriptors

586c65b

pythongh-152315: Rename metaclass test fixture

9b1d491

pythongh-152315: Align missing self docs wording

95039a6

pythongh-152315: Add agent guidance

32fb5e8

SurenNihalani force-pushed the missing_self branch from 0e0555e to 32fb5e8 Compare July 3, 2026 04:26

SurenNihalani added 2 commits July 2, 2026 22:01

pythongh-152315: Remove agent guidance

f4669fa

pythongh-152315: Clarify missing self heuristic

2b80cb2

SurenNihalani force-pushed the missing_self branch from 909c48e to 2b80cb2 Compare July 3, 2026 05:16

SurenNihalani requested review from ZeroIntensity, lul-cas, picnixz and sobolevn July 3, 2026 05:54

bedevere-app Bot added awaiting change review and removed awaiting changes labels Jul 3, 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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