Description
Feature or enhancement
Proposal:
Case1: When overriding new in a custom Meta class and forgetting to return a class object (e.g. by omitting return super().new(..)) Python raises the following error:
TypeError: 'NoneType' object is not callable
Case2: When returning something else other than class object it raises following error:
TypeError: 'str' object is not callable
This messages, while technically correct, lacks guidance for common scenarios and can confuse developers - especially those new to Meta class.
Steps to Reproduce:
Case1:
class NewMeta(type):
def new(cls, name, bases, dct):
print ("Forgot to return!")
class MyClass(metaclass = NewMeta):
def greet(self):
pass
m = MyClass()
Output: TypeError: 'NoneType' object is not callable
Case2:
class NewMeta(type):
def new(cls, name, bases, dct):
return ("Return non-callable object")
class MyClass(metaclass = NewMeta):
def greet(self):
pass
m = MyClass()
Output: TypeError: 'str' object is not callable
Suggested Improvement:
Improve the error message to something like:
TypeError: metaclass new must return a class object. Recommendation: super().new(metacls, name, bases, namespace)?
This small change could save significant debugging time and make advanced features like metaclasses more approachable.
Python Version:
Tested in Python 3.12.4 (tags/v3.12.4:8e8a4ba, Jun 6 2024, 19:30:16) [MSC v.1940 64 bit (AMD64)]
Why This Matters:
- Metaclass usage is already an advanced topic.
- Improving the DX (developer experience) here will help advanced learners and reduce friction.
- Similar improvements have already been adopted in recent Python versions for other common errors.
Has this already been discussed elsewhere?
This is a minor feature, which does not need previous discussion elsewhere
Links to previous discussion of this feature:
No response