Failed to inspect `__new__` and `__init_subclass__` methods generated by `warnings.deprecated`

[XuehaiPan]

Bug report

Bug description:

PEP 702 – Marking deprecations using the type system introduces a new API warnings.deprecated for deprecation.

While decorating a class object, it will update the __new__ method:

cpython/Lib/warnings.py

Lines 589 to 603 in 2268289

	original_new = arg.__new__
	@functools.wraps(original_new)
	def __new__(cls, *args, **kwargs):
	if cls is arg:
	warn(msg, category=category, stacklevel=stacklevel + 1)
	if original_new is not object.__new__:
	return original_new(cls, *args, **kwargs)
	# Mirrors a similar check in object.__new__.
	elif cls.__init__ is object.__init__ and (args or kwargs):
	raise TypeError(f"{cls.__name__}() takes no arguments")
	else:
	return original_new(cls)
	arg.__new__ = staticmethod(__new__)

and the __init_subclass__ method:

cpython/Lib/warnings.py

Lines 605 to 625 in 2268289

	original_init_subclass = arg.__init_subclass__
	# We need slightly different behavior if __init_subclass__
	# is a bound method (likely if it was implemented in Python)
	if isinstance(original_init_subclass, MethodType):
	original_init_subclass = original_init_subclass.__func__
	@functools.wraps(original_init_subclass)
	def __init_subclass__(*args, **kwargs):
	warn(msg, category=category, stacklevel=stacklevel + 1)
	return original_init_subclass(*args, **kwargs)
	arg.__init_subclass__ = classmethod(__init_subclass__)
	# Or otherwise, which likely means it's a builtin such as
	# object's implementation of __init_subclass__.
	else:
	@functools.wraps(original_init_subclass)
	def __init_subclass__(*args, **kwargs):
	warn(msg, category=category, stacklevel=stacklevel + 1)
	return original_init_subclass(*args, **kwargs)
	arg.__init_subclass__ = __init_subclass__

For a class (cls) that does not implement the __new__ and __init_subclass__ methods, the warnings.deprecated decorator will generate these two methods (based on object.__new__ and type.__init_subclass__ ) and assign them to cls.__dict__.

However, if users want to inspect the methods in cls.__dict__, the inspect module fails to get the correct definition of cls.__new__. It is defined in warnings.py rather than object.__new__.

# test.py

from warnings import deprecated  # or from typing_extensions import deprecated


class Foo:
    def __new__(cls):
        return super().__new__(cls)


@deprecated("test")
class Bar:
    pass

In [1]: import inspect

In [2]: from test import Foo, Bar

In [3]: inspect.getsourcelines(Foo.__new__)
Out[3]: (['    def __new__(cls):\n', '        return super().__new__(cls)\n'], 7)

In [4]: inspect.getsourcelines(Bar.__new__) 
TypeError: module, class, method, function, traceback, frame, or code object was expected, got builtin_function_or_method

Expected to have inspect.getsourcelines(Bar.__new__) to be source code in /.../lib/python3.13/warnings.py.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-119605: Respect follow_wrapped for __init__ and __new__ when getting class signature with inspect.signature #132055
• [3.13] gh-119605: Respect follow_wrapped for __init__ and __new__ when getting class signature with inspect.signature (GH-132055) #133277

[XuehaiPan]

inspect.signature(DeprecatedClass) is also related.

cpython/Lib/inspect.py

Lines 2574 to 2588 in 2268289

	new = _signature_get_user_defined_method(obj, '__new__')
	init = _signature_get_user_defined_method(obj, '__init__')
	# Go through the MRO and see if any class has user-defined
	# pure Python __new__ or __init__ method
	for base in obj.__mro__:
	# Now we check if the 'obj' class has an own '__new__' method
	if new is not None and '__new__' in base.__dict__:
	sig = _get_signature_of(new)
	if skip_bound_arg:
	sig = _signature_bound_method(sig)
	return sig
	# or an own '__init__' method
	elif init is not None and '__init__' in base.__dict__:
	return _get_signature_of(init)

Reproduce snippet:

# test.py

from warnings import deprecated  # or from typing_extensions import deprecated


class Foo:
    def __init__(self, x, y):
        self.x = x
        self.y = y


@deprecated("test")
class Bar:
    def __init__(self, x, y):
        self.x = x
        self.y = y

In [1]: import inspect

In [2]: from test import Foo, Bar

In [3]: inspect.signature(Foo)
Out[3]: <Signature (x, y)>

In [4]: inspect.signature(Bar)
Out[4]: <Signature (*args, **kwargs)>

[XuehaiPan]

I got a workaround to patch the __text_signature__ attribute of the generated __new__ and __init_subclass__ methods:

+ def update_signature(original_func):
+     # Ensure that the signature of the decorated callable matches the original one
+
+     def wrapper(func):
+         import inspect
+
+         try:
+             original_signature = inspect.signature(original_func)
+         except ValueError:
+             pass
+         else:
+             signature = inspect.signature(func)
+             if signature != original_signature:
+                 try:
+                     func.__text_signature__ = str(original_signature)
+                 except (AttributeError, TypeError):
+                     pass
+
+         return func
+
+     return wrapper

  if isinstance(arg, type):
      import functools
      from types import MethodType

      original_new = arg.__new__

+     @update_signature(original_new)
      @functools.wraps(original_new)
      def __new__(cls, *args, **kwargs):
          if cls is arg:
              warn(msg, category=category, stacklevel=stacklevel + 1)
          if original_new is not object.__new__:
              return original_new(cls, *args, **kwargs)
          # Mirrors a similar check in object.__new__.
          elif cls.__init__ is object.__init__ and (args or kwargs):
              raise TypeError(f"{cls.__name__}() takes no arguments")
          else:
              return original_new(cls)

      arg.__new__ = staticmethod(__new__)