Introspection

The inspect module will fully support duck-typing when introspecting callables.

The inspect.Signature.from_callable() function computes the signature of a callable. If an object has a __signature__ property, then inspect.Signature.from_callable() simply returns that. To further support duck-typing, if a callable has a __text_signature__ then the __signature__ will be created from that.

This means that 3rd party builtin-functions can implement __text_signature__ if sufficient, and the more expensive __signature__ if necessary.

Efficient calls to third-party callables

Currently the majority of calls are dispatched to functions and method_descriptors in custom code, using the “FastcallKeywords” internal calling convention. This PEP proposes that this calling convention is implemented via a C function pointer. Third-party callables which implement this binary interface will have the potential to be called as fast as a built-in function.

Python visible changes

1. A new built-in class, builtin_function, will be added.
2. types.BuiltinFunctionType will refer to builtin_function not builtin_function_or_method.
3. Instances of the builtin_function class will retain the __module__ property of builtin_function_or_method and gain the func_module and func_globals properties. The func_module allows access to the module to which the function belongs. Note that this is different from the __module__ property which merely returns the name of the module. The func_globals property is equivalent to func_module.__dict__ and is provided to mimic the Python function property of the same name.
4. When binding a method_descriptor instance to an instance of its owning class, a bound_method will be created instead of a builtin_function_or_method. This means that the method_descriptors now mimic the behaviour of Python functions more closely. In other words, [].append becomes a bound_method instead of a builtin_function_or_method.

C API changes

1. A new function PyBuiltinFunction_New(PyMethodDef *ml, PyObject *module) is added to create built-in functions.
2. PyCFunction_NewEx() and PyCFunction_New() are deprecated and will return a PyBuiltinFunction if able, otherwise a builtin_function_or_method.

Internal C changes

Two new flags will be allowed for the typeobject.tp_flags field. These are Py_TPFLAGS_EXTENDED_CALL and Py_TPFLAGS_FUNCTION_DESCRIPTOR

typedef PyObject *(*extended_call_ptr)(PyObject *callable, PyObject** args,
                   int positional_argcount, PyTupleObject* kwnames);

typedef struct {
    PyObject_HEAD
    extended_call_ptr ext_call;
} PyExtendedCallable;

Third-party built-in classes using the new extended call interface

To enable call performance on a par with Python functions and built-in functions, third-party callables should set the Py_TPFLAGS_EXTENDED_CALL bit of tp_flags and ensure that the corresponding C struct starts with the PyExtendedCallable. Any built-in class that has the Py_TPFLAGS_EXTENDED_CALL bit set must also implement the tp_call function and make sure its behaviour is consistent with the ext_call function.