diff --git a/Doc/howto/clinic.rst b/Doc/howto/clinic.rst index e8e6aace3..286623c24 100644 --- a/Doc/howto/clinic.rst +++ b/Doc/howto/clinic.rst @@ -1898,3 +1898,91 @@ blocks embedded in Python files look slightly different. They look like this: #[python start generated code]*/ def foo(): pass #/*[python checksum:...]*/ + + +.. _clinic-howto-deprecate-positional: + +How to deprecate passing parameters positionally +------------------------------------------------ + +Argument Clinic provides syntax that makes it possible to generate code that +deprecates passing :term:`arguments ` positionally. +For example, say we've got a module-level function :py:func:`!foo.myfunc` +that has three :term:`parameters `: +positional-or-keyword parameters *a* and *b*, and a keyword-only parameter *c*:: + + /*[clinic input] + module foo + myfunc + a: int + b: int + * + c: int + [clinic start generated output]*/ + +We now want to make the *b* parameter keyword-only; +however, we'll have to wait two releases before making this change, +as mandated by Python's backwards-compatibility policy (see :pep:`387`). +For this example, imagine we're in the development phase for Python 3.12: +that means we'll be allowed to introduce deprecation warnings in Python 3.12 +whenever the *b* parameter is passed positionally, +and we'll be allowed to make it keyword-only in Python 3.14 at the earliest. + +We can use Argument Clinic to emit the desired deprecation warnings +using the ``* [from ...]``` syntax, +by adding the line ``* [from 3.14]`` right above the *b* parameter:: + + /*[clinic input] + module foo + myfunc + a: int + * [from 3.14] + b: int + * + c: int + [clinic start generated output]*/ + +Next, regenerate Argument Clinic code (``make clinic``), +and add unit tests for the new behaviour. + +The generated code will now emit a :exc:`DeprecationWarning` +when an :term:`argument` for the :term:`parameter` *b* is passed positionally. +C preprocessor directives are also generated for emitting +compiler warnings if the ``* [from ...]`` line has not been removed +from the Argument Clinic input when the deprecation period is over, +which means when the alpha phase of the specified Python version kicks in. + +Let's return to our example and skip ahead two years: +Python 3.14 development has now entered the alpha phase, +but we forgot all about updating the Argument Clinic code +for :py:func:`!myfunc`! +Luckily for us, compiler warnings are now generated: + +.. code-block:: none + + In file included from Modules/foomodule.c:139: + Modules/clinic/foomodule.c.h:83:8: warning: Update 'b' in 'myfunc' in 'foomodule.c' to be keyword-only. [-W#warnings] + # warning "Update 'b' in 'myfunc' in 'foomodule.c' to be keyword-only." + ^ + +We now close the deprecation phase by making *b* keyword-only; +replace the ``* [from ...]``` line above *b* +with the ``*`` from the line above *c*:: + + /*[clinic input] + module foo + myfunc + a: int + * + b: int + c: int + [clinic start generated output]*/ + +Finally, run ``make clinic`` to regenerate the Argument Clinic code, +and update your unit tests to reflect the new behaviour. + +.. note:: + + If you forget to update your input block during the alpha and beta phases, + the compiler warning will turn into a compiler error when the + release candidate phase begins.