python-peps/peps/pep-0542.rst

PEP: 542
Title: Dot Notation Assignment In Function Header
Version: $Revision$
Last-Modified: $Date$
Author: Markus Meskanen <markusmeskanen@gmail.com>
Status: Rejected
Type: Standards Track
Content-Type: text/x-rst
Created: 10-Feb-2017
Resolution: https://mail.python.org/pipermail/python-dev/2019-March/156695.html


Abstract
========

Function definitions only allow simple function names to be used,
even though functions are assignable first class objects.

This PEP proposes adding support for assigning a function to
a class or instance attribute directly in the function
definition's header by using the dot notation to separate
the object from the function's name.

Although a similar feature, this PEP does not address general
assignment to anything that supports assignment, such as dict keys
and list indexes.

Rationale
=========

Currently if a function needs to be assigned to a class or instance
attribute, it requires an additional assignment statement to be made::

  class MyClass:
      ...

  my_instance = MyClass()

  def my_function(self):
      ...

  # Assign to class attribute
  MyClass.my_function = my_function

  # Or assign to instance attribute
  my_instance.my_function = my_function

While this isn't usually an inconvenience, using dot notation to
assign directly in the function's header would greatly simplify this::

  class MyClass:
      ...

  my_instance = MyClass()

  # Assign to class attribute
  def MyClass.my_function(self):
      ...

  # Or assign to instance attribute
  def my_instance.my_function(self):
      ...

There are multiple reasons to use this functionality over
a standard class method, for example when the class is referenced
inside the function's header (such as with decorators and typing).
This is also useful when an instance requires a callback attribute::

  class Menu:
      def __init__(self, items=None, select_callback=None):
          self.items = items if items is not None else []
          self.select_callback = select_callback

  my_menu = Menu([item1, item2])

  def my_menu.select_callback(item_index, menu):
      print(menu.items[item_index])

As opposed to::

  my_menu = Menu([item1, item2])

  def select_callback(item_index, menu):
      print(menu.items[item_index])
  my_menu.select_callback = select_callback

Or defining them in an "unnatural" order::

  def select_callback(item_index, menu):
      print(menu.items[item_index])

  my_menu = Menu([item1, item2], select_callback)

It reads better than the "unnatural" way, since you already know at
the time of the function definition what it's goig to be used for.
It also saves one line of code while removing visual complexity.

The feature would also avoid leaving the function's name into
the global namespace::

  eggs = 'something'

  def Spam.eggs(self):
      ...

  def Cheese.eggs(self):
      ...

  assert eggs == 'something'

Ideally this would be just syntastic sugar::

  def x.y():
      ...

  # Equals to

  def y():
      ...
  x.y = y

Similar to how decorators are syntastic sugar::

  @decorate
  def f():
      ...

  # Equals to

  def f():
      ...
  f = decorate(f)

Implementation
==============

The ``__name__`` would follow the principles of a normal function::

  class MyClass:
      def my_function1(self):
          ...

  def MyClass.my_function2(self):
      ...

  assert my_function1.__name__ == 'my_function1'
  assert my_function2.__name__ == 'my_function2'

The grammar would use ``dotted_name`` to support chaining of attributes::

  def Person.name.fset(self, value):
      self._name = value

Backwards Compatibility
=======================

This PEP is fully backwards compatible.

Copyright
=========

This document has been placed in the public domain.