[Python-ideas] Fwd: Define a method or function attributeoutsideof a class with the dot operator

Markus Meskanen markusmeskanen at gmail.com
Sun Feb 12 11:51:25 EST 2017

I will say, though, that you're right that we've over-reacted a bit to
the monkeypatching use case. Although maybe that's because no-one can
think of many *other* use cases that they'd need the new syntax for


Hi Paul, I believe at least two other use cases than monkey patching have
been mentioned already:

1. Allowing the class to be used in the method's header, f.e. for typing
and decorators:

  def MyClass.method(self, other: MyClass) -> List[MyClass]:

This is useful since you can't refer the class itself inside of its body.
At the moment the way to use typing is to write the class's name as a
string... It feels awful.

2. To register callbacks to objects, i.e. plain out set an attribute for an
instance. I've used the menu example above:

  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(['Pizza', 'Cake', 'Pasta'])
  def my_menu.select_callback(item_index):
      if item_index == 0:  # Pizza
      else:  # Cake or Pasta

This is just one example of using it to set an instance's variable to a
callback. It's just shorthand for:

  def select_callback(item_index):
  my_menu.select_callback = select_callback

This reads much easier and saves us from typing the same thing three times
(see decorators).
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