163 lines
3.7 KiB
Plaintext
163 lines
3.7 KiB
Plaintext
PEP: 542
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Title: Dot Notation Assignment In Function Header
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Version: $Revision$
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Last-Modified: $Date$
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Author: Markus Meskanen <markusmeskanen@gmail.com>
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Status: Rejected
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 10-Feb-2017
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Resolution: https://mail.python.org/pipermail/python-dev/2019-March/156695.html
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Abstract
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========
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Function definitions only allow simple function names to be used,
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even though functions are assignable first class objects.
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This PEP proposes adding support for assigning a function to
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a class or instance attribute directly in the function
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definition's header by using the dot notation to separate
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the object from the function's name.
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Although a similar feature, this PEP does not address general
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assignment to anything that supports assignment, such as dict keys
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and list indexes.
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Rationale
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=========
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Currently if a function needs to be assigned to a class or instance
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attribute, it requires an additional assignment statement to be made::
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class MyClass:
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...
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my_instance = MyClass()
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def my_function(self):
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...
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# Assign to class attribute
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MyClass.my_function = my_function
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# Or assign to instance attribute
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my_instance.my_function = my_function
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While this isn't usually an inconvenience, using dot notation to
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assign directly in the function's header would greatly simplify this::
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class MyClass:
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...
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my_instance = MyClass()
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# Assign to class attribute
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def MyClass.my_function(self):
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...
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# Or assign to instance attribute
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def my_instance.my_function(self):
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...
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There are multiple reasons to use this functionality over
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a standard class method, for example when the class is referenced
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inside the function's header (such as with decorators and typing).
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This is also useful when an instance requires a callback attribute::
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class Menu:
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def __init__(self, items=None, select_callback=None):
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self.items = items if items is not None else []
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self.select_callback = select_callback
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my_menu = Menu([item1, item2])
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def my_menu.select_callback(item_index, menu):
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print(menu.items[item_index])
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As opposed to::
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my_menu = Menu([item1, item2])
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def select_callback(item_index, menu):
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print(menu.items[item_index])
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my_menu.select_callback = select_callback
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Or defining them in an "unnatural" order::
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def select_callback(item_index, menu):
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print(menu.items[item_index])
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my_menu = Menu([item1, item2], select_callback)
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It reads better than the "unnatural" way, since you already know at
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the time of the function definition what it's goig to be used for.
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It also saves one line of code while removing visual complexity.
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The feature would also avoid leaving the function's name into
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the global namespace::
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eggs = 'something'
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def Spam.eggs(self):
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...
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def Cheese.eggs(self):
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...
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assert eggs == 'something'
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Ideally this would be just syntastic sugar::
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def x.y():
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...
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# Equals to
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def y():
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...
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x.y = y
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Similar to how decorators are syntastic sugar::
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@decorate
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def f():
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...
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# Equals to
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def f():
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...
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f = decorate(f)
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Implementation
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==============
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The `__name__` would follow the principles of a normal function::
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class MyClass:
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def my_function1(self):
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...
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def MyClass.my_function2(self):
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...
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assert my_function1.__name__ == 'my_function1'
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assert my_function2.__name__ == 'my_function2'
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The grammar would use `dotted_name` to support chaining of attributes::
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def Person.name.fset(self, value):
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self._name = value
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Backwards Compatibility
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=======================
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This PEP is fully backwards compatible.
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Copyright
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=========
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This document has been placed in the public domain.
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