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	PEP: 379
	Title: Adding an Assignment Expression
	Version: $Revision$
	Last-Modified: $Date$
	Author: Jervis Whitley <jervisau@gmail.com>
	Status: Withdrawn
	Type: Standards Track
	Content-Type: text/x-rst
	Created: 14-Mar-2009
	Python-Version: 2.7, 3.2
	Post-History:


	Abstract
	========

	This PEP adds a new assignment expression to the Python language
	to make it possible to assign the result of an expression in
	almost any place. The new expression will allow the assignment of
	the result of an expression at first use (in a comparison for
	example).


	Motivation and Summary
	======================

	Issue1714448 "if something as x:" [1]_ describes a feature to allow
	assignment of the result of an expression in an if statement to a
	name. It supposed that the ``as`` syntax could be borrowed for this
	purpose. Many times it is not the expression itself that is
	interesting, rather one of the terms that make up the
	expression. To be clear, something like this::

	if (f_result() == [1, 2, 3]) as res:

	seems awfully limited, when this::

	if (f_result() as res) == [1, 2, 3]:

	is probably the desired result.


	Use Cases
	=========

	See the Examples section near the end.


	Specification
	=============

	A new expression is proposed with the (nominal) syntax::

	EXPR -> VAR

	This single expression does the following:

	- Evaluate the value of ``EXPR``, an arbitrary expression;
	- Assign the result to ``VAR``, a single assignment target; and
	- Leave the result of ``EXPR`` on the Top of Stack (TOS)

	Here ``->`` or (``RARROW``) has been used to illustrate the concept that
	the result of ``EXPR`` is assigned to ``VAR``.

	The translation of the proposed syntax is::

	VAR = (EXPR)
	(EXPR)

	The assignment target can be either an attribute, a subscript or
	name::

	f() -> name[0] # where 'name' exists previously.

	f() -> name.attr # again 'name' exists prior to this expression.

	f() -> name

	This expression should be available anywhere that an expression is
	currently accepted.

	All exceptions that are currently raised during invalid
	assignments will continue to be raised when using the assignment
	expression. For example, a ``NameError`` will be raised when in
	example 1 and 2 above if ``name`` is not previously defined, or an
	``IndexError`` if index 0 was out of range.


	Examples from the Standard Library
	==================================

	The following two examples were chosen after a brief search
	through the standard library, specifically both are from ast.py
	which happened to be open at the time of the search.

	Original::

	def walk(node):
	from collections import deque
	todo = deque([node])
	while todo:
	node = todo.popleft()
	todo.extend(iter_child_nodes(node))
	yield node

	Using assignment expression::

	def walk(node):
	from collections import deque
	todo = deque([node])
	while todo:
	todo.extend(iter_child_nodes(todo.popleft() -> node))
	yield node

	Original::

	def get_docstring(node, clean=True):
	if not isinstance(node, (FunctionDef, ClassDef, Module)):
	raise TypeError("%r can't have docstrings"
	% node.__class__.__name__)
	if node.body and isinstance(node.body[0], Expr) and \
	isinstance(node.body[0].value, Str):
	if clean:
	import inspect
	return inspect.cleandoc(node.body[0].value.s)
	return node.body[0].value.s

	Using assignment expression::

	def get_docstring(node, clean=True):
	if not isinstance(node, (FunctionDef, ClassDef, Module)):
	raise TypeError("%r can't have docstrings"
	% node.__class__.__name__)
	if node.body -> body and isinstance(body[0] -> elem, Expr) and \
	isinstance(elem.value -> value, Str):
	if clean:
	import inspect
	return inspect.cleandoc(value.s)
	return value.s


	Examples
	========

	The examples shown below highlight some of the desirable features
	of the assignment expression, and some of the possible corner
	cases.

	1. Assignment in an if statement for use later::

	def expensive():
	import time; time.sleep(1)
	return 'spam'

	if expensive() -> res in ('spam', 'eggs'):
	dosomething(res)

	2. Assignment in a while loop clause::

	while len(expensive() -> res) == 4:
	dosomething(res)

	3. Keep the iterator object from the for loop::

	for ch in expensive() -> res:
	sell_on_internet(res)

	4. Corner case::

	for ch -> please_dont in expensive():
	pass
	# who would want to do this? Not I.


	References
	==========

	.. [1] Issue1714448 "if something as x:", k0wax
	http://bugs.python.org/issue1714448


	Copyright
	=========

	This document has been placed in the public domain.



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