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

Copyright

This document has been placed in the public domain.