python-peps/pep-0572.rst

PEP: 572
Title: Syntax for Statement-Local Name Bindings
Author: Chris Angelico <rosuav@gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 28-Feb-2018
Python-Version: 3.8
Post-History: 28-Feb-2018


Abstract
========

Programming is all about reusing code rather than duplicating it.  When
an expression needs to be used twice in quick succession but never again,
it is convenient to assign it to a temporary name with small scope.
By permitting name bindings to exist within a single statement only, we
make this both convenient and safe against name collisions.


Rationale
=========

When a subexpression is used multiple times in a list comprehension, there
are currently several ways to spell this, none of which is universally
accepted as ideal. A statement-local name allows any subexpression to be
temporarily captured and then used multiple times.


Syntax and semantics
====================

In any context where arbitrary Python expressions can be used, a named
expression can appear. This must be parenthesized for clarity, and is of
the form ``(expr as NAME)`` where ``expr`` is any valid Python expression,
and ``NAME`` is a simple name.

The value of such a named expression is the same as the incorporated
expression, with the additional side-effect that NAME is bound to that
value for the remainder of the current statement.

Just as function-local names shadow global names for the scope of the
function, statement-local names shadow other names for that statement.
They can also shadow each other, though actually doing this should be
strongly discouraged in style guides.


Example usage
=============

These list comprehensions are all approximately equivalent::

    # Calling the function twice
    stuff = [[f(x), x/f(x)] for x in range(5)]

    # External helper function
    def pair(x, value): return [value, x/value]
    stuff = [pair(x, f(x)) for x in range(5)]

    # Inline helper function
    stuff = [(lambda y: [y,x/y])(f(x)) for x in range(5)]

    # Extra 'for' loop - see also Serhiy's optimization
    stuff = [[y, x/y] for x in range(5) for y in [f(x)]]

    # Iterating over a genexp (similar to the above but reordered)
    stuff = [[y, x/y] for y in (f(x) for x in range(5))]

    # Expanding the comprehension into a loop
    stuff = []
    for x in range(5):
        y = f(x)
        stuff.append([y, x/y])

    # Using a statement-local name
    stuff = [[(f(x) as y), x/y] for x in range(5)]

If calling ``f(x)`` is expensive or has side effects, the clean operation of
the list comprehension gets muddled. Using a short-duration name binding
retains the simplicity; while the extra ``for`` loop does achieve this, it
does so at the cost of dividing the expression visually, putting the named
part at the end of the comprehension instead of the beginning.

Statement-local name bindings can be used in any context, but should be
avoided where regular assignment can be used, just as ``lambda`` should be
avoided when ``def`` is an option.  As the name's scope extends to the full
current statement, even a block statement, this can be used to good effect
in the header of an ``if`` or ``while`` statement::

    while (input("> ") as command) != "quit":
        print("You entered:", command)

    # See, for instance, Lib/pydoc.py
    if (re.search(pat, text) as match):
        print("Found:", match.group(0))

    while (sock.read() as data):
        print("Received data:", data)

Particularly with the ``while`` loop, this can remove the need to have an
infinite loop, an assignment, and a condition. It also creates a smooth
parallel between a loop which simply uses a function call as its condition,
and one which uses that as its condition but also uses the actual value.


Performance costs
=================

The cost of SLNBs must be kept to a minimum, particularly when they are not
used; the normal case MUST NOT be measurably penalized.  SLNBs are expected
to be uncommon, and using many of them in a single function should definitely
be discouraged.  Thus the current implementation uses a linked list of SLNB
cells, with the absence of such a list being the normal case. This list is
used for code compilation only; once a function's bytecode has been baked in,
execution of that bytecode has no performance cost compared to regular
assignment.

Other Python implementations may choose to do things differently, but a zero
run-time cost is strongly recommended, as is a minimal compile-time cost in
the case where no SLNBs are used.


Open questions
==============

1. What happens if the name has already been used? ``(x, (1 as x), x)``
   Currently, prior usage functions as if the named expression did not
   exist (following the usual lookup rules); the new name binding will
   shadow the other name from the point where it is evaluated until the
   end of the statement.  Is this acceptable?  Should it raise a syntax
   error or warning?

2. Syntactic confusion in ``except`` statements.  While technically
   unambiguous, it is potentially confusing to humans.  In Python 3.7,
   parenthesizing ``except (Exception as e):`` is illegal, and there is no
   reason to capture the exception type (as opposed to the exception
   instance, as is done by the regular syntax).  Should this be made
   outright illegal, to prevent confusion?  Can it be left to linters?
   It may also (and independently) be of value to use a subscope for the
   normal except clause binding, such that ``except Exception as e:`` will
   no longer unbind a previous use of the name ``e``.

3. Similar confusion in ``with`` statements, with the difference that there
   is good reason to capture the result of an expression, and it is also
   very common for ``__enter__`` methods to return ``self``.  In many cases,
   ``with expr as name:`` will do the same thing as ``with (expr as name):``,
   adding to the confusion.


Alternative syntax proposals
============================

Proposals of this nature have come up frequently on python-ideas. Below are
a number of alternative syntaxes, some of them specific to comprehensions,
which have been rejected in favour of the one given above.

1. ``where``, ``let``, ``given``::

       stuff = [(y, y) where y = f(x) for x in range(5)]
       stuff = [(y, y) let y = f(x) for x in range(5)]
       stuff = [(y, y) given y = f(x) for x in range(5)]

   This brings the subexpression to a location in between the 'for' loop and
   the expression. It introduces an additional language keyword, which creates
   conflicts. Of the three, ``where`` reads the most cleanly, but also has the
   greatest potential for conflict (eg SQLAlchemy and numpy have ``where``
   methods, as does ``tkinter.dnd.Icon`` in the standard library).

2. ``with``::

       stuff = [(y, y) with y = f(x) for x in range(5)]

   As above, but reusing the `with` keyword. Doesn't read too badly, and needs
   no additional language keyword. Is restricted to comprehensions, though,
   and cannot as easily be transformed into "longhand" for-loop syntax. Has
   the C problem that an equals sign in an expression can now create a name
   binding, rather than performing a comparison.

3. ``with... as``::

       stuff = [(y, y) with f(x) as y for x in range(5)]

   As per option 2, but using ``as`` in place of the equals sign. Aligns
   syntactically with other uses of ``as`` for name binding, but a simple
   transformation to for-loop longhand would create drastically different
   semantics; the meaning of ``with`` inside a comprehension would be
   completely different from the meaning as a stand-alone statement.

4. ``EXPR as NAME`` without parentheses::

       stuff = [[f(x) as y, y] for x in range(5)]

   Omitting the parentheses from this PEP's proposed syntax introduces many
   syntactic ambiguities.


Discrepancies in the current implementation
===========================================

1. SLNBs are implemented using a special (and mostly-invisible) name
   mangling.  This works perfectly inside functions (including list
   comprehensions), but not at top level.  Making this work at top-level
   and in class definitions would be very much of value.

2. The interaction with locals() is currently slightly buggy.  Ideally,
   statement-local names will appear in locals() while they are active (and
   shadow any other names from the same function); as an alternative, they
   could simply never appear, even when in scope.


References
==========

.. [1] Proof of concept / reference implementation
   (https://github.com/Rosuav/cpython/tree/statement-local-variables)


Copyright
=========

This document has been placed in the public domain.



..
   Local Variables:
   mode: indented-text
   indent-tabs-mode: nil
   sentence-end-double-space: t
   fill-column: 70
   coding: utf-8
   End: