python-peps/peps/pep-3125.rst

PEP: 3125
Title: Remove Backslash Continuation
Version: $Revision$
Last-Modified: $Date$
Author: Jim J. Jewett <JimJJewett@gmail.com>
Status: Rejected
Type: Standards Track
Content-Type: text/x-rst
Created: 29-Apr-2007
Post-History: 29-Apr-2007, 30-Apr-2007, 04-May-2007


Rejection Notice
================

This PEP is rejected.  There wasn't enough support in favor, the
feature to be removed isn't all that harmful, and there are some use
cases that would become harder.


Abstract
========

Python initially inherited its parsing from C.  While this has been
generally useful, there are some remnants which have been less useful
for Python, and should be eliminated.

This PEP proposes elimination of terminal ``\`` as a marker for line
continuation.


Motivation
==========

One goal for Python 3000 should be to simplify the language by
removing unnecessary or duplicated features.  There are currently
several ways to indicate that a logical line is continued on the
following physical line.

The other continuation methods are easily explained as a logical
consequence of the semantics they provide; ``\`` is simply an escape
character that needs to be memorized.


Existing Line Continuation Methods
==================================


Parenthetical Expression - ``([{}])``
-------------------------------------

Open a parenthetical expression.  It doesn't matter whether people
view the "line" as continuing; they do immediately recognize that the
expression needs to be closed before the statement can end.

Examples using each of ``()``, ``[]``, and ``{}``::

    def fn(long_argname1,
           long_argname2):
        settings = {"background": "random noise",
                    "volume": "barely audible"}
        restrictions = ["Warrantee void if used",
                        "Notice must be received by yesterday",
                        "Not responsible for sales pitch"]

Note that it is always possible to parenthesize an expression, but it
can seem odd to parenthesize an expression that needs parentheses only
for the line break::

    assert val>4, (
        "val is too small")


Triple-Quoted Strings
---------------------

Open a triple-quoted string; again, people recognize that the string
needs to finish before the next statement starts. ::

    banner_message = """
        Satisfaction Guaranteed,
        or DOUBLE YOUR MONEY BACK!!!





                                        some minor restrictions apply"""


Terminal ``\`` in the general case
----------------------------------

A terminal ``\`` indicates that the logical line is continued on the
following physical line (after whitespace).  There are no particular
semantics associated with this.  This form is never required, although
it may look better (particularly for people with a C language
background) in some cases::

    >>> assert val>4, \
            "val is too small"

Also note that the ``\`` must be the final character in the line.  If
your editor navigation can add whitespace to the end of a line, that
invisible change will alter the semantics of the program.
Fortunately, the typical result is only a syntax error, rather than a
runtime bug::

    >>> assert val>4, \
            "val is too small"

    SyntaxError: unexpected character after line continuation character

This PEP proposes to eliminate this redundant and potentially
confusing alternative.


Terminal ``\`` within a string
------------------------------

A terminal ``\`` within a single-quoted string, at the end of the
line.  This is arguably a special case of the terminal ``\``, but it
is a special case that may be worth keeping. ::

    >>> "abd\
     def"
    'abd def'

* Pro: Many of the objections to removing ``\`` termination were
  really just objections to removing it within literal strings;
  several people clarified that they want to keep this literal-string
  usage, but don't mind losing the general case.

* Pro: The use of ``\`` for an escape character within strings is well
  known.

* Contra: But note that this particular usage is odd, because the
  escaped character (the newline) is invisible, and the special
  treatment is to delete the character.  That said, the ``\`` of
  ``\(newline)`` is still an escape which changes the meaning of the
  following character.


Alternate Proposals
===================

Several people have suggested alternative ways of marking the line
end.  Most of these were rejected for not actually simplifying things.

The one exception was to let any unfinished expression signify a line
continuation, possibly in conjunction with increased indentation.

This is attractive because it is a generalization of the rule for
parentheses.

The initial objections to this were:

- The amount of whitespace may be contentious; expression continuation
  should not be confused with opening a new suite.

- The "expression continuation" markers are not as clearly marked in
  Python as the grouping punctuation "(), [], {}" marks are::

      # Plus needs another operand, so the line continues
      "abc" +
          "def"

      # String ends an expression, so the line does not
      # not continue.  The next line is a syntax error because
      # unary plus does not apply to strings.
      "abc"
          + "def"

- Guido objected for technical reasons.  [#dedent]_ The most obvious
  implementation would require allowing INDENT or DEDENT tokens
  anywhere, or at least in a widely expanded (and ill-defined) set of
  locations.  While this is of concern only for the internal parsing
  mechanism (rather than for users), it would be a major new source of
  complexity.

Andrew Koenig then pointed out [#lexical]_ a better implementation
strategy, and said that it had worked quite well in other
languages. [#snocone]_ The improved suggestion boiled down to:

    The whitespace that follows an (operator or) open bracket or
    parenthesis can include newline characters.

    It would be implemented at a very low lexical level -- even before
    the decision is made to turn a newline followed by spaces into an
    INDENT or DEDENT token.

There is still some concern that it could mask bugs, as in this
example [#guidobughide]_::

    # Used to be y+1, the 1 got dropped.  Syntax Error (today)
    # would become nonsense.
    x = y+
    f(x)

Requiring that the continuation be indented more than the initial line
would add both safety and complexity.


Open Issues
===========

* Should ``\``-continuation be removed even inside strings?

* Should the continuation markers be expanded from just ([{}]) to
  include lines ending with an operator?

* As a safety measure, should the continuation line be required to be
  more indented than the initial line?


References
==========

..  [#dedent] (email subject) PEP 30XZ: Simplified Parsing, van Rossum
    https://mail.python.org/pipermail/python-3000/2007-April/007063.html

..  [#lexical] (email subject) :pep:`3125` -- remove backslash
    continuation, Koenig
    https://mail.python.org/pipermail/python-3000/2007-May/007237.html

..  [#snocone] The Snocone Programming Language, Koenig
    http://www.snobol4.com/report.htm

..  [#guidobughide] (email subject) :pep:`3125` -- remove backslash
    continuation, van Rossum
    https://mail.python.org/pipermail/python-3000/2007-May/007244.html


Copyright
=========

This document has been placed in the public domain.



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