[Python-Dev] updated PEP3125, Remove Backslash Continuation (original) (raw)

Jim Jewett jimjjewett at gmail.com
Fri May 4 21:09:46 CEST 2007

Major rewrite.

The inside-a-string continuation is separated from the general continuation.

The alternatives section is expaned to als list Andrew Koenig's improved inside-expressions variant, since that is a real contender.

If anyone feels I haven't acknowledged their concerns, please tell me.

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

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.

An 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 recieved 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
them 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'

+ 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.

+ The use of ``\`` for an escape character within strings is well
  known.

- 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 unfished 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 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 expanced 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 http://mail.python.org/pipermail/python-3000/2007-April/007063.html

.. [#lexical] (email subject) PEP-3125 -- remove backslash continuation, Koenig http://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 http://mail.python.org/pipermail/python-3000/2007-May/007244.html

Copyright

This document has been placed in the public domain.

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