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.[1] 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[2] a better implementation strategy, and
said that it had worked quite well in other languages.[3] 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[4]:

    # 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

Copyright

This document has been placed in the public domain.



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

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

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

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