PEP 3125 - Remove Backslash Continuation

PEP: 3125
Title: Remove Backslash Continuation
Author: Jim J. Jewett <JimJJewett at gmail.com>
Status: Rejected
Type: Standards Track
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

[1] (email subject) PEP 30XZ: Simplified Parsing, van Rossum http://mail.python.org/pipermail/python-3000/2007-April/007063.html
[2] (email subject) PEP-3125 -- remove backslash continuation, Koenig http://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 http://mail.python.org/pipermail/python-3000/2007-May/007244.html
Source: https://hg.python.org/peps/file/tip/pep-3125.txt

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