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PEP 259 -- Omit printing newline after newline

PEP: 259
Title: Omit printing newline after newline
Author: guido at python.org (Guido van Rossum)
Status: Rejected
Type: Standards Track
Created: 11-Jun-2001
Python-Version: 2.2
Post-History: 11-Jun-2001

Abstract

Currently, the print statement always appends a newline, unless a trailing comma is used. This means that if we want to print data that already ends in a newline, we get two newlines, unless special precautions are taken.

I propose to skip printing the newline when it follows a newline that came from data.

In order to avoid having to add yet another magic variable to file objects, I propose to give the existing 'softspace' variable an extra meaning: a negative value will mean "the last data written ended in a newline so no space or newline is required."

Problem

When printing data that resembles the lines read from a file using a simple loop, double-spacing occurs unless special care is taken:

>>> for line in open("/etc/passwd").readlines():
... print line
...
root:x:0:0:root:/root:/bin/bash

bin:x:1:1:bin:/bin:

daemon:x:2:2:daemon:/sbin:

(etc.)

>>>

While there are easy work-arounds, this is often noticed only during testing and requires an extra edit-test roundtrip; the fixed code is uglier and harder to maintain.

Proposed Solution

In the PRINT_ITEM opcode in ceval.c , when a string object is printed, a check is already made that looks at the last character of that string. Currently, if that last character is a whitespace character other than space, the softspace flag is reset to zero; this suppresses the space between two items if the first item is a string ending in newline, tab, etc. (but not when it ends in a space). Otherwise the softspace flag is set to one.

The proposal changes this test slightly so that softspace is set to:

  • -1 -- if the last object written is a string ending in a newline
  • 0 -- if the last object written is a string ending in a whitespace character that's neither space nor newline
  • 1 -- in all other cases (including the case when the last object written is an empty string or not a string)

Then, the PRINT_NEWLINE opcode, printing of the newline is suppressed if the value of softspace is negative; in any case the softspace flag is reset to zero.

Scope

This only affects printing of 8-bit strings. It doesn't affect Unicode, although that could be considered a bug in the Unicode implementation. It doesn't affect other objects whose string representation happens to end in a newline character.

Risks

This change breaks some existing code. For example:

print "Subject: PEP 259\n"
print message_body

In current Python, this produces a blank line separating the subject from the message body; with the proposed change, the body begins immediately below the subject. This is not very robust code anyway; it is better written as:

print "Subject: PEP 259"
print
print message_body

In the test suite, only test_StringIO (which explicitly tests for this feature) breaks.

Implementation

A patch relative to current CVS is here:

http://sourceforge.net/tracker/index.php?func=detail&aid=432183&group_id=5470&atid=305470

Rejected

The user community unanimously rejected this, so I won't pursue this idea any further. Frequently heard arguments against included:

  • It is likely to break thousands of CGI scripts.
  • Enough magic already (also: no more tinkering with 'print' please).
Source: https://github.com/python/peps/blob/master/pep-0259.txt