PEP 259 – Omit printing newline after newline
- Guido van Rossum <guido at python.org>
- Standards Track
Table of Contents
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.”
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.
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)
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.
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.
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.
A patch relative to current CVS is here:
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).
This document has been placed in the public domain.
Last modified: 2022-10-05 16:48:43+00:00 GMT