Unexpected format string behavior

Question

I just ran into what seemed to me to be bizarre string formatting behavior in python. It turned out to be caused by carriage return characters ('\r') I didn't know were there. Here's an example:

>>> text = 'hello\r' >>> '(SUBJECT "%s")' % (text) '(SUBJECT "hello\r")' >>> print '(SUBJECT "%s")' % (text) ")UBJECT "hello

I tried the same thing in C (on a couple machines) as a sanity check.

#include <stdio.h> int main() { char *text = "hello\r"; printf("(SUBJECT \"%s\")\n", text); return 0; }

Output:

% ./a.out ")UBJECT "hello

Is this desired behavior? If so, can someone explain what is going on?

"carriage return" is a quaint old fashioned term meaning "go to beginning of line". — Jim Balter
– Jim Balter, Commented Jul 13, 2013 at 22:01

tinman · Accepted Answer · 2013-07-13 21:44:55Z

It (\r) is a carridge return without a linefeed so the cursor moves back to the start of the current line without moving onto a new line and therefore overwriting what is already displayed.

The behaviour depends on your console and whether it interprets CR and LF as individual operations.

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