Str.format() for Python 2.6 gives error where 2.7 does not

Question

I have some code which works well in Python 2.7.

Python 2.7.3 (default, Jan  2 2013, 13:56:14) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from sys import stdout
>>> foo = 'Bar'
>>> numb = 10
>>> stdout.write('{} {}\n'.format(numb, foo))
10 Bar
>>>

But in 2.6 I get a ValueError exception.

Python 2.6.8 (unknown, Jan 26 2013, 14:35:25) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from sys import stdout
>>> foo = 'Bar'
>>> numb = 10
>>> stdout.write('{} {}\n'.format(numb, foo))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: zero length field name in format
>>>

When looking through the documentation (2.6, 2.7), I can see no mention of changes having been done between the two versions. What is happening here?

Answer 1

Python 2.6 and before (as well as Python 3.0) require that you number the placeholders:

'{0} {1}\n'.format(numb, foo)

The numbering, if omitted in Python 2.7 and Python 3.1 and up, is implicit, see the documentation:

Changed in version 2.7: The positional argument specifiers can be omitted, so '{} {}' is equivalent to '{0} {1}'.

The implicit numbering is popular; a lot of examples here on Stack Overflow use it as it is easier to whip up a quick format string that way. I have forgotten to include them more than once when working on projects that must support 2.6 still.

Answer 2

This is in the docs here:
http://docs.python.org/2/library/string.html#format-string-syntax

About halfway through that section:

Changed in version 2.7: The positional argument specifiers can be omitted, so '{} {}' is equivalent to '{0} {1}'.