String formatting: % vs. .format vs. string literal

Question

Python 2.6 introduced the str.format() method with a slightly different syntax from the existing % operator. Which is better and for what situations?

Pyt

Answer 1

But please be careful, just now I've discovered one issue when trying to replace all % with .format in existing code: '{}'.format(unicode_string) will try to encode unicode_string and will probably fail.

Just look at this Python interactive session log:

Python 2.7.2 (default, Aug 27 2012, 19:52:55) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-48)] on linux2
; s='й'
; u=u'й'
; s
'\xd0\xb9'
; u
u'\u0439'

s is just a string (called 'byte array' in Python3) and u is a Unicode string (called 'string' in Python3):

; '%s' % s
'\xd0\xb9'
; '%s' % u
u'\u0439'

When you give a Unicode object as a parameter to % operator it will produce a Unicode string even if the original string wasn't Unicode:

; '{}'.format(s)
'\xd0\xb9'
; '{}'.format(u)
Traceback (most recent call last):
  File "", line 1, in 
UnicodeEncodeError: 'latin-1' codec can't encode character u'\u0439' in position 0: ordinal not in range(256)

but the .format function will raise "UnicodeEncodeError":

; u'{}'.format(s)
u'\xd0\xb9'
; u'{}'.format(u)
u'\u0439'

and it will work with a Unicode argument fine only if the original string was Unicode.

; '{}'.format(u'i')
'i'

or if argument string can be converted to a string (so called 'byte array')