Python string formatting - the move from % to str.format

The % operator in Python has been a very clever and easy-to-use formatter, but has lacked the extensibility that's been incumbent on its structure and mirroring of the C sprintf function. Or - put another way - it was rather irritatingly running out of steam for some of the more complex formatting jobs, and users were having to write code and use modules to do whatshould really have been some fairly basic formatting.

At Python 2.6 the format method was added onto string object to provide a more flexible alternative, and as from Python 3 this will be / is the new standard.

The parentage of sprintf can still be seen in the new format method, although it's slighly longer to use.

Python 2.5:
  print "The value %.2f is the result" % val

Python 2.6:
  print "The value {0:.2f} is the result".format(val)

Python 2.7:
  print "The value {:.2f} is the result".format(val)
The position parameter 0 from 2.6 has become optional

Python 3.x:
  print ("The value {:.2f} is the result".format(val))
print is a function - as it always should have been - from Python 3.0

The string on which the fomat method is run is a template within which elements to be inserted are defined in curly braces. Each of these elements may comprise a number of parts - the main two of which specify what is to be inserted and how it is to be formatted. For example:

  val = 1000 / 7 ; # integer example

  print "The results is {0:4d} if you divide 1000 by 7".format(val)

uses the string {0:4d} to say "place the zeroth parameter to format - which is the value held in the val variable in my example - here. Output it to base 10 (decimal) and take up at least 4 character positions". There are a number of sensible defaults - the value will be right justified, left space filled if it doesn't need all the characer positions requested, for example.

With a formatter that's so flexible, you'll want to see plenty of examples and I have added an annotated program to show you a lot more - together with sample output - [here]. And we're moving over to using the String Formatter in new examples we write now that Python 2.5 and earlier is starting to get a little rarer. The "old stuff" still works so there's no need to go rushing to make changes to your existing codebase. In Python 3:

  val = 12.0 / 17.0

  print ("The value is %.3f - old style" % val)

  print ("The value is {:.3f} - new style".format(val))

Which runs as follows:

  wizard:oct11 graham$ python3 spy3.py 

  The value is 0.706 - old style

  The value is 0.706 - new style

  wizard:oct11 graham$

As even .. "as taught on our Python Courses" ;-)
(written 2011-10-08) 236c

Associated topics are indexed under

Y108 - Python - String Handling
  [4027] Collections in Python - list tuple dict and string. - (2013-03-04)
  [3886] Formatting output - why we need to, and first Python example - (2012-10-09)
  [3796] Backquote, backtic, str and repr in Python - conversion object to string - (2012-07-05)
  [3469] Teaching dilemma - old tricks and techniques, or recent enhancements? - (2011-10-08)
  [3349] Formatting output in Python through str.format - (2011-07-07)
  [3218] Matching a license plate or product code - Regular Expressions - (2011-03-28)
  [3090] Matching to a string - what if it matches in many possible ways? - (2010-12-17)
  [2814] Python - splitting and joining strings - (2010-06-16)
  [2780] Formatted Printing in Python - (2010-05-25)
  [2765] Running operating system commands from your Python program - (2010-05-14)
  [2721] Regular Expressions in Python - (2010-04-14)
  [2692] Flexible search and replace in Python - (2010-03-25)
  [2406] Pound Sign in Python Program - (2009-09-15)
  [2284] Strings as collections in Python - (2009-07-12)
  [1876] Python Regular Expressions - (2008-11-08)
  [1608] Underlining in Perl and Python - the x and * operator in use - (2008-04-12)
  [1517] Python - formatting objects - (2008-01-24)
  [1195] Regular Express Primer - (2007-05-20)
  [1110] Python - two different splits - (2007-03-15)
  [970] String duplication - x in Perl, * in Python and Ruby - (2006-12-07)
  [954] Splitting Pythons in Bradford - (2006-11-29)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [903] Pieces of Python - (2006-10-23)
  [773] Breaking bread - (2006-06-22)
  [560] The fencepost problem - (2006-01-10)
  [496] Python printf - (2005-11-15)
  [463] Splitting the difference - (2005-10-13)
  [324] The backtick operator in Python and Perl - (2005-05-25)

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