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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
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)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y108 - Python - String Handling
  [324] The backtick operator in Python and Perl - (2005-05-25)
  [463] Splitting the difference - (2005-10-13)
  [496] Python printf - (2005-11-15)
  [560] The fencepost problem - (2006-01-10)
  [773] Breaking bread - (2006-06-22)
  [903] Pieces of Python - (2006-10-23)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [954] Splitting Pythons in Bradford - (2006-11-29)
  [970] String duplication - x in Perl, * in Python and Ruby - (2006-12-07)
  [1110] Python - two different splits - (2007-03-15)
  [1195] Regular Express Primer - (2007-05-20)
  [1517] Python - formatting objects - (2008-01-24)
  [1608] Underlining in Perl and Python - the x and * operator in use - (2008-04-12)
  [1876] Python Regular Expressions - (2008-11-08)
  [2284] Strings as collections in Python - (2009-07-12)
  [2406] Pound Sign in Python Program - (2009-09-15)
  [2692] Flexible search and replace in Python - (2010-03-25)
  [2721] Regular Expressions in Python - (2010-04-14)
  [2765] Running operating system commands from your Python program - (2010-05-14)
  [2780] Formatted Printing in Python - (2010-05-25)
  [2814] Python - splitting and joining strings - (2010-06-16)
  [3090] Matching to a string - what if it matches in many possible ways? - (2010-12-17)
  [3218] Matching a license plate or product code - Regular Expressions - (2011-03-28)
  [3349] Formatting output in Python through str.format - (2011-07-07)
  [3469] Teaching dilemma - old tricks and techniques, or recent enhancements? - (2011-10-08)
  [3796] Backquote, backtic, str and repr in Python - conversion object to string - (2012-07-05)
  [3886] Formatting output - why we need to, and first Python example - (2012-10-09)
  [4027] Collections in Python - list tuple dict and string. - (2013-03-04)
  [4152] Why are bus fares so high? - (2013-08-18)
  [4213] Formatting options in Python - (2013-11-16)
  [4307] Identifying and clearing denial of service attacks on your Apache server - (2014-09-27)
  [4360] Python - comparison of old and new string formatters - (2014-12-22)
  [4593] Command line parameter handling in Python via the argparse module - (2015-12-08)
  [4595] Python formatting update - including named completions - (2015-12-10)
  [4659] Prining a pound sign from Python AND running from the command line at the same time - (2016-03-03)


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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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