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Convering from Python 2 to Python 3 - an update, and the 2to3 utility

Python 3 has now been with us for a number of years, but the move across from Python 2 to Python 3 is still far from complete. And as Python 3 is not source code compatible with Python 2, that's no big surprise to anyone. The very good reason for the changes is that the correct decisions made on langauge syntax in 1988 would not be the correct decisions for the language for 2018 or 2028, but there's a huge user bas of code out there which people have invested in and need to maintain and keep running, and a huge number of modules and packages that need to be converted and tested before end user programmers can make full use of them.

In order to help with the move in the future - so that Python 2 support can eventually (2019?) be phased out, some of the new facilities have been backported to Python 2. The new format method on a string, for example, and the with keyword. But some things cannot be easily backported.

Recommmendation is that you stick with developmement in 2.7 while you are still supporting Python 2 strema machines, writing your code with an eye on Python 3. It is possible to write code that will compile and run in both - our json parser demo is an example - but there are time that's not the optimum thing to do.

Supplied with Python3 is a utility called 2to3 which will do a lot of the work for you; be wary though, because there are some changes in a dynamic language like Python which it's simply not possible to automate.

Here's an example of 2to3 in use:

 WomanWithCat:q2 grahamellis$ 2to3 -w dx
 RefactoringTool: Skipping implicit fixer: buffer
 RefactoringTool: Skipping implicit fixer: idioms
 RefactoringTool: Skipping implicit fixer: set_literal
 RefactoringTool: Skipping implicit fixer: ws_comma
 RefactoringTool: Refactored dx
 --- dx (original)
 +++ dx (refactored)
 @@ -19,9 +19,9 @@
  edges = getEdges(width,height,depth)
  corners = getCorners(width,height,depth)
 
 -print "Our",what,"is",width,"by",height,"by",depth
 -print "Volume is",volume
 -print "Surface ares is",surface
 -print "Edge length is",edges
 -print "Corners",corners
 +print("Our",what,"is",width,"by",height,"by",depth)
 +print("Volume is",volume)
 +print("Surface ares is",surface)
 +print("Edge length is",edges)
 +print("Corners",corners)
 
 RefactoringTool: Files that were modified:
 RefactoringTool: dx


And here's a note of the changes it's made:

 WomanWithCat:q2 grahamellis$ diff dx dx.bak
 22,26c22,26
 < print("Our",what,"is",width,"by",height,"by",depth)
 < print("Volume is",volume)
 < print("Surface ares is",surface)
 < print("Edge length is",edges)
 < print("Corners",corners)
 ---
 > print "Our",what,"is",width,"by",height,"by",depth
 > print "Volume is",volume
 > print "Surface ares is",surface
 > print "Edge length is",edges
 > print "Corners",corners
 WomanWithCat:q2 grahamellis$


Our public Python Courses are moving from Python 2 to Python 3 over the next year; examples are run in both at present and that will continue, but the bias will switch - and with typical group sizes being just three or four delegates, I'm well able to tailor the presentation to suit the group.

Private courses are run using Python 2, or Python 3, after a discussion with the technical lead prior to the course. Almost inevitably, any courses in Python 2 will have a strong element of "do it this way to prepare for the future" about them, but it's rare for a course to be jumping back and forth between every example as that serves more to baffle than to clarify!
(written 2016-10-30)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y300 - Python 3 - What is new, what's changed and why
  [4712] A reminder of the key issues to consider in moving from Python 2 to Python 3 - (2016-10-30)
  [4650] Why populate object with values as you construct them? - (2016-02-18)
  [4649] Object and Static methods - what is the difference; example in Python 3 - (2016-02-17)
  [4590] Progress on moving from Python 2 to Python 3 - training for both versions - (2015-12-01)
  [4470] Testing in Python 3 - unittest, doctest and __name__ == __main__ too. - (2015-04-21)
  [4469] Sorting in Python 3 - and how it differs from Python 2 sorting - (2015-04-20)
  [2871] Moving from Python 2.6 to Python 3 - (2010-07-14)
  [2778] Learning to program in Python 2 ... and / or in Python 3 - (2010-05-24)
  [2559] Moving the product forward - ours, and MySQL, Perl, PHP and Python too - (2010-01-01)
  [2285] Great new diagrams for our notes ... Python releases - (2009-07-13)
  [2277] Python classes / courses - what version do we train on? - (2009-07-10)
  [1791] The road ahead - Python 3 - (2008-09-10)
  [1788] Python 2 to Python 3 / Python 3000 / Py3k - (2008-09-07)
  [753] Python 3000 - the next generation - (2006-06-09)


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Searching a Json or XML structure for a specific key / value pair in Python
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Horse's mouth home
Forward to
A reminder of the key issues to consider in moving from Python 2 to Python 3
Some other Articles
Well House Manor - Still five out of five!
The technical article feed continues - personal updates more proactive on Facebook now!
Equality (in Python)
Convering from Python 2 to Python 3 - an update, and the 2to3 utility
Searching a Json or XML structure for a specific key / value pair in Python
Some gems from Intermediate Python
Scons - a build system in Python - building hello world
Some gems from an introduction to Python
Melksham trial train service is to be made permanent
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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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