GAC coding dicussion
October 11, 2017

Why are there two versions of Python?

Guido van Rossum (the original creator of the Python language) decided to clean up Python 2.x properly, with less regard for backwards compatibility than is the case for new releases in the 2.x range.

• Python 3.0 was released in 2008, with lots of major improvements, but not backwards-compatible
• Python 2.7 has been maintained ever since as an "end of life" release to support existing code
  • Some 3.x features have been backported to 2.7
  • 2.6 was the last release before 3.0 came out; 2.7 was released in 2010 to ease the transition
  • Loses support in 2020
  • Current support is only for bugs and security fixes (no more backporting)

Python 2.x is legacy, Python 3.x is the present and future of the language.

• Wiki.python.org

• 2008: lots of effort, few libraries, little awareness
• 2012: some effort, some libraries, some awareness, major debate

• 2017: some effort, most libraries, major awareness, debate is over

• For an alternative perspective: Python 3 is killing Python

• you never learned 2.7 anyway
• you're writing a major new project
• an important library requires 3.x (e.g. Django)

• an important library is still in 2.7
• you're working with legacy code
• you don't have time to learn changes for 3.0
  • (these changes are easy to adjust to, but you may not have the time/focus to do so)
• you're deploying to an environment you don't control (e.g. a server or cluster where only 2.7 is available)

6,000 stable and actively developed packages on PyPI

The 28% of packages that are Python 2 only

Current status of 360 most popular PyPI packages

Python 3 Readiness

Most systems currently maintain python 2.7 and python 3.x simultaneously

03:34 PM emeryl@fisher ~ $ which python
/usr/bin/python
03:34 PM emeryl@fisher ~ $ which python2
/usr/bin/python2
03:34 PM emeryl@fisher ~ $ which python3
/usr/local/bin/python3

03:34 PM emeryl@fisher ~ $ ls -lhA /usr/local/bin/python*
lrwxrwxrwx 1 root root 35 Sep 10  2012 /usr/local/bin/python2.7 -> /usr/local/python-2.7/bin/python2.7
lrwxrwxrwx 1 root root 42 Sep 10  2012 /usr/local/bin/python2.7-config -> /usr/local/python-2.7/bin/python2.7-config
lrwxrwxrwx 1 root root  9 Sep 11  2012 /usr/local/bin/python3 -> python3.1
lrwxrwxrwx 1 root root 37 Sep 11  2012 /usr/local/bin/python3.1 -> /usr/local/python-3.1.2/bin/python3.1
lrwxrwxrwx 1 root root 44 Sep 11  2012 /usr/local/bin/python3.1-config -> /usr/local/python-3.1.2/bin/python3.1-config
lrwxrwxrwx 1 root root 35 Jan  4  2016 /usr/local/bin/python3.4 -> /usr/local/python-3.4/bin/python3.4
lrwxrwxrwx 1 root root 36 Jan  4  2016 /usr/local/bin/python3.4m -> /usr/local/python-3.4/bin/python3.4m
lrwxrwxrwx 1 root root 43 Jan  4  2016 /usr/local/bin/python3.4m-config -> /usr/local/python-3.4/bin/python3.4m-config

03:35 PM emeryl@fisher ~ $ ls -lhA /usr/bin/python*
-rwxr-xr-x 2 root root 8.9K May 22  2015 /usr/bin/python
lrwxrwxrwx 1 root root    6 Jun 29  2016 /usr/bin/python2 -> python
-rwxr-xr-x 2 root root 8.9K May 22  2015 /usr/bin/python2.6
-rwxr-xr-x 1 root root 1.4K May 22  2015 /usr/bin/python2.6-config
lrwxrwxrwx 1 root root   16 Jun 29  2016 /usr/bin/python-config -> python2.6-config

Apple's python

01:00 PM emeryl@gcc-mac-004 ~ $ /usr/bin/python2.6
Python 2.6.9 (unknown, Feb  7 2017, 00:08:08) 
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.34)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 

Homebrew

12:58 PM emeryl@gcc-mac-004 ~ $ port select --summary
Name        Selected      Options
====        ========      =======
mysql       mysql57       mysql57 none
pip         pip34         none
python      python34      python26-apple python27 python27-apple python34 python35 none
python2     python27      python26-apple python27 python27-apple none
python3     python34      python34 python35 none
virtualenv  virtualenv34  virtualenv34 virtualenv35 none

12:58 PM emeryl@gcc-mac-004 ~ $ which python
/Users/emeryl/macports/bin/python
12:59 PM emeryl@gcc-mac-004 ~ $ which python2
/Users/emeryl/macports/bin/python2

12:59 PM emeryl@gcc-mac-004 ~ $ ls -lhA ~/macports/bin/python*
lrwxr-xr-x  1 emeryl  staff    36B Jun 29 17:29 /Users/emeryl/macports/bin/python -> /Users/emeryl/macports/bin/python3.4
lrwxr-xr-x  1 emeryl  staff    43B Jun 29 17:29 /Users/emeryl/macports/bin/python-config -> /Users/emeryl/macports/bin/python3.4-config
lrwxr-xr-x  1 emeryl  staff    18B Oct 11 12:59 /Users/emeryl/macports/bin/python2 -> /usr/bin/python2.6
lrwxr-xr-x  1 emeryl  staff    25B Oct 11 12:59 /Users/emeryl/macports/bin/python2-config -> /usr/bin/python2.6-config
lrwxr-xr-x  1 emeryl  staff    85B Sep 27 09:57 /Users/emeryl/macports/bin/python2.7 -> /Users/emeryl/macports/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7
lrwxr-xr-x  1 emeryl  staff    92B Sep 27 09:57 /Users/emeryl/macports/bin/python2.7-config -> /Users/emeryl/macports/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7-config
lrwxr-xr-x  1 emeryl  staff    36B Sep 28 11:03 /Users/emeryl/macports/bin/python3 -> /Users/emeryl/macports/bin/python3.4
lrwxr-xr-x  1 emeryl  staff    43B Sep 28 11:03 /Users/emeryl/macports/bin/python3-config -> /Users/emeryl/macports/bin/python3.4-config
lrwxr-xr-x  1 emeryl  staff    85B Sep 27 15:23 /Users/emeryl/macports/bin/python3.4 -> /Users/emeryl/macports/Library/Frameworks/Python.framework/Versions/3.4/bin/python3.4
lrwxr-xr-x  1 emeryl  staff    92B Sep 27 15:23 /Users/emeryl/macports/bin/python3.4-config -> /Users/emeryl/macports/Library/Frameworks/Python.framework/Versions/3.4/bin/python3.4-config
lrwxr-xr-x  1 emeryl  staff    86B Sep 27 15:23 /Users/emeryl/macports/bin/python3.4m -> /Users/emeryl/macports/Library/Frameworks/Python.framework/Versions/3.4/bin/python3.4m
lrwxr-xr-x  1 emeryl  staff    93B Sep 27 15:23 /Users/emeryl/macports/bin/python3.4m-config -> /Users/emeryl/macports/Library/Frameworks/Python.framework/Versions/3.4/bin/python3.4m-config
...

...
lrwxr-xr-x  1 emeryl  staff    85B Sep 27 15:24 /Users/emeryl/macports/bin/python3.5 -> /Users/emeryl/macports/Library/Frameworks/Python.framework/Versions/3.5/bin/python3.5
lrwxr-xr-x  1 emeryl  staff    92B Sep 27 15:24 /Users/emeryl/macports/bin/python3.5-config -> /Users/emeryl/macports/Library/Frameworks/Python.framework/Versions/3.5/bin/python3.5-config
lrwxr-xr-x  1 emeryl  staff    86B Sep 27 15:24 /Users/emeryl/macports/bin/python3.5m -> /Users/emeryl/macports/Library/Frameworks/Python.framework/Versions/3.5/bin/python3.5m
lrwxr-xr-x  1 emeryl  staff    93B Sep 27 15:24 /Users/emeryl/macports/bin/python3.5m-config -> /Users/emeryl/macports/Library/Frameworks/Python.framework/Versions/3.5/bin/python3.5m-config
lrwxr-xr-x  1 emeryl  staff    37B Sep 28 11:03 /Users/emeryl/macports/bin/python3m -> /Users/emeryl/macports/bin/python3.4m
lrwxr-xr-x  1 emeryl  staff    44B Sep 28 11:03 /Users/emeryl/macports/bin/python3m-config -> /Users/emeryl/macports/bin/python3.4m-config
lrwxr-xr-x  1 emeryl  staff    19B Oct 11 12:59 /Users/emeryl/macports/bin/pythonw2 -> /usr/bin/pythonw2.6
lrwxr-xr-x  1 emeryl  staff    86B Sep 27 09:57 /Users/emeryl/macports/bin/pythonw2.7 -> /Users/emeryl/macports/Library/Frameworks/Python.framework/Versions/2.7/bin/pythonw2.7

Any suggestions?

http://sebastianraschka.com/Articles/2014_python_2_3_key_diff.html

Dictionary iterator methods are gone

• dict.iterkeys(), dict.itervalues(), and dict.iteritems() are gone
  
• These methods used to return iterator versions of the lists that were returned by dict.keys(), dict.values(), and dict.items()
  
• Python 3 dicts return "dictionary views" from dict.keys(), dict.values(), and dict.items(), and these views are similar to iterators, so the iterator methods have been removed

• Use the 2to3 conversion utility

2.x code can be a lot like 3.x code.

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