Who's Afraid of the Big Bad 3

Lennart Regebro

PyCon Ireland, Dublin 2014

<Cue dramatic music!>

Who wants Python 3 anyway?

Python 2 is good enough!

You will have to spend hundreds of man-hours!

There are no third-party modules!

Python 3 was a mistake!

No other language ever break backwards compatibility!

Everything breaks!

Python 3 is really a new language!

The core developers don't listen!


Time to Third-party!

You want Python 3!

Although you might not know it yet

Extended Iterable Unpacking

>>> first, second, *rest, last = \
...     "a b c d e f".split()
>>> first, second, last
('a', 'b', 'f')

Keyword only arguments

>>> def foo(a, *args, b, **kw):
...   print(a, args, b, kw)

>>> foo(1, 2, 3, b=4, c=5)
1 (2, 3) b {'c': 5}

Chained exceptions

>>> raise KeyError("wut?") from ZeroDivisionError()

The above exception was the direct cause of
the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'wut?'

Better OS Exceptions

       BlockingIOError ChildProcessError
       ConnectionError BrokenPipeError
ConnectionAbortedError ConnectionRefusedError
  ConnectionResetError FileExistsError
     FileNotFoundError InterruptedError
     IsADirectoryError NotADirectoryError
       PermissionError ProcessLookupError

File handle warnings

__main__:1: ResourceWarning: unclosed file

Yield from

>>> def my_generator():
...     yield from range(1,5)
...     yield from range(10,15)
>>> list(my_generator())
[1, 2, 3, 4, 10, 11, 12, 13, 14]

Simply super

Python 2

super(ClassName, self).method(foo, bar)

Python 3

super().method(foo, bar)


Supporting Python 3 is not so bad

Many changes are handled by 2to3

Some changes need no handling at all

If you need Python 2 compatibility

>>> from __future__ import division
>>> from __future__ import print_function
>>> print("Three halves is written", 3/2, "with decimals.")
Three halves is written 1.5 with decimals.

u'' is back!

So what IS hard?

API changes

Example 1: zope.interface

class TheComponent(object):

Example 1: zope.interface

class TheComponent(object):

Example 2: icalendar

ical = str(icalendarobject)

Example 2: icalendar

ical = icalendarobject.to_ical()


You gotta keep'em separated!

Practical Experiences


Tool 1: caniusepython3


$ caniusepython3 --project diazo

You need 3 projects to transition to Python 3.
Of those 3 projects, 2 has no direct
dependencies blocking its transition:

  repoze.xmliter (which is blocking diazo)
  experimental.cssselect (which is blocking diazo)

Adding Python 3 support to collective.checkdocs

Tool 2: 2to3

$ 2to3 -w .


Time spent: ~4h

Adding Python 3 support to repoze.xmliter

Tool 3a: Tox

$ tox

__________________ summary __________________
  py26: commands succeeded
  py27: commands succeeded
ERROR:   py34: commands failed
  pep8: commands succeeded

Tool 3b: Virtualenv + bash

$ virtualenv-2.7 .venv/py27
$ virtualenv-3.4 .venv/py34

And a small script:

.venv/py27/bin/python setup.py test
.venv/py34/bin/python setup.py test

The Unicode problem

if sys.version_info > (3,):
    unicode = str

doctype_re_b = re.compile(
    b"^<!DOCTYPE\\s[^>]+>\\s*", re.MULTILINE)
doctype_re_u = re.compile(
    u"^<!DOCTYPE\\s[^>]+>\\s*", re.MULTILINE)

if isinstance(result, unicode):
    result, subs = doctype_re_u.subn(
        self.doctype, result, 1)
    result, subs = doctype_re_b.subn(
        self.doctype.encode(), result, 1)


Time spent: < 6h

Adding Python 3 support to Diazo

Tool 4: Futurize

$ pip install future

Running a single fixer

futurize -w -f <thespecificfixer> .

Updating the Diazo documentation

Updating the documentation

  1. Replace the WSGI server with gearbox
  2. Replace Paster#urlmap with rutter
  3. Replace Paster#static with ???
  4. Replace Paster#proxy with ???

Introducing webobentrypoints

  1. Replace the WSGI server with gearbox
  2. Replace Paster#urlmap with rutter
  3. Replace Paster#static with webobentrypoints#staticdir
  4. Replace Paster#proxy with webobentrypoints#proxy


Adding Python 3 support to Diazo: ~3h

Switching from Paster: < 6h

Total time: Less than 20 hours!

General workflow

  1. caniusepython3
  2. Make sure you have tests
  3. Fix one problem at a time

Fixing problems

  1. Run fixers one by one
  2. 2to3 or Futurize
  3. Review the changes


Just Do It!

Yes We Can!


Been there, done that!

You can do it!


Props to


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