The Ups and Downs of Migrating to Python 3

A Pragmatic Approach

• Julian Berman
• http://github.com/Julian
• tos9 on Freenode (Come visit #python!)

Overview

What's New

What's Touchy

What's Coming

Catching Up

• unittest / unittest2

self.assertRaises(IndexError, foo, [1, 2, 3], bar=12)

with self.assertRaises(IndexError):
    [1, 2, 3][42]

self.assertRaises(ValidationError, getattr, thing, "attr")

with self.assertRaises(ValidationError):
    thing.attr

argparse

Catching Up

str.format enhancements

"{} {} {}".format("hello", "world", "!")

subprocess.check_output

>>> subprocess.check_output(["echo", "What a pretty kitty!"]) 
'What a pretty kitty!'

context manager propagation
• bz2.BZ2File
• zipfile.ZipFile
• ...
set literals and comprehensions

>>>{i for i in xrange(10) if i % 2}
set([1, 3, 5, 7, 9])
      

dict views

>>> d = {1 : 2, 3 : 4}
>>> d.viewkeys() & {1}
set([1])

Conception

def input():
    eval(sys.stdin.readline())
      

    

Wat

    > 1 + "0"
    '10'

Wat

>>> 1 < "0"
True

A million names to be cruel

url...wat?

Wat

>>> d = {u"foo" : 12, b"bar" : "monkeys"}

>>> d[b"foo"], d[u"bar"]
(12, "monkeys")

Wat

def add(s1, s2):            
    return " ".join(s1, s2) 

>>> print add(u"hello", "world")
"hello world"

>>> print add(u"hello", "w ⓞ rld")
Traceback (most recent call last):
    ...
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 3: ordinal not in range(128)

Walk the Line

Python 2 only

Python 3 only

Support everything

Something in between

So you want to write 2.x+3.x code

Figure out how!

• 2to3 (python-modernize)

• 3to2

• unified codebase

So you want to write 2.x+3.x code

Ignore <2.6 and <(=)3.1

    try:
        int("Arg!")
    except ValueError, e:
        pass
    

So you want to write 2.x+3.x code

Be clever.

    from __future__ import unicode_literals, division

    if PY3:
        from urllib import parse as urlparse
        from urllib.parse import unquote
        from urllib.request import urlopen
        basestring = unicode = str
        iteritems = operator.methodcaller("items")
    else:
        from itertools import izip as zip
        from urllib import unquote
        from urllib2 import urlopen
        import urlparse
        iteritems = operator.methodcaller("iteritems")
    

Lighten Up

The Stragglers

• alternate implementations

• networking

• large frameworks

• backward compatible conscious

The Tragic Tale of twisted.internet.test.test_tcp

    $ bin/trial twisted.test.test_tcp
    ...
    Ran 36 tests in 0.875s

$ admin/run-python3-tests twisted.test.test_tcp
    ...
    Ran 36 tests in 38.698s

zerozerozerozero

>>> bytes(14)
'14'

>>> bytes(14)
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

contextlib.ExitStack

    def process(file=None):
        if file is None:
            file = open("a/default/file")
            cleanup_needed = True

        try:
            result = perform_operation()
            if result:
                cleanup_needed = False
        finally:
            if cleanup_needed:
                cleanup_resources()
    

contextlib.ExitStack

    from contextlib import ExitStack

    def process(file=None):
        with ExitStack() as stack:
            if file is None:
                file = open("a/default/file")
                stack.enter_context(file)
            process(file)
    

Oh no! An exception!

    try:
        with open("document.txt") as f:
            content = f.read()
    except IOError as err:
        if err.errno == ENOENT:
            pass
        raise

try:                                
    with open("document.txt") as f: 
        content = f.read()          
except FileNotFoundError:           
    pass                            

Subtests

    class TestMultiplication(unittest.TestCase):
        def test_multiply(self):
            for i, j in itertools.product(range(2, 5), range(1, 3)):
                    with self.subTest(i=i, j=j):
                        self.assertEqual(multiply(i, j), i * j)
    

Optional Subparsers

    $ git --version
    $ git commit -m "Hi!"
    $ git --color commit -m "Hi!"
    

Thanks