Move to Python 3!

Graham Markall - @gmarkall

Ian Ozsvald - @ianozsvald, ianozsvald.com

https://github.com/gmarkall/py3lightning

Py3 Improvements over Py2

An (incomplete) list:

• Unicode improvements, Exception chaining, unpacking generalisations, asyncio, async/await, fine-grained OSError subclasses, yield from, fault handlers, unorderable types, type hints, iterators everywhere...

Brief demos of:

• Matrix multiplication @ operator
• Advanced unpacking
• Keyword-only args

Matrix Multiplication - @ operator

Old "syntax":

M.dot(M.T).dot(N.dot(v))

New syntax:

M @ M.T @ N @ v

Defining matrix multiplication for your own classes:

def __matmul__(self, other)  # @
def __rmatmul__(self, other) # @
def __imatmul__(self, other) # @=

Advanced Unpacking

Get "everything else" from an iterable:

>>> a, b, *rest = range(10)
>>> a
0
>>> b
1
>>> rest
[2, 3, 4, 5, 6, 7, 8, 9]

Get the first and last lines of a file:

>>> with open('using_python_to_profit.txt') as f:
        first, *_, last = f.readlines()
>>> first
'Step 1: Use Python 3\n'
>>> last
'Step 10: Profit!\n'

Keyword-only Args

def cleanup(folder, *, extreme=False):
    if extreme:
        shutil.rmtree('/')
    else:
        shutil.rmtree(folder)

>>> cleanup('/home/gmarkall/tmp', '/home/gmarkall/tmp2')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: cleanup() takes 1 positional argument but 2 were given

Compatibility with common libraries

Py3-only libraries

“Ubuntu 15.10 (Wily Werewolf) to Switch to Python 3.5 Ahead of Ubuntu 16.04 LTS”

How to upgrade

• 2to3: mechanical conversion of Py2 code to Py3 code
• six / future: compatibility layers for both 2 and 3
  • use Python 3 features, keep compatibility with Python 2
• conda package manager: Easily switch between Python 2 and 3 environments
  • conda create -n py34 python=3.4 anaconda
• Resource:: python3porting.com
• Unit tests: have lots of them!
  • Talk to Dave MacIver about this (and Hypothesis)!
• Use CI: e.g. Travis for testing with multiple versions
• New projects: Start in Py3, backport only if necessary