Python: the coolest is yet to come

The COOLEST is yet
TO COME

{

“event”: “PDI DEVCON 2013”
“author”: “Pablo Enfedaque”
“twi5er”: “pablitoev56”

> 

Today we are going to see a quick tour through
Python’s newest features
> 

The new super function

> 

The yield from statement

> 

The new exception chaining

> 

Single-‐‑dispatch generic functions

> 

Python virtual environments

> 

The enum module

> 

The asyncio module

> 

And lots of other new features

Welcome!
{ “event”: “PDI DEVCON 2013”, “author”: “Pablo Enfedaque”, “twi5er”: “pablitoev56”}

The super function

class SilentDict(dict):
def __getitem__(self, key):
"""Called to implement evaluation of self[key]
:returns: None by default
"""
try:
return super(SilentDict, self).__getitem__(key)
except KeyError:
return None

>>> d = SilentDict()
>>> print (d[“a”])
None

>>> d = SilentDict({"a": 1, "b": 2})
>>> print(d["a"])
1
>>> print(d["a"])
None

Let’s implement a SilentDict

class SilentDict(dict):
def __getitem__(self, key):
"""Called to implement evaluation of self[key]
:returns: None by default
"""
try:
return super().__getitem__(key)
except KeyError:
return None

>>> d = SilentDict()
>>> print (d[“a”])
None

>>> d = SilentDict({"a": 1, "b": 2})
>>> print(d["a"])
1
>>> print(d["a"])
None

Let’s implement a SilentDict

> 

h5p://docs.python.org/3/library/
functions.html#super
> 

PEP 3135: New Super
> 

> 

h5p://www.python.org/dev/peps/pep-‐‑3135/

Introduced in Python 3.0

> 

Standard Library updated to use it

> 

No oﬃcial backport

The super function

Let’s implement itertools.chain

def chain(*iterables):
# chain('ABC', 'DEF') --> A B C D E F
for it in iterables:
yield from it

>>> list(chain('ABC', 'DEF'))
['A', 'B', 'C', 'D', 'E', 'F']

>>> list(chain([1, 2, 3], (4, 5, 6), [7, 8, 9]))
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Let’s implement itertools.chain

> 

PEP 380: Syntax for Delegating to a Subgenerator
> 


> 


> 

No backport


from urllib.request import urlopen
from urllib.error import URLError

class CustomException(Exception):
"""Out custom exception for this example""”

def requester(url):
"""Open and read given url"""
try:
return urlopen(url).readall()
except URLError:
raise CustomException

def call_the_requester(url):
return requester(url)

def main_app(url):
print(call_the_requester(url))

if __name__ == "__main__":
main_app("http://unknown_host:8765")


$ python3.4 raise_from_none_examples.py
Traceback (most recent call last):
...
File "/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/urllib/
request.py", line 1255, in do_open
raise URLError(err)
urllib.error.URLError: <urlopen error [Errno 8] nodename nor servname provided,
or not known>

During handling of the above exception, another exception occurred:

File "raise_from_none_examples.py", line 39, in <module>
File "raise_from_none_examples.py", line 35, in main_app
File "raise_from_none_examples.py", line 31, in call_the_requester
File "raise_from_none_examples.py", line 27, in requester
raise CustomException
__main__.CustomException




def requester(url):
try:
except URLError as exc:
raise CustomException from exc


def main_app(url):

if __name__ == "__main__":


...
File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/urllib/
request.py", line 1242, in do_open
raise URLError(err)
urllib.error.URLError: <urlopen error [Errno 8] nodename nor servname provided,
or not known>

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

File "raise_from_none_examples.py", line 40, in <module>
File "raise_from_none_examples.py", line 36, in main_app
File "raise_from_none_examples.py", line 32, in call_the_requester
File "raise_from_none_examples.py", line 28, in requester
raise CustomException from exc




def requester(url):
try:
except URLError:
raise CustomException from None


def main_app(url):

if __name__ == "__main__":


File "raise_from_none_examples.py", line
call_the_requester
raise CustomException from None

40, in <module>
36, in main_app
32, in

28, in requester


> 

Several PEPs involved:
> 

PEP 3134: Exception Chaining and Embedded
Tracebacks
> 

> 

PEP 409: Suppressing exception context
> 

> 

h5p://www.python.org/dev/peps/pep-‐‑3134

PEP 415: Implement context suppression with
exception a5ributes
> 

h5p://www.python.org/dev/peps/pep-‐‑0415

> 

Introduced in Python 3.0 and 3.3

> 



class Duck:
def quack(self):
print("quack")

class Cat:
def meow(self):
print("meow")

class Dog:
def bark(self):
print("bark”)

class Cow:
def moo(self):
print("moo")

donald = Duck()
garfield = Cat()
milu = Dog()
milka = Cow()

Let’s create some animals

def make_sound(animal):
if isinstance(animal, Duck):
animal.quack()
elif isinstance(animal, Cat):
animal.meow()
elif isinstance(animal, Dog):
animal.bark()
else:
raise NotImplementedError("Unknown animal")

>>> make_sound(donald)
quack
>>> make_sound(garfield)
meow
>>> make_sound(milu)
bark
>>> make_sound(milka)
File "<stdin>", line 1, in <module>
File "<stdin>", line 11, in make_sound
NotImplementedError: Unknown animal

Let’s play animals’ sounds

import functools

@functools.singledispatch
def make_sound(animal):
raise NotImplementedError("Unknown animal")

@make_sound.register(Duck)
def make_duck_sound(animal):
animal.quack()

@make_sound.register(Cat)
def make_cat_sound(animal):
animal.meow()

@make_sound.register(Dog)
def _(animal):
animal.bark()


>>> make_sound(donald)
quack

>>> make_sound(garfield)
meow

>>> make_sound(milu)
bark

>>> make_sound(milka)
File "/Library/Frameworks/Python.framework/Versions/3.4/lib/
python3.4/functools.py", line 632, in wrapper
return dispatch(args[0].__class__)(*args, **kw)
File "<stdin>", line 5, in make_sound
NotImplementedError: Unknown animal


> 

h5p://docs.python.org/dev/library/
functools.html#functools.singledispatch
> 

PEP 443: Single-‐‑dispatch generic functions
> 

> 
> 





$ pyvenv /tmp/test_venv

$ ls /tmp/test_venv/
bin
include
lib
pyvenv.cfg

$ ls /tmp/test_venv/bin/
activate
activate.csh activate.fish python
python3
python3.4

$ cat /tmp/test_venv/pyvenv.cfg
home = /Library/Frameworks/Python.framework/Versions/3.4/bin
include-system-site-packages = false
version = 3.4.0

$ pyvenv /tmp/test_venv --symlinks --clear --system-site-packages

$ cat /tmp/test_venv/pyvenv.cfg
home = /Library/Frameworks/Python.framework/Versions/3.4/bin
include-system-site-packages = true
version = 3.4.0

$ ls -lAhtr /tmp/test_venv/bin/python3.4
lrwxr-xr-x 1 pev wheel
63B 5 nov 00:21 /tmp/test_venv/bin/python3.4 -> /
Library/Frameworks/Python.framework/Versions/3.4/bin/python3.4


$ source /tmp/test_venv/bin/activate

(test_venv) ...$ which python
/tmp/test_venv/bin/python

(test_venv) ...$ which python3.4
/tmp/test_venv/bin/python3.4

(test_venv) ...$ python
Python 3.4.0a4 (v3.4.0a4:e245b0d7209b, Oct 20 2013, 02:43:50)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.prefix, sys.base_prefix
('/private/tmp/test_venv', '/Library/Frameworks/Python.framework/Versions/3.4')
>>> sys.exec_prefix, sys.base_exec_prefix
('/private/tmp/test_venv', '/Library/Frameworks/Python.framework/Versions/3.4')

(test_venv) ...$ deactivate

$ which python
/usr/bin/python


> 

Features
> 

Supports symbolic links (recommended)

> 

Access both environment and system site-‐‑packages

> 

pyvenv.cfg environment configuration file:
> 
> 

> 

> 

home: path to original Python installation bin
include-‐‑system-‐‑site-‐‑packages: access system site-‐‑packages
after the environment site-‐‑packages
version: Python version in use

Public API to let 3rd party virtual environments customise
the environment creation
> 

Subclass EnvBuilder and redefine desired methods


> 

h5p://docs.python.org/3/library/venv.html
> 

PEP 405: Python Virtual Environments
> 

> 
> 


No oﬃcial backport, although quite similar to
virtualenv


The enum module

RED, ORANGE, GREEN = range(3) # Colors
STOPPED, RUNNING = range(2)
# Statuses

class TrafficLight:
def __init__(self, light_color, status):
assert light_color in (RED, ORANGE, GREEN), "Not a valid color"
assert status in (RUNNING, STOPPED), "Not a valid status"
self.color = light_color
self.status = status

def print_state(self):
color_name = "red" if self.color == RED else "green" if self.color ==
GREEN else "orange"
status_name = "running" if self.status == RUNNING else "stopped"
print("Traffic light is {} in color {}".format(status_name,
color_name))

>>> my_semaphore = TrafficLight(RUNNING, RED)
>>> my_semaphore.print_state()
Traffic light is stopped in color orange

Let’s implement a traﬃc light

RED, ORANGE, GREEN = range(3) # Colors
STOPPED, RUNNING = range(2)
# Statuses

class TrafficLight:
assert light_color in (RED, ORANGE, GREEN), "Not a valid color"
assert status in (RUNNING, STOPPED), "Not a valid status"

color_name = "red" if self.color == RED else "green" if self.color ==
GREEN else "orange"
status_name = "running" if self.status == RUNNING else "stopped"
color_name))

>>> STOPPED is RED and RUNNING is ORANGE
True
>>> (RUNNING + ORANGE) * GREEN
4

Let’s implement a traﬃc light

from enum import Enum

Color = Enum("Color", "red orange green")
# Colors
Status = Enum("Status", "stopped running") # Statuses

class TrafficLight:
assert light_color in Color, "Not a valid color"
assert status in Status, "Not a valid status"

color_name = "red" if self.color == Color.red else "green" if
self.color == Color.green else "orange"
status_name = "running" if self.status == Status.running else "stopped"
color_name))

my_semaphore = TrafficLight(Status.running, Color.red)

Introducing the enum module

>>> my_semaphore = TrafficLight(Status.running, Color.red)
File "<stdin>", line 3, in __init__
AssertionError: Not a valid color

>>> my_semaphore = TrafficLight(Color.red, Status.running)
Traffic light is running in color red

>>> Status.stopped is Color.red
False
>>> Status.running is Color.orange
False

>>> (Status.running + Color.orange) * Color.green
TypeError: unsupported operand type(s) for +: 'Status' and 'Color'

Introducing the enum module

class Color(Enum): # enumaration
red = 0
orange = 10 # enumeration members
green = 20
# custom non-consecutive values
ruby = 0
# aliases

>>> print(Color.green)
Color.green
>>> print(repr(Color.green))
<Color.green: 20>

>>> print(repr(Color.ruby))
# this is an alias
<Color.red: 0>

>>> print(Color.orange.name)
# we can retrieve a member name
orange
>>> print(Color["orange"])
# we can get the enum member from its name
Color.orange

>>> print(Color.orange.value) # we can retrieve a member value
10
>>> print(Color(10))
# we can get the enum member from its value
Color.orange

More about enum module

class Color(Enum):
red = 0
orange = 10
green = 20

class Status(Enum):
running = 0
stopped = 1

class TrafficLight:
assert light_color in Color, "Not a valid color"
assert status in Status, "Not a valid status"

print("Traffic light is {} in color {}".format(self.status.name,
self.color.name))

>>> my_semaphore = TrafficLight(Color.red, Status.running)
Traffic light is running in color red


> 

And still more features
> 

Iteration over enumeration members
> 

> 

Ordered dictionary __members__ special a5ribute

Define methods and class methods
> 

Even redefine __str__ or __repr__

> 

IntEnum to preserve old int behaviour

> 

@enum.unique to enforce no aliases are defined

> 

Possibility to define other custom Enum sublcasses

> 

Functional API


> 

h5p://docs.python.org/3.4/library/enum.html
> 

PEP 435: Adding an Enum type to the Python
standard library
> 

> 

> 



Standard Library updated to use enums
> 
> 

> 

IntEnum when backwards-‐‑compatibility broken
Enum in new developments and internal modules

Oﬃcial backport available in PyPi
> 

h5ps://pypi.python.org/pypi/enum34/

The enum module

The asyncio module

> 

Python’s asynchronous IO Support Rebooted

> 

No new language features, no new lib extensions

> 

Based on a customisable event loop
> 

Interface to manage the event loop and its policies

> 

Interface to implement or adapt event loops
> 

Diﬀerent 3rd party async frameworks could interoperate

> 

Coroutines, futures and tasks

> 

Transports and protocols

> 

Use yield from to delegate or callbacks

The asyncio module (aka. Tulip)

> 

h5p://docs.python.org/3.4/library/enum.html
> 

PEP 3156: Asynchronous IO Support Rebooted:
the "ʺasyncio"ʺ Module
> 


> 
> 

> 

Reference implementation in Python 3.4
Expected ﬁnal status in Python 3.5

Backport to 3.3 available in PyPi
> 

h5ps://pypi.python.org/pypi/asyncio/0.1.1

The asyncio module

> 

unicode, unicode everywhere

> 

generators, generators everywhere
> 

or iterators

> 

uniNest.mock (port of Michael Foord’s mock)

> 

contextlib.ExitStack and contextlib.ContextDecorator

> 

Be5er GIL (but still Global Interpreter Lock)

> 

concurrent.futures for async computation

> 

Qualiﬁed name for classes and functions

> 

selectors module

> 

…

And still lots of new features

Q&A

Are we done yet?

Q&A

Are we done yet?
> 

Not yet
> 

Only 22 slides missing
> 

16 only with pictures, and even 1 animated GIF
> 

Sorry, no ki5ens

Q&A

Should we move now to
Python 3?

Q&A

Should we move now to
Python 3?
> 

YES
> 

Well, maybe NO
> 

Well, let me explain

Q&A

h5p://www.python.org/download/releases/2.7/

Release schedule:
>  …
>  2.7.0 ﬁnal: July 3, 2010
>  …
>  2.7.6 candidate 1: October 26, 2013
>  ???
>  2.7.X end of support: ≈ 2016

Python 2.7

h5p://www.python.org/download/releases/3.4.0/

Release schedule:
>  3.4.0 alpha 4: October 20, 2013
>  3.4.0 beta 1 (feature freeze): November 24, 2013
>  3.4.0 beta 2: January 5, 2014
>  3.4.0 candidate 1: January 19, 2014
>  3.4.0 candidate 2: February 2, 2014
>  3.4.0 ﬁnal: February 23, 2014

Python 3.4

The long journey to Python 3

Meanwhile, most of the community

> 

Great features are waiting us in Python 3

> 

We are beyond half the planned life of Python 2.7
> 

> 

Most 3rd party dependencies already in Python 3
> 

> 

Python 2.6 is oﬃcially retired with 2.6.9 release
Or we could return to OSS all that we have been given

Porting to Python 3 is not such a big deal
> 

In most of the cases

> 

So, no more excuses

> 

It’s time to start moving to Python 3

Conclusions

Conclusions

Thanks for coming!

Slides: h5p://goo.gl/Yrzt0Q
Code: h5p://goo.gl/V9bv72

Q&A

Python: the coolest is yet to come

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