Python Tricks That You Can't Live Without

Audrey Roy
audreyr@cartwheelweb.com
http://www.audreymroy.com
@audreyr
PYCON PHILIPPINES 2012

PYTHON TRICKS THAT
YOU CAN’T LIVE
WITHOUT

ABOUT ME

• Principal at Cartwheel Web
• Massachusetts Institute of
Technology EECS (winter 2005)

• Filipina-American and very
proud to be here

ﬂickr.com/photos/chrisjrn/6102009780/

I ♥ PYTHON
• OpenComparison core dev, and contributor to
various open-source projects

• Co-founded PyLadies
• Helped organize #pyconph
• Python Software Foundation member
• I even met my fiancé Daniel Greenfeld at PyCon!
• I even met my fiancé Daniel Greenfeld at PyCon!
•
Audrey Roy
@audreyr

http://www.ﬂickr.com/photos/47628826@N05/4374285165/

OVERVIEW

• Code readability
• Linters and code checkers
• Where to find free reusable Python libraries
• How to package your code for reuse

Audrey Roy
@audreyr

CODE READABILITY
The #1 trick to being a great Python developer is
writing clear, understandable code.

CODE READABILITY

• The best Python code is compact, but not
too compact

• Write self-documenting code
• And document it anyway :)

Audrey Roy
@audreyr

CODE READABILITY
Can this be made cleaner?
def is_even(x):
if x % 2 == 0:
return True
else:
return False

CODE READABILITY
Can this be made even cleaner?
def is_even(x):
if x % 2 == 0:
return True
return False

CODE READABILITY
That’s better, but what’s missing?
def is_even(x):
return x % 2 == 0

CODE READABILITY
Don’t forget your docstrings
def is_even(x):
""" Returns True if x is even, and
False if x is odd. """

return x % 2 == 0

ZEN OF PYTHON
Keep in mind Python’s philosophy as you code.

>>> import this

The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
...

PEP8

• Python style guide
• 4 spaces. No tabs!
• Blank lines between function & class defs
• Much more...

Audrey Roy
@audreyr

TOOLS FOR CODE
READABILITY
Kind of like spell check, but for code

SUBLIME TEXT 2 + PLUGINS
SublimeLinter

Highlights lines of
code that are not
PEP8-compliant.

Catches potential
style issues or errors.

(also for CSS, JS, PHP,
Ruby, etc.)

SUBLIME TEXT 2 + PLUGINS

By the way, Sublime Text 2 plugins are
simple Python files

To write a plugin, you put a Python file in
Sublime’s “Packages” directory

PEP8.PY
A command-line PEP8 checker.

$ pep8 test2.py
test2.py:13:1: E302 expected 2 blank lines, found 1
test2.py:20:1: W391 blank line at end of file

http://pypi.python.org/pypi/pep8

PYLINT
Advanced Python source code analyzer.
$ pylint test2.py
No config file found, using default configuration
************* Module test2
C: 1,0: Missing docstring
F: 1,0: Unable to import 'django.db.models'
C: 3,0: Invalid name "compa2lookup" (should match (([A-Z_][A-
Z0-9_]*)|(__.*__))$)
C: 13,0:p_expression_ID: Invalid name "p_expression_ID" (should
match [a-z_][a-z0-9_]{2,30}$)
C: 13,0:p_expression_ID: Invalid name "p" (should match [a-z_][a-
z0-9_]{2,30}$)
C: 13,20:p_expression_ID: Invalid name "p" (should match [a-z_][a-
z0-9_]{2,30}$)
C: 18,4:p_expression_ID: Invalid name "d" (should match [a-z_][a-
z0-9_]{2,30}$)
W: 19,11:p_expression_ID: Used * or ** magic

http://pypi.python.org/pypi/pylint

PYLINT
Advanced Python source code analyzer.
Report
======
8 statements analysed.

Messages by category
--------------------
+-----------+-------+---------+-----------+
|type |number |previous |difference |
+===========+=======+=========+===========+
|convention |6 |NC |NC |
+-----------+-------+---------+-----------+
|refactor |0 |NC |NC |
+-----------+-------+---------+-----------+
|warning |1 |NC |NC |
+-----------+-------+---------+-----------+

http://pypi.python.org/pypi/pylint

FINDING PYTHON
LIBRARIES
“Free stuff for Python developers!”

FINDING CODE TO REUSE
Where to get FREE reusable Python libraries:
1. Python Standard Library
• Many great essentials, already on your system!

• http://docs.python.org/library/index.html

2. Python Package Index
• 21,000+ packages to download!

• http://pypi.python.org/

Audrey Roy
@audreyr

WHY REUSE CODE?

• Python helps you avoid reinventing the
wheel

• “Not Invented Here” syndrome

Audrey Roy
@audreyr

MORE ABOUT THE
PYTHON STDLIB

A collection of highly useful modules

• No need to install
• Just import and start using them!

Audrey Roy
@audreyr

STDLIB EXAMPLE: MATH
>>> import math
>>> math.ceil(2.03)
3.0
>>> math.floor(2.99)
2.0
>>> math.log(32,2)
5.0
>>> math.erf(0.5)
0.5204998778130465

Mathematical functions defined by the C standard

STDLIB EXAMPLE: RANDOM
>>> import random
>>> random.random()
0.12863367604888531
>>> random.uniform(0,100)
25.374019279313988
>>> math.floor(random.uniform(0,100))
77.0
>>> random.randrange(0,100)
69

MORE ABOUT PYPI

• PyPI is “Python Package Index”
• 21,000+ packages
• All created by community members like you
• http://pypi.python.org

Audrey Roy
@audreyr

PYPI EXAMPLES

• You saw some great examples already
from PyPI (Python Package Index)

• pep8: Simple PEP8 syntax checker
• pylint: Advanced source code analyzer

Audrey Roy
@audreyr

STDLIB VS. PYPI

• The stdlib is conservative
• Few additions/changes/deprecations
• On PyPI, anything goes!

Audrey Roy
@audreyr

STDLIB VS. PYPI

• Sometimes PyPI packages are better than
the equivalent stdlib ones

• e.g. requests is better than urllib2
• If in doubt, ask around

Audrey Roy
@audreyr

INSTALLING
PYTHON PACKAGES
The wrong way, and the right way

THE WRONG WAY

• Systemwide installation of Python
libraries is generally bad

• You can make a mess of your system
Audrey Roy
@audreyr

THE RIGHT WAY

You really should be using these 2 tools:

• pip - a good package installer
• virtualenv - create isolated Python envs

I strongly recommend virtualenvwrapper too.

Audrey Roy
@audreyr

THE RIGHT WAY:
VIRTUALENV
Create isolated virtualenvs for different projects.

$ workon consumer_io
(consumer_io) $ cd consumer_io/proj/
(consumer_io) $ python manage.py runserver
(consumer_io) $ ...
(consumer_io) $ deactivate
$ cd ../../experiments
$ workon experiments
(experiments) $ python somethingelse.py
(experiments) $ ...

THE RIGHT WAY: PIP

Use pip to install packages into virtualenvs.

(experiments) $ pip install Django==1.4

pip is like easy_install, but much better.

THE RIGHT WAY:
PIP+VIRTUALENV
SCENARIO:
You use Django 1.3 for work, but you want to
experiment with Django 1.4.

With pip and virtualenv, you can switch between
1.3 and 1.4 on the same computer.

PIP REQUIREMENTS FILES
You should pin your dependencies in requirements.txt!
$ pip install -r requirements.txt

# Your requirements.txt file
Flask==0.8
glue==0.2.5
Pillow==1.7.7
Django==1.4

Use pip install PackageName==1.0.4 for
experimentation only.

AFTER INSTALLATION?
Once installed, you can import Python code
from modules:
from collections import deque

Or from submodules:
from os.path import abspath

WRITING REUSABLE
CODE
How code reuse works in Python

MODULES
A module is a file containing Python
definitions and statements.

Like this:

# divisible.py

def is_even(x):
""" Returns True if x is even, and
False if x is odd. """

return x % 2 == 0

PACKAGES
A Python package is a collection of modules.
sound/
__init__.py
formats/
__init__.py
wav.py
aiff.py
effects/
__init__.py
echo.py
surround.py

PACKAGES
A sample import from this package:
from sound.formats.wav import read_wav

sound/
__init__.py
formats/
__init__.py
wav.py
aiff.py
effects/
__init__.py
echo.py
surround.py

INTRA-PACKAGE IMPORTS
Relative imports work between submodules of
a package:
from . import echo
from .. import formats
from ..filters import equalizer

INTRA-PACKAGE IMPORTS
Absolute imports work between submodules of
a package:
# Use this from anywhere in the package
from sound.effects import echo
package root

IMPORTING FROM
OUTSIDE A PACKAGE
• Can’t use absolute/relative imports
• What to do? One of these:
• Good: Add the package to PYTHONPATH
[edit env var or use sys.path.append()]

• Better: Install the package into your active
virtualenv.

Audrey Roy
@audreyr

BETTER PACKAGING

• Recommended reading: “The Hitchhiker’s
Guide To Packaging”

• http://guide.python-distribute.org
• Learn to make packages that are
downloadable & installable from PyPI

Audrey Roy
@audreyr

THANK YOU

• Find me if you have questions
• Introduce yourself - I’d love to meet you!
• Twitter: @audreyr
• Email: audreyr@cartwheelweb.com

Audrey Roy
@audreyr

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