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Quick Reference of Python

Last modified: Aug, 2016

This page is for Python in general. There are also specific pages for numpy, scipy and matplotlib.

Infinity number in Python

Use this

float("inf")

Sort a list of list/tuples by certain position

Below is an example of sorting a list of tuples by the 2nd item

>>> from operator import itemgetter
>>> data = [('a', 10),('b', 7),('c', 1), ('d',9)]
>>> sorted(data,key=itemgetter(1))
[('c', 1), ('b', 7), ('d', 9), ('a', 10)]

Reference:

Virtualenv

A virtualenv environment will have access to global (system-level) Python packages outside but keep the local packages separately. So it is a good idea to minimize the installation of global packages for minimal dependency hell.

To create and activate a virtualenv environment on Linux:

sudo apt-get install python-virtualenv  # works on Ubuntu/Debian
virtualenv my_env
source my_env/bin/activate

After this, which python and which pip should give local Python and Pip instances.

To create and activate a virtualenv environment on Windows with Anaconda which comes with virtualenv by default) and MINGW shell:

virtualenv my_env
source my_env/Scripts/activate

(Optional) Upgrade pip first:

pip install --upgrade pip

Installing with the “requirements” feature of Pip can be useful when configuring a new virtualenv container:

env/bin/pip install -r requirements.txt

Refer to Pip user guide and requirement file format for more information.

Get JSON data after certain inquiry:

import urllib2
import simplejson

# Search Kickstarter with a keyword
url = 'http://www.kickstarter.com/projects/search.json?search=&term=USB'
res = urllib2.urlopen(url) 
data = simplejson.load(res)
print data # will be a dictionary

Generators

This is such a good article that explained in an out of generators and keyword yield in Python.

Another highly upvoted StackOverflow post on what Python generator is and what keyword yield does.

Traps

Some “traps” below seem “naive” but caused lots of head-scratching

When testing the existence of a key in a dictionary, do:

if key in dictionary:

but not this

if key in dictionary.keys():

because the latter will generate an intermediate array and can be very slow if the dictionary has lots of keys.