Introduction to Python

##Python Shell Python at it's core is a fancy calculator. Typing python into the command line will give you a prompt

python

or

ipython

IPython provides a more interactive shell and provides a few nice utility functions that make life easier. For example, starting with a ?,

?sys

will give documentation for function. Also, IPython provides autocomplete as well. Additional it provides a few "magic" functions like %time and %paste, the former of which times function and the latter allows easy copy and paste of code.

##Defining functions In Python we define functions by using the def keyword. The value of the function is given in the return statement.

def addition(x, y):
	return x + y

x and y are the inputs to the function and the return keyword tells us what the function returns. The most important thing to note here is that Python respects whitespace, the second line must be indented for this to run.

##Data Structures

Python has a variety of data structures available. The most basic of them are the basic integer, float and strings. Python is dynamically typed, so types do not need to be specified and variables can be respecified to different types.

x = 5.0
x = [5.0]
x = "Five Point 0"

###Lists

Lists are one of the most common data structures in Python.

l = [1,2,3,4] #Setting up a list
l.append(5) #Adding to it using the append function
l += [6] #Adding to it using the addition operation

###Dictionaries

Dictionaries in Python are equivalent to Hash Tables in any other language.

x = {} # Empty Dictionary
y = {'key' : 'value'}
y['key'] = "new-value"

##Loop Constructs

Like any other programming language, Python has loop constructs, the ability to iterate over different elements.

l = [1,2,3,4]
for i in l:
	print i

x = [1,2,3,4]

#The following are equivalent
y = []
for i in x:
	y.append(i**2)

#OR

y = [i**2 for i in x]

Also, Python can iterate over generators, these are functions that act as iterators, meaning they give access to elements of a sequence.

For example, xrange

for i in xrange(10):
    print i

or enumerate

for i, el in enumerate(x):
   print i, el

This will print out the index of the element (starting from 0) and the element.

##Pandas Pandas is a Python library for data analysis that resembles the tools available in R. It is also optimized for fast I/O and data manipulation.

import pandas as pd

data = pd.read_csv('https://github.com/pburkard88/DS_BOS_07/Data/538model/data/2012_poll_data_states.csv', sep='\t')
#Get a preview of the first six rows
data.head()

Get descriptive statistics for any or all columns

data.describe()

Select certain columns

data[['Poll', 'Obama (D)', 'Romney (R)']]

Select a range of rows

data[:3]
data[1:3]

Bin the values of a column

data['Poll'].value_counts()

Aggregate over a particular column

data.groupby('Poll')['Obama (D)'].mean()