Working with Dictionaries

What is a Python dictionary?

A dictionary is a data structure that lets you associate a key with a value

For example, you could assign a name (key) to a telephone number (value)

In Python, you can do it like this:

>>> tel_nums = {'Bob':'555-1111', 'Mary':'555-2222'}

>>> tel_nums

{'Bob':'555-1111', 'Mary':'555-2222'}

In this example, tel_nums is a dictionary with 2 items

An item is a key-value pair separated by a colon

We can refer to values in the dictionary by their keys like this:

>>> tel_nums['Bob']

'555-1111'

>>> tel_nums['Mary']

'555-2222'

So what?

If you know the key of the value you�re looking for, you can often find it much faster using a dictionary than by some other method

Let�s assume you had a list set up like this to store names and numbers:

>>> tel_num_list = ['Bob', '555-1111', \

�� �Mary', '555-2222', 'Jill', '555-1234']

>>> tel_num_list

['Bob', '555-1111', 'Mary', '555-2222', 'Jill', '555-1234']

How might you find Jill�s phone number?

You could search through the list for Jill�s name, then select the next item

Try this in Sublime:

def getPhoneNum(theList, name):

� pos = 0

� for n in theList:

��� pos += 1

��� if n == name:

������ print 'found ' + name + '\'s phone number at pos ' + str(pos)

������ return theList[pos]

tel_num_list = ['Bob', '555-1111', 'Mary', '555-2222', \

'Jill', '555-1234']

print 'Mary: ', getPhoneNum(tel_num_list, 'Mary')

I get the following output:

Mary:� found Mary's phone number at pos 3

555-2222

This works but is difficult to understand and will take a lot longer to perform on large data sets�use the dictionary approach instead!!

Using dictionaries as keyword arguments to functions (kwargs)

The code here makes use of dictionaries as keyword argument lists (and other argument types as well)

Professional python programmers make extensive use of the **kwargs technique

Here is a typical use of kwargs in an imported library module (the acorr() function in the pyplot library)

Hash codes

Dictionaries implement hash codes�a fundamental type of algorithm for classifying a value with a key

You can read about hash codes here and in many other places

This link refers specifically to Java but the idea of a hash code is generic to all languages