Introduction to Python

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Dictionaries: Lookup Tables

dictionaries

A dictionary (or dict) is a collection of unique key/value pairs of objects.

mydict = {}                      # empty dict

mydict = {'a':1, 'b':2, 'c':3}   # dict with str keys and int values

print(mydict['a'])               # look up 'a' to get 1

• each item is a pair
• the pair consists of a key and a value
• the keys in a dict are unique
• each key is associated with a value
• a dict is addressible by key, in the same way that a list is addressable by index

example uses: dictionaries

Pairs describe data relationships that we often want to consider:

• companies paired with annual revenue for each company
• employees paired with contact information for each employee
• students paired with grade point averages
• dates paired with the high temperature for each
• web pages paired with the number of times each was accessed

You yourself may consider data in pairs, even in your personal life:

• home projects and the amount of time you think each might take
• different items you might want to buy at the grocery and the price for each
• stores and their distance from your house
• restaurants and their ratings
• your siblings and their names
• your family members and their ages

types of dictionaries

There are a few main ways dictionaries are used:

• a lookup table: pairing clients with their addresses allows you to look up the address of any client
• a ranking: pairing companies with their market capitalizations allows you to rank them by market cap
• an aggregation: pairing each city with the number of students listed from that city allows you to see which cities have the most

initialize a dict

Dicts are marked by curly braces. Keys and values are separated with colons.

initialize a dict

mydict = {}                        # empty dict

mydict = {'a':1, 'b':2, 'c':3}     # dict with str keys and int values

add a key/value pair to a dict

We use subscript syntax to assign a value to a key.

mydict = {'a':1, 'b':2, 'c':3}

mydict['d'] = 4                 # setting a new key and value

print(mydict)                   # {'a': 1, 'c': 3, 'b': 2, 'd': 4}

retrieve a value from a dict using a key

We also use subscript syntax to retrieve a value.

mydict = {'a':1, 'b':2, 'c':3, 'd': 4}

dval = mydict['d']                 # value for 'd' is 4

xxx = mydict['c']                  # value for 'c' is 3

You might notice that this subscripting is very close in syntax to list subscripting. The only difference is that instead of an integer index we are using the dict key (most often a string).

the KeyError exception

This exception is raised when we request a key that does not exist in the dict.

mydict = {'a': 1, 'b': 2, 'c': 3}

val = mydict['d']       # KeyError:  'd'

Like the IndexError exception, which is raised if we ask for a list item that doesn't exist, KeyError is raised if we ask for a dict key that doesn't exist.

check for key membership

If we're not sure whether a key is in the dict, before we subscript we can check to confirm.

mydict = {'a': 1, 'b': 2, 'c': 3}

if 'a' in mydict:
    print("'a' is a key in mydict")