Dictionary Manipulation in Python

Overview

A dictionary is a collection of key-value pairs.

A dictionary is a set of key:value pairs.

All keys in a dictionary must be unique.

In a dictionary, a key and its value are separated by a colon.

The key, value pairs are separated with commas.

The key & value pairs are listed between curly brackets ” { } ”

We query the dictionary using square brackets ” [ ] ”

Dictionary Manipulation

Dictionaries are useful whenever you have to items that you wish to link together,
and for example storing results for quick lookup.

Create an empty dictionary

months = {}

Create a dictionary with some pairs

# Note: Each key must be unique

months = { 1 : "January", 
     	2 : "February", 
    	3 : "March", 
        4 : "April", 
     	5 : "May", 
     	6 : "June", 
    	7 : "July",
        8 : "August",
     	9 : "September", 
    	10 : "October", 
        11 : "November",
    	12 : "December" } 

months[1-12] are keys and “January-December” are the values

Print all keys

print "The dictionary contains the following keys: ", months.keys()

Output:

The dictionary contains the following keys: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
11, 12]

Accessing

To get a value out of a dictionary, you must supply its key, you cannot provide
the value and get the key

whichMonth = months[1]
print whichMonth

Output: January

To delete an element from a dictionary, use del

del(months[5])
print months.keys()

Output:
[1, 2, 3, 4, 6, 7, 8, 9, 10, 11, 12]

To add a new element to a dictionary, assign a value to a new key

months[5] = "May"
print months.keys()

Output:
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

To update an element of a dictionary, assign a new value to its key

months[1] = "Jan"
print months

Output:
{1: ‘Jan’, 2: ‘February’, 3: ‘March’, 4: ‘April’, 5… }

Sorting

sortedkeys = months.keys()
print sortedkeys

Output:
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

Dictionaries and Loops

Iterating over keys

for key in months:
    print key, months[key]

Output:
1 January
2 February
3 March
4 April
5 May
6 June
7 July
8 August
9 September
10 October
11 November
12 December

Iterating over (key, value) pairs

for key, value in months.iteritems():
    print key, value

print "The entries in the dictionary are:"
for item in months.keys():
    print "months[ ", item, " ] = ", months[ item ]

Combining List and Dictionary

Example of a list of dictionaries

customers = [{"uid":1,"name":"John"},
    {"uid":2,"name":"Smith"},
           {"uid":3,"name":"Andersson"},
            ]
print customers

Output:
[{‘uid’: 1, ‘name’: ‘John’}, {‘uid’: 2, ‘name’: ‘Smith’}, {‘uid’: 3, ‘name’:
‘Andersson’}]

Print the uid and name of each customer

for x in customer:
    print x["uid"], x["name"]

Output:
1 John
2 Smith
3 Andersson

Modify an entry

This will change the name of customer 2 from Smith to Charlie

customers[2]["name"]="charlie"
print customers

Output:
[{‘uid’: 1, ‘name’: ‘John’}, {‘uid’: 2, ‘name’: ‘Smith’}, {‘uid’: 3, ‘name’:
‘charlie’}]

Add a new field to each entry

for x in customers:
    x["password"]="123456" # any initial value

print customers

Output:
[{‘password’: ‘123456’, ‘uid’: 1, ‘name’: ‘John’}, {‘password’: ‘123456’, ‘uid’:
2, ‘name’: ‘Smith’}, {‘password’: ‘123456’, ‘uid’: 3, ‘name’: ‘Andersson’}]

Delete a field

del customers[1]
print customers

Output:
[{‘uid’: 1, ‘name’: ‘John’}, {‘uid’: 3, ‘name’: ‘Andersson’}]

Delete all fields

# This will delete id field of each entry.
for x in customers:
    del x["id"]

Output:
[{‘name’: ‘John’}, {‘name’: ‘Smith’}, {‘name’: ‘Andersson’}]

For more information about Dictionary, please see this article.