Dictionaries

We've been learning about sequences in Python but now we're going to switch gears and learn about mappings in Python. If you're familiar with other languages you can think of these Dictionaries as hash tables.

This section will serve as a brief introduction to dictionaries and consist of:

1.) Constructing a Dictionary
2.) Accessing objects from a dictionary
3.) Nesting Dictionaries
4.) Basic Dictionary Methods

So what are mappings? Mappings are a collection of objects that are stored by a key, unlike a sequence that stored objects by their relative position. This is an important distinction, since mappings won't retain order since they have objects defined by a key.

A Python dictionary consists of a key and then an associated value. That value can be almost any Python object.

Constructing a Dictionary

Let's see how we can construct dictionaries to get a better understanding of how they work!



In [1]:

    
# Make a dictionary with {} and : to signify a key and a value
my_dict = {'key1':'value1','key2':'value2'}



In [2]:

    
# Call values by their key
my_dict['key2']









    Out[2]:





'value2'

Its important to note that dictionaries are very flexible in the data types they can hold. For example:



In [13]:

    
my_dict = {'key1':123,'key2':[12,23,33],'key3':['item0','item1','item2']}



In [4]:

    
#Lets call items from the dictionary
my_dict['key3']









    Out[4]:





['item0', 'item1', 'item2']



In [5]:

    
# Can call an index on that value
my_dict['key3'][0]









    Out[5]:





'item0'



In [7]:

    
#Can then even call methods on that value
my_dict['key3'][0].upper()









    Out[7]:





'ITEM0'

We can effect the values of a key as well. For instance:



In [14]:

    
my_dict['key1']









    Out[14]:





123



In [15]:

    
# Subtract 123 from the value
my_dict['key1'] = my_dict['key1'] - 123



In [16]:

    
#Check
my_dict['key1']









    Out[16]:





0

A quick note, Python has a built-in method of doing a self subtraction or addition (or multiplication or division). We could have also used += or -= for the above statement. For example:



In [17]:

    
# Set the object equal to itself minus 123 
my_dict['key1'] -= 123
my_dict['key1']









    Out[17]:





-123

We can also create keys by assignment. For instance if we started off with an empty dictionary, we could continually add to it:



In [21]:

    
# Create a new dictionary
d = {}



In [22]:

    
# Create a new key through assignment
d['animal'] = 'Dog'



In [24]:

    
# Can do this with any object
d['answer'] = 42



In [25]:

    
#Show
d









    Out[25]:





{'animal': 'Dog', 'answer': 42}

Nesting with Dictionaries

Hopefully you're starting to see how powerful Python is with its flexibility of nesting objects and calling methods on them. Let's see a dictionary nested inside a dictionary:



In [26]:

    
# Dictionary nested inside a dictionary nested in side a dictionary
d = {'key1':{'nestkey':{'subnestkey':'value'}}}

Wow! That's a quite the inception of dictionaries! Let's see how we can grab that value:



In [29]:

    
# Keep calling the keys
d['key1']['nestkey']['subnestkey']









    Out[29]:





'value'

A few Dictionary Methods

There are a few methods we can call on a dictionary. Let's get a quick introduction to a few of them:



In [30]:

    
# Create a typical dictionary
d = {'key1':1,'key2':2,'key3':3}



In [35]:

    
# Method to return a list of all keys 
d.keys()









    Out[35]:





['key3', 'key2', 'key1']



In [36]:

    
# Method to grab all values
d.values()









    Out[36]:





[3, 2, 1]



In [33]:

    
# Method to return tuples of all items  (we'll learn about tuples soon)
d.items()









    Out[33]:





[('key3', 3), ('key2', 2), ('key1', 1)]

Hopefully you now have a good basic understanding how to construct dictionaries. There's a lot more to go into here, but we will revisit dictionaries at later time. After this section all you need to know is how to create a dictionary and how to retrieve values from it.