7/25/2019 05-Dictionaries
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 [3]:
my_dict = {'key1':123,'key2':[12,23,33],'key3':['item0','item1','item2']}
In [4]:
# Let's call items from the dictionary
my_dict['key3']
Out[4]:
['item0', 'item1', 'item2']
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In [5]:
# Can call an index on that value
my_dict['key3'][0]
Out[5]:
'item0'
In [6]:
# Can then even call methods on that value
my_dict['key3'][0].upper()
Out[6]:
'ITEM0'
We can affect the values of a key as well. For instance:
In [7]:
my_dict['key1']
Out[7]:
123
In [8]:
# Subtract 123 from the value
my_dict['key1'] = my_dict['key1'] - 123
In [9]:
#Check
my_dict['key1']
Out[9]:
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 [10]:
# Set the object equal to itself minus 123
my_dict['key1'] -= 123
my_dict['key1']
Out[10]:
-123
We can also create keys by assignment. For instance if we started off with an empty dictionary, we could
continually add to it:
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In [11]:
# Create a new dictionary
d = {}
In [12]:
# Create a new key through assignment
d['animal'] = 'Dog'
In [13]:
# Can do this with any object
d['answer'] = 42
In [14]:
#Show
d
Out[14]:
{'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 [15]:
# Dictionary nested inside a dictionary nested inside 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 [16]:
# Keep calling the keys
d['key1']['nestkey']['subnestkey']
Out[16]:
'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 [17]:
# Create a typical dictionary
d = {'key1':1,'key2':2,'key3':3}
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In [18]:
# Method to return a list of all keys
d.keys()
Out[18]:
dict_keys(['key1', 'key2', 'key3'])
In [19]:
# Method to grab all values
d.values()
Out[19]:
dict_values([1, 2, 3])
In [20]:
# Method to return tuples of all items (we'll learn about tuples soon)
d.items()
Out[20]:
dict_items([('key1', 1), ('key2', 2), ('key3', 3)])
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.
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