Objectives

By the end of this chapter, you should be able to:

• Create a Python dictionary
• Access values in an dictionary
• Modify the values stored in a dictionary
• Determine if a key exists in a dictionary
• Use common dictionary methods

Dictionaries

Dictionaries in Python are key-value pairs. To access a value in a dictionary, you pass the key in using square brackets.

There are a few different ways to create a dictionary. One is to use curly braces, separating key-value pairs by a comma, and placing a colon between the key and the value:

authors = {
    'Great Gatsby': 'F Scott Fitzgerald',
    'Slaughterhouse Five': 'Kurt Vonnegut',
    'Of Mice and Men': 'John Steinbeck'
}

authors['Great Gatsby'] == 'F Scott Fitzgerald' # True

Another approach is to use the dict function. In this case, you can assign values to keys directly using =:

another_dictionary = dict(key = 'value')
another_dictionary # {'key': 'value'}
another_dictionary['key'] == 'value' # True
another_dictionary['another_key'] # KeyError

Note that if you try to access a value with a key that doesn't exist in the dictionary, Python will throw an error.

As with lists, we can reassign values to existing keys by using =. We can also use this to create new key-value pairs:

another_dictionary = dict(key = 'value')
another_dictionary['key'] = 'new value'
another_dictionary['another_key'] = 'another value'
another_dictionary # {'another_key': 'another value', 'key': 'new value'}

Here are some common methods on dictionaries (in alphabetical order):

clear

Clears all the keys and values in a dictionary:

d = dict(a=1,b=2,c=3)
d.clear()
d # {}

copy

Makes a copy of a dictionary:

d = dict(a=1,b=2,c=3)
c = d.copy()
c # {'a': 1, 'b': 2, 'c': 3}
c is d # False - this really is a copy, not the original dictionary

fromkeys

Creates key-value pairs from comma separated values:

{}.fromkeys("a","b") # {'a': 'b'}
{}.fromkeys("a",[1,2,3,4,5]) # {'a': [1, 2, 3, 4, 5]}

get

Retrieves a key in an object and return None instead of a KeyError if the key does not exist:

d = dict(a=1,b=2,c=3)
d['a'] # 1
d.get('a') # 1
d['b'] # 2
d.get('b') # 2
d['no_key'] # KeyError
d.get('no_key') # None

items

Returns a list of tuples with each key-value pair:

d = dict(a=1,b=2,c=3)
d.items() # dict_items([('a', 1), ('b', 2), ('c', 3)])

keys

Returns a dict_keys object containing all of the keys in an object.

d = dict(a=1,b=2,c=3)
d.keys() # dict_keys(['a', 'b', 'c'])

pop

Takes a single argument corresponding to a key and removes that key-value pair from the dictionary. Returns the value corresponding to the key that was removed. Unlike pop on lists, you must supply an argument to the dictionary pop method or you'll get an error. You'll also get an error if you try to pop a key that doesn't exist in the dictionary.

d = dict(a=1,b=2,c=3)
d.pop() # TypeError: pop expected at least 1 arguments, got 0
d.pop('a') # 1
d # {'c': 3, 'b': 2}
d.pop('e') # KeyError

popitem

Removes a random key in a dictionary:

d = dict(a=1,b=2,c=3,d=4,e=5)
d.popitem() # ('d', 4)
d.popitem('a') # TypeError: popitem() takes no arguments (1 given)

update

Update keys and values in a dictionary with another set of key value pairs.

first = dict(a=1,b=2,c=3,d=4,e=5)
second = {}

second.update(first)
second # {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5}

# let's overwrite an existng key
second['a'] = "AMAZING"

# if we update again
second.update(first) # {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5}

# the update overrides our values
second # {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5}