Using Dictionaries

So far we dealt with some different standard data types of Python, including numerical values (integers and floats), strings and lists. Additionally there were the arrays provided by numpy. There is one last, very important data type: Dictionaries (which can be though of as hashes, if you come from another programming language).

The name Dictionary already gives you an idea of what this data type does: It stores values associated with a given key, just as a foreign language dictionary would do:

In [1]:
dictionary = {"my":"mein",
              "hovercraft":"Luftkissenboot",
              "is":"ist","full":"voll","of":"von",}
print(dictionary["hovercraft"])

Luftkissenboot

Accessing the values of a dictionary is similar to accessing a given position in a list, you pass it to the dictionary using square brackets. You can also use this to create new entries in your dictionary.

In [2]:
dictionary["eels"]

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-2-e059547cb67b> in <module>()
----> 1 dictionary["eels"]

KeyError: 'eels'

The key eels does not exist yet, this is why we are getting a key error. So we create it.

In [3]:
dictionary["eels"] = "Aale"
print(dictionary["eels"])

Aale

To initialize an empty dictionary you can use dictionary = {}. You can use strings, integers and floats as your keys for the dictionary. Your values can be basically everything. It can also be a list or even a dictionary.

In [4]:
example_dictionary = {}

example_dictionary[1.0] = "1,0"
example_dictionary[2] = 1
example_dictionary["list"] = ["yes","this","is","possible"]
example_dictionary["another_dictionary?"] = {"yes":"that","works":"as well"}

print(example_dictionary["another_dictionary?"])

{'yes': 'that', 'works': 'as well'}

Unlike lists, dictionaries are not ordered. So if you loop over them you are not guaranteed that they will always have the same order.

By default the loop will iterate over the keys.

In [5]:
for key in example_dictionary:
    print(key)

1.0
2
another_dictionary?
list

Often you want to have both, the key and value at the same time. For this you can use the dictionary function .items():

In [6]:
for key,value in example_dictionary.items():
    print(key,value)

1.0 1,0
2 1
another_dictionary? {'yes': 'that', 'works': 'as well'}
list ['yes', 'this', 'is', 'possible']

Questions / Tasks

  • How does your dictionary look like when you fill it like this: dictionary = {1.0 : "a", 1:"b"}?

  • You have two dictionaries, shopping_list and prices. Use it to calculate the total cost of your shopping trip.

shopping_list = {"banana":2,
    "apple": 4,
    "orange": 1,
    "pear": 3,
    "chocolate": 1}
prices = { "banana":0.39,
    "apple": 0.20,
    "orange": 0.30,
    "pear":0.20,
    "chocolate":0.90
    }
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