Dictionaries¶

Dictionaries are unordered, associative arrays. They consist of key/value pairs. They are very versatile data structures, but more difficult to use than lists if you are new to Python. As the name implies, dictionaries are good for looking up things, or searching in general.

Creating dictionaries¶

Python dictionaries are defined with winged brackets. On the left side of each entry is the key, on the right side the value:

ratios = {
    'Alice': 0.75,
    'Bob': 0.55,
    'Charlie': 0.80
    }

Accessing elements in dictionaries¶

By using square brackets and a key, you can retrieve the values from a dictionary. At least if the key is present:

ratios['Alice']    # 0.75
ratios['Ewing']    # KeyError!

Retrieving values in a fail-safe way:¶

With the get() method you can assign an alternative value if the key was not found.

ratios.get('Alice')
ratios.get('Ewing', 'sorry not found')

Changing values in a dictionary¶

The contents of a dictionary can be modified. For instance if you start with an empty dictionary:

persons = {}

Now you can add values one key/value pair at a time:

persons['Emily'] = 1977

Setting values only if they dont exist yet:¶

persons.setdefault('Alice', 1980)
persons.setdefault('Emily', 1898)
# for 'Emily', nothing happens

Getting all keys or values:¶

ratios.keys()
ratios.values()
ratios.items()

Checking whether a key exists¶

The in operators checks whether a key exists in the dictionary.

if 'Bob' in ratios:
    print('found it')

Note that you can use in for the same with a list as well. The dictionary is much faster!

Loops over a dictionary¶

You can access the keys of a dictionary in a for loop.

for name in ratios:
    print(name)

However, there is no stable order unless you sort the keys explicitly:

for name in sorted(ratios):
    print(name)

What data can I use as keys?¶

Valid types for keys are:

integers
floats
strings
tuples
booleans

You may mix keys of different type in one dictionary. However, mutable data types such as lists and other dictionaries are not allowed as keys.

The concept behind this phenomenon is that dictionaries use a hash function to sort the keys internally. The hash function is what allows to look up values very quickly.