dictionaries.md

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:

:::python
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:

:::python
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.

:::python
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:

:::python
persons = {}

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

:::python
persons['Emily'] = 1977

Setting values only if they dont exist yet:

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

Getting all keys or values:

:::python
ratios.keys()
ratios.values()
ratios.items()

Checking whether a key exists

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

:::python
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.

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

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

:::python
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.