dictionary.adoc

Dictionaries

Dictionaries are sometimes found in other languages as “associative memories” or “associative arrays”. Unlike sequences, which are indexed by a range of numbers, dictionaries are indexed by keys, which can be any immutable type; strings and numbers can always be keys.

It is best to think of a dictionary as a set of key: value pairs, with the requirement that the keys are unique (within one dictionary). A pair of braces creates an empty dictionary: {}. Placing a comma-separated list of key:value pairs within the braces adds initial key:value pairs to the dictionary; this is also the way dictionaries are written on output.

Creating a Dictionary

Creating a dictionary is easy:

phonenumbers = {'Sam': 6554098, 'Jade': 6554139}

Modifying a Dictionary

Modifying a dictionary and getting values from them is very easy.

# get Sam's number
currentPhone = phonenumbers["Sam"]

# we can also update a value
phonenumbers['Jade'] = 6551439

# you can delete pairs too
del phonenumbers["Sam"]

You can add another dictionary key:value pairs.

# we can also add a new key:value pair.
phonenumbers['Sadie'] = 6551004

We can print it out.

# we can also add a new key:value pair.
print(phonenumbers)

# output is
# {'Sam': 6554098, 'Jade': 6554139, 'Sadie': 6551004}

Testing for a Key

And one of the very cooler aspects is the testing of whether or not a key is in the dictionary. You use in and not in to determine

if 'Sam' in phonenumbers: # -> True
    doSomething()

if 'Guido' not in phonenumbers: # -> False
    figureOutWhyNot()

We could use them to hold information about some object we’re holding data about.

vehicle1 = {
    'Name': "Mars Lander"
    'Altitude': 8000,
    'Speed': 1000,
    'Fuel': 12000,
}

Or this dictionary

katniss = {
    'firstname': "Katniss",
    'lastname': "Everdene",
    'homedistrict': 12,
    'skills': ["foraging", "wildlife", "hunting", "survival"]
}

Dictionaries are a tremendous tool. They are used in many, many places within Python systems. Dictionaries are built-in data types in Python that associate (map) keys to values, forming key-value pairs. You can access, add, modify, and delete key-value pairs.