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19 - Dictionaries

In the last lesson we learned about sequenced iterables. In this lesson we will learn about one of the most powerful iterables that does not exist in a sequence. This is called a dictionary. The following lesson will introduce and illustrate how to use a dictionary.

# dictionary_practice.py

# curly braces or just "braces" define a dictionary
player_data = {"Name":"Jack", "Team":"Wildcats", "Wins":3}

# we subscript with a keyword instead of an index,
print(player_data["Name"])
print(player_data["Team"])
print(player_data["Wins"],"\n")

# a dictionary is an iterable
for key in player_data:
    print(key, ":", player_data[key])

print("")

# another possibly better way to do the same thing
for key, value in player_data.items():
    print(key, ":", value)

print("")

# dictionaries are mutable and extendable
player_data["Wins"]  += 1
player_data["Height"] = "6'2\""

for key, value in player_data.items():
    print(key, ":", value)

print("")

# you can also delete items too
del player_data["Height"]

for key, value in player_data.items():
    print(key, ":", value)

Here is what should happen

$ python dictionary_practice.py
Jack
Wildcats
3 

Name : Jack
Team : Wildcats
Wins : 3

Name : Jack
Team : Wildcats
Wins : 3

Name : Jack
Team : Wildcats
Wins : 4
Height : 6'2"

Name : Jack
Team : Wildcats
Wins : 4
$

Dictionaries

A dictionary is an iterable and also a mapping type, that is, a data type that takes one kind of object and connects it to another. In other programming languages this concept of a mapping type is called a "hash map" or a "hash table". Dictionaries are nothing short of magic in programming for their efficiency and usefulness. A dictionary can be like a mini database or a convenient way to describe almost any object.

As we saw above, dictionaries can be thought of as a list of unique keys that, when passed in the subscript operator, return a corresponding value. These key-value pairs allow a data structure that acts like a literal dictionary. This short lesson is only do introduce dictionaries but there are many things you can do with them.

Hone Your Skills

  • Look up the documentation on Mapping Types (dictionaries) and experiment with all the dictionary methods. Think about how you might go about making a dictionary that can turn English into Python code.

Advanced Mastery

  • Search the Internet and learn about hash maps and how they work. From what you learn, why are dictionaries/hash maps so efficient? How can you use this to make your code better?

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