Dynamic programming with dictionaries

Completed

In your program, you want to perform various calculations, like totaling the number of moons. Additionally, as you get into more advanced programming, you might find that you're loading this type of information from files or a database, rather than coding directly into Python.

To help support these scenarios, Python enables you to treat both the keys and values inside of a dictionary as a list. You can dynamically determine keys and values, and perform various calculations.

Imagine a dictionary storing monthly rainfall amounts. You would likely have keys for each month and the associated rainfall. You want to add up the total rainfall, and writing the code to perform the operation by using each individual key would be rather tedious.

Retrieve all keys and values

The keys() method returns a list object that contains all the keys. You can use this method to iterate through all items in the dictionary.

Imagine you have the following dictionary, storing the last three months of rainfall.

rainfall = {
    'october': 3.5,
    'november': 4.2,
    'december': 2.1
}

Let's say you want to display the list of all rainfall. You can type out the name of each month, but that would be tedious.

for key in rainfall.keys():
    print(f'{key}: {rainfall[key]}cm')

october: 3.5cm
november: 4.2cm
december: 2.1cm

Note

You can still use square brackets ([ ]) with a variable name, rather than the hard-coded string literal.

Determine if a key exists in a dictionary

When you update a value in a dictionary, Python will either overwrite the existing value or create a new one, if the key doesn't exist. If you wish to add to a value rather than overwriting it, you can check to see if the key exists by using in. For example, if you want to add a value to December or create a new one if it doesn't exist, you can use the following:

if 'december' in rainfall:
    rainfall['december'] = rainfall['december'] + 1
else:
    rainfall['december'] = 1

# Because december exists, the value will be 3.1

Retrieve all values

Similar to keys(), values() returns the list of all values in a dictionary without their respective keys. values() can be helpful when you're using the key for labeling purposes, such as the preceding example, in which the keys are the name of the month. You can use values() to determine the total rainfall amount:

total_rainfall = 0
for value in rainfall.values():
    total_rainfall = total_rainfall + value

print(f'There was {total_rainfall}cm in the last quarter.')

There was 10.8cm in the last quarter.