Dictionary methods and operators

Python Dictionary comes with a slew of methods that we can use. The most common is len to return the number of elements, and str which will return a printable version of the dict.

Quick Summary

Here is a list of the more common methods that you might use with a dict,

• dict.clear() -> Removes all elements of dictionary dict
• dict.pop(key) -> Removes the element from the dictionary
• dict.items() -> Returns a list of dict’s (key, value) tuple pairs
• dict.keys() -> Returns list of dictionary dict’s keys
• dict.values() -> Returns list of dictionary dict’s values

in operator

The in operator is really useful for a dictionary. It returns true or false depending on whether a key exists in the dictionary. The example below illustrates this.

myData = {
    'Name': 'Zara', 
    'Age': 7, 
    'Class': 'First'
    }

if 'Age' in myData:
    print('You have age data')
else:
    print('Age data missing')

Note you can also use the not in operator as well - to see if a key is not in a dictionary!

Dictionary - Guided Exercise

In an earlier chapter on lists we followed a guided exercise on synchronized lists.

students = ['brent', 'paul', 'andrea']
grades   = ['High Distinction', 'Pass', 'Credit']

How do we get the following output?

We want to follow on from this exercise but use a combination of a dictionary and list to describe our data.

Solution

First step is figuring out what our data is and how we are going to map it. This is how I envisage the structure.

• first name and grade which can be combined into a dictionary
• a list that can contains many of the dictionary above.

A first go will look something like this:

students = [
    {
    'firstname': 'brent',
    'grade': 'High Distinction'
    },
    {
    'firstname': 'paul',
    'grade': 'Pass'
    },
    {
    'firstname': 'andrea',
    'grade': 'Credit'
    }
]

print(students)

Looping thru a list with a dictionary

Looping thru a list with a dictionary is the same as looping thru a list with any other data type. So we can do this:

students = [
    {
    'firstname': 'brent',
    'grade': 'High Distinction'
    },
    {
    'firstname': 'paul',
    'grade': 'Pass'
    },
    {
    'firstname': 'andrea',
    'grade': 'Credit'
    }
]

for student in students:
    print(student['firstname'], 'has a grade of', student['grade'])