Intro to Python 2: Lists & Dictionaries

Week 1 | Lesson 1.4

LEARNING OBJECTIVES

After this lesson, you will be able to:

• Create lists and operate on them
• Create dictionaries and operate on them

STUDENT PRE-WORK

Before this lesson, you should already be able to:

• Define/describe/explain a list and a dictionary

STARTER CODE

Code Along

Dict Practice List Practice Bonus Starter

LESSON GUIDE

Introduction: Lists and dictionaries (10 mins)

Lists are lists of arbitrary items in python.

Lists are:

• expressed with square brackets: [ ]
• indexed starting at 0
• "mutable", meaning they can be modified such as appended to
• can contain any type of python type
• can contain a mixture of types

Examples:

numbers = [1, 2, 3, 4, 5]
letters = ['a','b','c','d']
mixture = ['hi',12.13,1,(1,2),['a','b','c']]

Dictionaries contain multiple items, like lists, but are organized in key:value pairs

Dictionaries are:

• expressed with curly brackets: { }
• follow the pattern: {key1:value1, key2:value2, ...}
• indexed by their keys rather than numeric index
• are mutable
• can contain mixtures of key and value types

Examples:

telephone_nums = {'john':5103315523, 'andrew':4156678890}
lists = {0:[1,2,3], 1:[4,5,6]}
mixture = {'animals':['dog','cat'], 1:2, empty_dicts:{'a':{}, 'b':{}}}

Lists and Dictionaries

Demo / Guided Practice: Lists (25 mins)

In nearly all cases lists are preferable to tuples because we can change their values. Let's practice creating and modifying python lists

Creating lists

Lists are defined like tuples. Say you have 5 friends called Curly, Moe, Larry, Tweedle Dee and Tweedle Dumb. Create a list named "friends" with the 5 friends.

Lists can be assigned to variables. Make the list of friends and assign it to a variable named "friends"

Recall that lists are indexed with integers.

You can use two indices separated by a colon to access a range of elements within a list. For example: my_list[0:5] would print out the first 5 elements of my_list.

Modifying lists

Lists have pre-defined functions which are very useful for manipulating them. The build in functions of list work in place, meaning that you do not have to assign the result to a new variable.

The .append() function will add an element to the end of a list. Use the function like: [...].append(item)

You can also combine lists by simply adding them together with the + operator.

The .remove() function can remove an specific element of a list, and the del command will remove the item of a list at a specific index. For example, del my_list[2] removes the 3rd element of my_list.

More information about lists

Independent Practice: Lists (10 minutes)

Explore the built in functions of lists:

• use .append(item) to add item to a list
• use .extend([item1, item2]) to join another list to the end of the list
• use .insert(index, item) to add an item to a list at an index
• use .remove(item) to remove an item from a list
• use .sort() to sort a list
• use .count(item) to count the number of times an item appears in a list
• use .index(item) to find the index of an element in a list
• use .pop() to extract (and remove) the last element of a list
• use .reverse() to reverse a list

Demo / Guided Practice: Dictionaries (25 mins)

Dictionaries are useful when you want to associate items with a specific reference, or key. An example of this would be storing phone numbers of your friends with the key values as your friends' names.

Remember that dictionaries are created with curly brackets, using {key:value, key2:value2} syntax.

One potentially confusing aspect of dictionaries is that although they are created with the curly brackets { }, accessing a value with a key is done using square brackets [ ], for example: my_dict['key'] returns the value for "key" in my_dict.

Making and using dictionaries

Dictionaries are created the same as lists, with assignment to a variable. Create a dictionary called "zipcodes" with the following district:zip code pairs:

'sunset':94122 'presidio':94129 'soma':94105 'marina':94123

As you can see, when you print the dictionary, the key:value pairs are not guaranteed to be ordered the way you entered them like a list.

Values for a key can be accessed using the square bracket [ ] syntax or with the .get() method.

You can list the current keys in a dictionary with the .keys() function. The keys will be returned in a list.

The .items() function will create a list of tuples, where each of the tuples in the list is a key:value pair in the dictionary.

Modifying dictionaries

Adding new key:value pairs to a dictionary is similar to how you access an existing entry. The syntax for adding a new entry is: my_dict['new_key'] = new_value.

Removing a dictionary key:value pair can be done with the del command, like with a list, or with the .pop() function which will remove a key from the dictionary and return the value for that key.

Independent Practice: Dictionaries (10 minutes)

Build your own dictionary with key:value pairs of your choosing.

Explore some of the built in functions for dictionaries:

• use .pop(key) to remove a key:value pair from the dictionary and return the value
• use .get(key) to get the value for a key
• use .has_key(key) to check if a key is in the dictionary
• use .keys() to get a list of the keys in the dictionary
• use .items() to get a list of the key:value pairs in the dictionary
• use .update(other_dictionary) to merge a 2nd dictionary into the current dictionary
• use .clear() to remove all key:value pairs from the dictionary

Conclusion (10 mins)

• Partner up and explain what a list and a dictionary are to your partners
• What are the differences between a list and a dictionary?
• What are the two things you need for a dictionary?

Bonus Challenges

Once you've mastered the basics, further your understanding of Python by attempting "Alternate Code Challenges 2".