K.1 Lists

Lists are a fundamental data structure in Python. They are used to store heterogeneous data and are created with a pair of square brackets, [ ].

Click here to view code image

my_list = ['a', 1, True, 3.14]
print(my_list)

['a', 1, True, 3.14]

We can subset the list using square brackets and provide the index of the item we want.

Click here to view code image

# get the first item - index 0
print(my_list[0])

a

We can also pass in a range of values (Appendix P).

# get the first 3 values
print(my_list[:3])

['a', 1, True]

We can reassign values when we subset values from the list.

# reassign the first value
my_list[0] = 'zzzzz'
print(my_list)

['zzzzz', 1, True, 3.14]

Lists are objects in Python (Appendix S), so they will have methods that they can perform. For example, we can .append() values to the list.

Click here to view code image

my_list.append('appended a new value!')
print(my_list)

['zzzzz', 1, True, 3.14, 'appended a new value!']

More about lists and their various methods can be found in the documentation.¹

1. https://docs.python.org/3/tutorial/datastructures.html#more-on-lists

K.2 Tuples

A tuple is similar to a list, in that both can hold heterogeneous bits of information. The main difference is that the contents of a tuple are “immutable”, meaning they cannot be changed. They are created with a pair of round parentheses, ( ).

Click here to view code image

my_tuple  =('a', 1, True, 3.14)
print(my_tuple)

('a', 1, True, 3.14)

Subsetting items can be accomplished in the same ways as for a list (i.e., you use square brackets).

# get the first item
print(my_tuple[0])

a

However, if we try to change the contents of an index, we will get an error.

Click here to view code image

# this will cause an error
my_tuple[0] = 'zzzzz'

TypeError: 'tuple' object does not support item assignment

More information about tuples can be found in the documentation.²

2. https://docs.python.org/3/tutorial/datastructures.html#tuples-and-sequences

K.3 Dictionaries

Python dictionaries (dict) are efficient ways of storing information. Just as an actual dictionary stores a word and its corresponding definition, a Python dict stores some key and a corresponding value. Using dictionaries can make your code more readable because a label is assigned to each value in the dictionary. Contrast this with list objects, which are unlabeled. Dictionaries are created by using a set of curly braces, { }.

my_dict = {}
print(my_dict)

{}

print(type(my_dict))

<class 'dict'>

When we have a dict, we can add values to it by using square brackets, [ ]. We put the key inside these square brackets. Usually, it is some string, but it can actually be any immutable type (e.g., a Python tuple, which is the immutable form of a Python list). Here we create two keys, fname and lname, for a first name and last name, respectively.

my_dict['fname'] = 'Daniel'
my_dict['lname'] = 'Chen'

We can also create a dictionary directly, with key–value pairs instead of adding them one at a time. To do this, we use our curly braces, { }, with the key–value pairs being specified by a colon.

Click here to view code image

my_dict = {'fname': 'Daniel', 'lname': 'Chen'}
print(my_dict)

{'fname': 'Daniel', 'lname': 'Chen'}

To get the values from our keys, we can use the square brackets with the key inside.

fn = my_dict['fname']
print(fn)

Daniel

We can also use the .get() method.

ln = my_dict.get('lname')
print(ln)

Chen

The main difference between these two ways of getting the values from the dictionary is the behavior that occurs when you try to get a nonexistent key. When using the square-bracket notation, trying to get a key that does not exist will return an error.

# will return an error
print(my_dict['age'])

KeyError: 'age'

In contrast, the .get() method will return None.

# will return None
print(my_dict.get('age'))

None

To get all the keys from the dict, we can use the .keys() method.

Click here to view code image

# get all the keys in the dictionary
print(my_dict.keys())

dict_keys(['fname', 'lname'])

To get all the values from the dict, we can use the .values() method.

Click here to view code image

# get all the values in the dictionary
print(my_dict.values())

dict_values(['Daniel', 'Chen'])

To get every key–value pair, you can use the .items() method. This can be useful if you need to loop through a dictionary.

Click here to view code image

print(my_dict.items())

dict_items([('fname', 'Daniel'), ('lname', 'Chen')])

Each key–value pair is returned in a form of a tuple, as indicated by the use of round parentheses, ( ).

More on dictionaries can be found in the official documentation on data structures.³

3. https://docs.python.org/3/tutorial/datastructures.html#dictionaries