Lecture 4: More Python Datatypes

Eric Brauer

Introduction

This week we are introducing more iterable datatypes. Recall that iterables contain multiple elements that can be looped through with a for loop.

We will also discuss more ways of interacting with strings, and introduce many new methods for strings.

method: a special function that ‘belongs’ to a particular datatype object.

DATATYPE: List

Lists have many elements, can be combination of many datatypes
Can be changed with insert(), remove(), append() or pop() # removes last item while also returning it
Use index (starting at 0) to pick elements
**Is the first datatype we’ve used which is mutable.

Object Mutability

Most datatypes we’ve worked with are immutable. That means they cannot be changed (“mutated”) after creation.

The proof:

>>>> string_obj = 'hi'  # creates a string
>>>> print(string_obj.upper())  # returns the string but UPPER CASE
HI
>>>> print(string_obj)  # the original version is unchanged
hi
>>>> x = 3  # integers are also immutable
>>>> x + 1
4
>>>> x  # the original value of x unchanged
3
>>>> x = x + 1  # this is why we have to use x = x + 1 here, we are creating a new version of x

Lists Are Different

Lists are mutable. Using a method will change them permanently.

>>>> mylist = [ 'dog', 'cat', 'aardvark' ]
>>>> mylist.append('bat')
>>>> mylist
[ 'dog', 'cat', 'aardvark', 'bat' ]
>>>> mylist.sort()  # notice no need start with
>>>> mylist
['aardvark', 'bat', 'cat', 'dog' ]

New Datatype: Tuple

Tuples have many elements (like lists)
Tuples areimmutable (can’t be changed)
Use ()

# A Tuple of one element:
my_tup = ('capybara', )

# More:
my_tup2 = ('first', 'second')

# Can't be changed, can only be created
my_tup3 = my_tup + my_tup2

Applications of Tuples

Faster to access than lists
Data protection (subprocess.Popen)
Work for situations when something needs to be immutable

Datatype: Sets

Sets contain many elements
Cannot be sliced (no index numbers!)
Every element is unique
Can be modified, use add()
Use {}

>>>> myset = {'orange', 'blue', 'green'}
>>>> myset.add('red')
>>>> myset
{'blue', 'green', 'orange', 'red'}  # order isn't remembered, it's just a group of things

Applications Of Sets

Faster performance than a list
Often used for input validation
Consider whenever there is no inherent sort order

>>>> provs = {'AB', 'BC', 'SK', 'ON', 'QC', 'MN', 'PE', 'NL', 'NS', 'NB'}
>>>> 'AB' in provs  # 'in' will see if an item exists inside of an iterable
True
>>>> 'ON' in provs
True
>>>> 'Alaska' in provs
False

Set Combinations

Set Combinations: Union

zoo_animals = {'tiger', 'bat', 'rabbit', 'fox'}
predators = {'tiger', 'dog', 'fox', 'shark'}
print(zoo_animals | predators)
{'tiger', 'bat', 'rabbit', 'fox', 'dog', 'shark'}

Set Combinations: Intersection

zoo_animals = {'tiger', 'bat', 'rabbit', 'fox'}
predators = {'tiger', 'dog', 'fox', 'shark'}
print(zoo_animals & predators)
{'tiger', 'fox'}

Set Combinations: Difference

zoo_animals = {'tiger', 'bat', 'rabbit', 'fox'}
predators = {'tiger', 'dog', 'fox', 'shark'}
print(zoo_animals - predators)
{'bat', 'rabbit'}

Set Combinations: Symmetric Difference

zoo_animals = {'tiger', 'bat', 'rabbit', 'fox'}
predators = {'tiger', 'dog', 'fox', 'shark'}
print(zoo_animals ^ predators)
{'bat', 'rabbit', 'dog', 'shark'}

Datatype: Dictionaries

Dicts contain many elements, like lists
Assume they don’t have an order (only in recent Python!)
Instead of index numbers, we use values associated with keys
Can be modified
Use {}

>>>> mydict = {'name': 'Chris', 'age': 42}
>>>> mydict['name']
'Chris'

Datatype: Dictionaries

Dicts can be confusing, so think of it like this:

my_list_dict = {0:'cat', 1:'dog', 2:'human'}

There we go, we have made a dictionary that behaves like a list! The only difference is now we get to replace numbers with meaningful keys to access our data!

Dictionaries: Keys And Values

Dictionary keys and values can be different data types, with some caveats.
Keys are immutable, and can’t be changed. Only created or destroyed.
Values can be changed.

>>>> mydict = {'name': 'Chris', 'age': 42}
>>>> mydict['age'] = 43  # changes value for existing key
>>>> mydict['hobby'] = 'hiking'  # adds new key/value pair
>>>> mydict
{'name': 'Chris', 'age': 43, 'hobby': 'hiking'}

Iteration

You can iterate through sets and tuples, just like a list!
You can iterate through a dictionary, but it’s different:

for key in my_dict:  # iterating variable contains key
    print(key)
    print(my_dict[key])  # with the key, get the value

# another method
for k, v in my_dict.items():  # .items() will generate a tuple for each item in the dictionary
    print(k + ': ' + v)  # what is k, and what is v do you think?

String Slicing

By the way, strings are iterables as well. You can iterate through them with a for loop. They have index numbers, just like lists. Here are more ways of using index numbers:

Consider the string PYTHON

0	1	2	3	4	5	left-to-right: positive numbers
P	Y	T	H	O	N	string
-6	-5	-4	-3	-2	-1	right-to-left: negative numbers

>>>> test = 'PYTHON'
>>>> test[3:-2]
'HO'
>>>> test[-5:]  # leaving second number blank means slice includes to the end 
'YTHON'
>>>> test[:4]  # leaving first number blank includes the beginning
'PYTH'

String Methods

As discussed before, methods are functions that belong to a particular variable that you create.

>>>> x = 'hi' # create variable x, which is a string
>>>> x.upper()  # all strings contain a method called upper 
'HI'

You know you’re dealing with a method when you see them start with a dot (.).

Discovering Methods

>>>> dir(str)
'center',
'upper',
'strip',
'lower',
...

Some Useful Methods

strip: remove leading and trailing whitespace
split: convert a string into a list, by splitting on a delimiter
center: opposite of strip
join: opposite of split
replace: do a substitution

Examples

>>>> raw_data = '  this is a test \n'
>>>> raw_data.strip()
'this is a test'

>>>> raw_output = 'file001.jpg\nfile002.jpg'
>>>> raw_lines = raw_output.split('\n')
['file001.jpg', 'file002.jpg']

Examples

>>>> 'hello'.center(9)  
'  hello  '

>>>> mylist = ['toast', 'eggs']
>>>> '&'.join(mylist)
'toast&eggs'

>>>> 'color'.replace('o', 'ou')  
'coulour'