Python Tutorial - 3 - List, Tuples, and Sets

Lists, Tuples, and Sets

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• File: list-tuple-set.md
• Project: 3_list-tuple-set
• File Created: Wednesday, 8th March 2023 8:16:17 am
• Author: Hanlin Gu (hg_fine_codes@163.com)

---

• Last Modified: Saturday, 11th March 2023 4:31:11 pm
• Modified By: Hanlin Gu (hg_fine_codes@163.com>)
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1. Lists

In python, the square brackets [] indicates a list, and the values are separated by comma ,.

courses = ['History', 'Math', 'Physics', 'CompSci']

print(courses)
print(len(courses))
print(type(courses))

Output:

['History', 'Math', 'Physics', 'CompSci']
4
<class 'list'>

1.1. Locate the Elements in the List with index

The index rule is the same as in string, such that var_name[idx] gives you the element or part of the list with index as idx in the variable var_name.

• Python counting from 0. The negative index [-num] means counting from the end. So the last item is [-1].

courses = ['History', 'Math', 'Physics', 'CompSci']

print(courses[0])
print(courses[-1])

Output:

History
CompSci

It’s convenient to use [-1] as the last item of a list, since this is always true even with the length of the list is changed i.e. adding new elements into the list.

If an index does not exist, it will return an IndexError.

print(courses[4])

Output:

Traceback (most recent call last):
  File "d:\Documents\Code\python\tutorial\list-tuple-set.py", line 11, in <module>
    print(courses[4])
IndexError: list index out of range

• Slicing

The index [start:stop] include index [start] but exclude [stop] and return as a list. If the index is [:stop], this will give you all the elements before the index [stop], excluding the stop element. If the index is [start:], this gives you all the elements starting from index [start], including the start element, all the way to the end.

courses = ['History', 'Math', 'Physics', 'CompSci']
print(courses[0:3])
print(courses[:3])
print(courses[2:])

Output:

['History', 'Math', 'Physics']
['History', 'Math', 'Physics']
['Physics', 'CompSci']

1.2. List Methods and Functions

1.2.1. Add an Element

• Add the element to the end: var_name.append()

courses = ['History', 'Math', 'Physics', 'CompSci']
courses.append('Art') 
print(courses)

Output:

['History', 'Math', 'Physics', 'CompSci', 'Art']

• Add the element to specific location in the list: var_name.insert()

.insert() takes two arguments, the first is the location, and the second is the element, i.e. var_name.insert(loc,element).

courses = ['History', 'Math', 'Physics', 'CompSci']
courses.insert(0, 'Art')
print(courses)

Output:

['Art', 'History', 'Math', 'Physics', 'CompSci']

• Multiple values to add to the list: var_name.extend()

If one still use .insert() method,

courses = ['History', 'Math', 'Physics', 'CompSci']
courses_2 = ['Art', 'Education']
courses.insert(0, courses_2)
print(courses)

Output:

[['Art', 'Education'], 'History', 'Math', 'Physics', 'CompSci']

the new values are added directly as a list in the beginning rather than adding the individual values to the list.

And .append() also does the similar operation by putting the whole list at the end.

courses = ['History', 'Math', 'Physics', 'CompSci']
courses_2 = ['Art', 'Education']
courses.append(courses_2)
print(courses)

Output:

['History', 'Math', 'Physics', 'CompSci', ['Art', 'Education']]

The correct way to add all the values in the list var_2 to the list var_1 is to use var_1.extend(var_2).

courses = ['History', 'Math', 'Physics', 'CompSci']
courses_2 = ['Art', 'Education']
courses.extend(courses_2)
print(courses)

Output:

['History', 'Math', 'Physics', 'CompSci', 'Art', 'Education']

1.2.2. Remove Items from the List

• Direct remove of an element: var_name.remove()

courses = ['History', 'Math', 'Physics', 'CompSci']
courses.remove('Math')
print(courses)

Output:

['History', 'Physics', 'CompSci']

• .pop() method

var_name.pop() by default will remove the last element of the var_name, and it saved the removed value.

courses = ['History', 'Math', 'Physics', 'CompSci']

popped = courses.pop()

print(popped)
print(courses)

Output:

CompSci
['History', 'Math', 'Physics']

If one has a stack or a queue, then one can go through and keep popping out values until your list is empty.

1.2.3. Sort List

• Reverse List: var_name.reverse()

courses = ['History', 'Math', 'Physics', 'CompSci']
courses.reverse()
print(courses)

Output:

['CompSci', 'Physics', 'Math', 'History']

• Sorting the list: var_name.sort()

Sort the list in alphabetic order by var_name.sort().

If the list contains numbers, it will sort those in ascending order.

courses = ['History', 'Math', 'Physics', 'CompSci']
nums = [1, 53, 5, 244, 72]

courses.sort()
nums.sort()

print(courses)
print(nums)

Output:

['CompSci', 'History', 'Math', 'Physics']
[1, 5, 53, 72, 244]

If we want to sort the list in descending order, one could set the reverse in sort as true var_name.sort(reverse=True).

courses = ['History', 'Math', 'Physics', 'CompSci']
nums = [1, 53, 5, 244, 72]

courses.sort(reverse=True)
nums.sort(reverse=True)

print(courses)
print(nums)

Output:

['Physics', 'Math', 'History', 'CompSci']
[244, 72, 53, 5, 1]

• Sorted the list without altering the original list: sorted() function

courses = ['History', 'Math', 'Physics', 'CompSci']

sorted_courses = sorted(courses)

print(sorted_courses)
print(courses)

Output:

['CompSci', 'History', 'Math', 'Physics']
['History', 'Math', 'Physics', 'CompSci']

This is very useful, since a lot of time you don’t want to alter your original list.

1.2.4. Other List Functions

• Minimum value: min()

• Maximum value: max()

• Sum of the sequence: sum()

nums = [1, 53, 5, 244, 72]

print(min(nums))
print(max(nums))
print(sum(nums))

Output:

1
244
375

1.2.5. Find the index of certain value

• To find the index of a certain value one can use var_name.index().

courses = ['History', 'Math', 'Physics', 'CompSci']

print(courses.index('CompSci'))

Output:

3

If one search a value that is not in the list, there’ll be an ValueError.

courses = ['History', 'Math', 'Physics', 'CompSci']

print(courses.index('Art'))

Output:

Traceback (most recent call last):
  File "d:\Documents\Code\python\tutorial\list-tuple-set.py", line 53, in <module>
    print(courses.index('Art'))
ValueError: 'Art' is not in list

• Find if a value is in the list or not by in operator

courses = ['History', 'Math', 'Physics', 'CompSci']

print('Art' in courses)
print('Math' in courses)

Output:

False
True

This is going to be specially useful when go over the topics of conditionals and if-else statements. This can also be used to loop through values of the list by using a for loop.

courses = ['History', 'Math', 'Physics', 'CompSci']

for item in courses:
    print(item)

Output:

History
Math
Physics
CompSci

• Get the index and value simultaneously: enumerate() function

The enumerate() function returns two values which are the index that we are on and its value.

courses = ['History', 'Math', 'Physics', 'CompSci']

for item in enumerate(courses):
    print(item)

Output:

(0, 'History')
(1, 'Math')
(2, 'Physics')
(3, 'CompSci')

courses = ['History', 'Math', 'Physics', 'CompSci']

for index, course in enumerate(courses):
    print(index, course)

Output:

0 History
1 Math
2 Physics
3 CompSci

If one don’t want to start the index at 0, one can pass a start value for enumerate function as enumerate(var_list, start=)

courses = ['History', 'Math', 'Physics', 'CompSci']

for index, course in enumerate(courses, start=1):
    print(index, course)

Output:

1 History
2 Math
3 Physics
4 CompSci

1.2.6. Turn List into Strings

• Turn a list into strings separated by certain values using string method .join() and passes the list as argument.

To turn the list of courses into a string of comma separated values.

courses = ['History', 'Math', 'Physics', 'CompSci']

course_str = ', '.join(courses)
print(course_str)

Output:

History, Math, Physics, CompSci

• Turn a string into a list by .split() method

courses = ['History', 'Math', 'Physics', 'CompSci']

course_str = ' - '.join(courses)

new_list = course_str.split(' - ')

print(course_str)
print(new_list)

Output:

History - Math - Physics - CompSci
['History', 'Math', 'Physics', 'CompSci']

2. Tuples

Tuples are very similar to list, but with one major difference that we can’t modify tuples. In programming, this is called mutable and immutable. Lists are mutable, but tuples are immutable .

If you need to modify your list, then this mutability is what you want.

# Mutable
list_1 = ['History', 'Math', 'Physics', 'CompSci']
list_2 = list_1

print(list_1)
print(list_2)

list_1[0] = 'Art'

print(list_1)
print(list_2)

Output:

['History', 'Math', 'Physics', 'CompSci']
['History', 'Math', 'Physics', 'CompSci']
['Art', 'Math', 'Physics', 'CompSci']
['Art', 'Math', 'Physics', 'CompSci']

If you want a list of values that you know not to change, then we can use a tuple. Tuple uses parentheses () instead of square brackets [] in list.

# Immutable
tuple_1 = ('History', 'Math', 'Physics', 'CompSci')
tuple_2 = tuple_1

print(tuple_1)
print(tuple_2)

tuple_1[0] = 'Art'

print(tuple_1)
print(tuple_2)

Output:

Traceback (most recent call last):
  File "d:\Documents\Code\python\tutorial\test.py", line 20, in <module>
    tuple_1[0] = 'Art'
TypeError: 'tuple' object does not support item assignment
('History', 'Math', 'Physics', 'CompSci')
('History', 'Math', 'Physics', 'CompSci')

Since tuple is immutable, it doesn’t have nearly as many methods as list. One can’t add, delete, append values in tuple. But for loop, access values, index and all the other things expect for what mutates the elements works in tuple.

• .sort(), .append(), .insert(), .extend(), .remove(), .pop() can not be used to tuples.

• sorted(), .index(), in, enumerate(), and slicing can apply to tuples.

tuple_1 = ('Math', 'Art', 'Physics', 'MatSci')

print(sorted(tuple_1))

print(tuple_1.index('Art'))

print(tuple_1[1:])

for index, course in enumerate(tuple_1, start=1):
    print(index, course)

Output:

['Art', 'MatSci', 'Math', 'Physics']
1
('Art', 'Physics', 'MatSci')
1 Math
2 Art
3 Physics
4 MatSci

3. Sets

Sets are values that are unordered and with no duplicates. Sets use curly braces {}.

cs_courses = {'History', 'Math', 'Physics', 'CompSci'}

print(cs_courses)

Output:

{'Math', 'History', 'CompSci', 'Physics'}

However, if one run the above code again, one gets a different result.

{'History', 'Physics', 'Math', 'CompSci'}

Unlike list and tuple, set doesn’t care about orders. The main uses for a set is either to test a value is part of the set or to remove duplicated values because sets throw away duplicates.

3.1. Remove duplicates

Sets automatically throw away the duplicates.

cs_courses = {'History', 'Math', 'Physics', 'CompSci', 'Math'}

print(cs_courses)

Output:

{'History', 'CompSci', 'Physics', 'Math'}

3.2. Membership test

Sets do this test a lot more efficiently than lists and tuples.

cs_courses = {'History', 'Math', 'Physics', 'CompSci', 'Math'}

print('Math' in cs_courses)

Output:

True

Even list and tuple also has in operation, set is optimized to search for the existence of a value.

3.3. Determine what values are shared or not shared with other set

Notes that .intersection() and .difference() of sets return a set.

• The common values of two sets

To find the common values of sets var_1 and var_2, one can use .intersection() method as var_1.intersection(var_2).

cs_courses = {'History', 'Math', 'Physics', 'CompSci'}
art_courses = {'History', 'Math', 'Art', 'Design'}

print(cs_courses.intersection(art_courses))

Output:

{'Math', 'History'}

• Values in set var_1 but not in set var_2

To get the Values in set var_1 but not in set var_2, .difference() can be used as var_1.difference(var_2).

cs_courses = {'History', 'Math', 'Physics', 'CompSci'}
art_courses = {'History', 'Math', 'Art', 'Design'}

print(cs_courses.difference(art_courses))

Output:

{'CompSci', 'Physics'}

• Combine the two sets

To combine the two sets var_1 and var_2, one can use .union() method as var_1.union(var_2).

cs_courses = {'History', 'Math', 'Physics', 'CompSci'}
art_courses = {'History', 'Math', 'Art', 'Design'}

print(cs_courses.union(art_courses))

Output:

{'Design', 'Math', 'CompSci', 'Physics', 'Art', 'History'}

4. Create empty lists, tuples, and Sets

# Empty Lists
empty_list = []
empty_list = list()

# Empty Tuples
empty_tuple = ()
empty_tuple = tuple()

# Empty Sets
empty_set = {}  # This isn't right! It's a dictionary
empty_set = set()