Lesson 4 - Complex Data Structures in Python - Tuples, Sets and Dictionaries

4.1 Tuples

Python Tuples in W3Schools

• Tuples are used to store multiple items in a single variable. They are ordered, immutable/unchangeable collections of elements.
• They allow duplicate elements.
• We generally use tuples when we have a lot of data and we don’t want to change it.
• We write them inside round brackets - (el1, el2, el3)

# tuple example
my_tuple = (1, 2, 3)
print(my_tuple)
print(my_tuple[1])  # access the second element, element on the index 1

(1, 2, 3)
2

4.1.1 Methods with tuples

• these are the methods we can use with tuples:
  • count - returns the number of times an element appears
  • index - returns the index of an element
  • len - returns the length of the tuple
  • max - returns the maximum value
  • min - returns the minimum value
  • sum - returns the sum of all elements
  • sorted - sorts the tuple
  • reverse - reverses the tuple
  • tuple - converts a list to a tuple

# forbidden methods examples

# appending the tuple
my_tuple = (1, 2, 3)
my_tuple.append(4)  # AttributeError: 'tuple' object has no attribute 'append'

# changing the elements in the tuple
my_tuple = (1, 2, 3)
my_tuple[1] = 4  # TypeError: 'tuple' object does not support item assignment

# deleting the tuple
del my_tuple  # NameError: name 'my_tuple' is not defined

---------------------------------------------------------------------------

AttributeError                            Traceback (most recent call last)

Cell In[5], line 5
      1 # forbidden methods examples
      2 
      3 # appending the tuple
      4 my_tuple = (1, 2, 3)
----> 5 my_tuple.append(4)  # AttributeError: 'tuple' object has no attribute 'append'
      7 # changing the elements in the tuple
      8 my_tuple = (1, 2, 3)


AttributeError: 'tuple' object has no attribute 'append'

# allowed methods examples

# len
my_tuple = (1, 2, 3)
print(len(my_tuple))

# max
my_tuple = (1, 2, 3)
print(max(my_tuple))

# min
my_tuple = (1, 2, 3)
print(min(my_tuple))

# sum
my_tuple = (1, 2, 3)
print(sum(my_tuple))

3
3
1
6

4.1.2 Tuple Packing and Unpacking

• packing means assigning multiple values to the tuple
  • syntax: my_tuple = (value1, value2, value3)
• under the term unpacking we mean extracting the values from the tuple to the variables
  • syntax: value1, value2, value3 = my_tuple
  • in order to do this we need to have the same number of variables as the number of elements in the tuple

# unpacking the tuple
my_tuple = (1, 2, 3)
print(my_tuple)

a, b, c = my_tuple
print(a)
print(b)
print(c)

(1, 2, 3)
1
2
3

• There’s also a special sytax for unpacking a tuple that can be used when the number of variables is not equal to the number of elements in the tuple.
  • syntax: value1, *value2, value3 = my_tuple
    • value1 will be the first element in the tuple
    • value2 will be a list of all the elements that were not unpacked
    • value3 will be the last element in the tuple
• or we cane use it like this:
  • syntax: value1, value2, value3, *value4 = my_tuple
    • value4 will be a list of all the remaining elements that were not unpacked

# example unpacking with asterisk
my_tuple = (1, 2, 3, 4, 5)
print(my_tuple)
a, b, *c = my_tuple
print(a)
print(b)
print(c)

# example unpacking first and last element
first, *middle, last = my_tuple
print(first)
print(middle)
print(last)

(1, 2, 3, 4, 5)
1
2
[3, 4, 5]
1
[2, 3, 4]
5

Important! We can use the same unpacking methods with lists and sets.

• Tuples can be concatenated with the + operator, but the result must be a new tuple and if stored, it has to be stored in a new variable, or if given to the same variable, the original tuple will be overwritten.

• If we wish to change the original tuple, we can convert it to a list and change the elements in the list, after that we can convert it back to a tuple.

# conversion tuple to list, change the list and convert back to tuple
my_tuple = (1, 2, 3)
print(my_tuple)
my_list = list(my_tuple)
my_list[0] = 5
my_tuple = tuple(my_list)
print(my_tuple)

# or
new_tuple = (1, 2, 3)
print(new_tuple)
new_list = list(new_tuple)
new_list.append(4)
new_tuple = tuple(new_list)
print(new_tuple)

(1, 2, 3)
(5, 2, 3)
(1, 2, 3)
(1, 2, 3, 4)

4.2 Sets

• Python Sets in W3Schools
• Python Sets in Real Python

• Sets are used to store multiple items in a single variable. They are unordered, and do not allow duplicate values.
• A set itself may be modified, but the elements contained in the set must be of an immutable type (lists and dictionaries cannot be inside a set).
• We generally use them when we need to have a collection of unique elements.
• We write them inside curly brackets - {el1, el2, el3}

# set
my_set = {1, 2, 3, 4, 5}
print(my_set)

{1, 2, 3, 4, 5}

• a set can created by converting a list or tuple to a set

# converting to set
my_list = [1,2,3,4,5,6,7,8,9,10]
my_set = set(my_list)
print(my_set)
print(type(my_set))

{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
<class 'set'>

# if we have more than one equal element in the set, any iteration will be ignored
my_list = [1,2,3,4,5,6,7,8,9,10,1,2,3,4,5,6,7,8,9,10]
my_set = set(my_list)
print(my_set)
print(type(my_set))

{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
<class 'set'>

4.2.1 Methods with sets

• We can make a union of two sets with the | operator. The same can be done withe the union method.
• We can make an intersection of two sets with the & operator. The same can be done with the intersection method.

# union of sets
set1 = {1, 2, 3, 4, 5}
set2 = {4, 5, 6, 7, 8}
print(set1 | set2)
print(set1.union(set2))

{1, 2, 3, 4, 5, 6, 7, 8}
{1, 2, 3, 4, 5, 6, 7, 8}

# intersection of sets
a={1,2,3,4,5}
b={4,5,6,7,8}
print(a & b)
print(a.intersection(b))

{4, 5}
{4, 5}

• We can also make a difference of two sets with the - operator. The same can be done with the difference method.

# difference of sets
a={1,2,3,4,5}
b={4,5,6,7,8}
print(a - b)
print(a.difference(b))

{1, 2, 3}
{1, 2, 3}

• We can also check if one set is a subset of another with the issubset method or with <= operator.
  • If the first set is a subset of the second, the result will be True, otherwise it will be False.
    
• We can also check if one set is a superset of another with the issuperset method or with >= operator.
  • If the first set is a superset of the second, the result will be True, otherwise it will be False.

# issubset
a={4,5,6}
b={4,5,6,7,8}
print(a.issubset(b))

# issuperset
print(a.issuperset(b))

True
False

Modification of Sets

• We can add elements to a set with the add method.
• We can remove elements from a set with the remove method.
• We can clear the set with the clear method.
• We can delete the set with the del keyword.
• We can copy the set with the copy method.
• We can update the set with the update method.
• We can remove the set with the pop method.

# Create a set
my_set = {1, 2, 3, 4, 5}

# Add elements to a set
my_set.add(6)
print(my_set)  # Output: {1, 2, 3, 4, 5, 6}

# Remove elements from a set
my_set.remove(3)
print(my_set)  # Output: {1, 2, 4, 5, 6}

# Clear the set
my_set.clear()
print(my_set)  # Output: set()

# Delete the set
del my_set
print(my_set)  # Output: NameError: name 'my_set' is not defined

# Copy the set
set1 = {1, 2, 3}
set2 = set1.copy()
print(set2)  # Output: {1, 2, 3}

# Update the set
set1 = {1, 2, 3}
set2 = {3, 4, 5}
set1.update(set2)
print(set1)  # Output: {1, 2, 3, 4, 5}

# Remove an element from the set using pop()
set1 = {1, 2, 3, 4, 5}
set1.pop()
print(set1)  # Output: {2, 3, 4, 5}

{1, 2, 3, 4, 5, 6}
{1, 2, 4, 5, 6}
set()
{1, 2, 3}
{1, 2, 3, 4, 5}
{2, 3, 4, 5}

4.2.2 Frozen Sets

• Frozen sets are immutable sets that cannot be modified after creation.
• We can create a frozen set with the frozenset function.
• We can convert a list or a tuple to a frozen set with the frozenset function.
• all the methods of sets can be used with frozen sets except the ones that modify the set.

# frozen set
my_frozen_set = frozenset([1, 2, 3, 4, 5])
print(my_frozen_set)
my_frozen_set.add(6)  # AttributeError: 'frozenset' object has no attribute 'add'

frozenset({1, 2, 3, 4, 5})



---------------------------------------------------------------------------

AttributeError                            Traceback (most recent call last)

Cell In[25], line 4
      2 my_frozen_set = frozenset([1, 2, 3, 4, 5])
      3 print(my_frozen_set)
----> 4 my_frozen_set.add(6)  # AttributeError: 'frozenset' object has no attribute 'add'


AttributeError: 'frozenset' object has no attribute 'add'

4.3 Dictionaries

Dictionaries in W3Schools

• dictionary is a collection which is ordered and changeable. It doesn’t allow duplicates.
• dictionaries are written with curly brackets, and they are composed of key-value pairs, that are called items.
• keys are unique within a dictionary, and they can be of any immutable data type such as strings, numbers, or tuples.
• syntax:
  • my_dict = {key1: value1, key2: value2, key3: value3}

# create a dictionary
my_dict = {'name': 'John', 'age': 25, 'city': 'New York'}
print(my_dict)

{'name': 'John', 'age': 25, 'city': 'New York'}

4.3.1 Methods with dictionaries

• these are the methods we can use with dictionaries:
  • clear - removes all the elements from the dictionary
  • copy - returns a copy of the dictionary
  • fromkeys - returns a new dictionary with the specified keys and values
  • get - returns the value of the specified key
  • items - returns a list containing the dictionary’s (key, value) tuple pairs
  • keys - returns a list containing the dictionary’s keys
  • pop - removes the element with the specified key
  • popitem - removes the last inserted item
  • setdefault - returns the value of the specified key. If the key does not exist: insert the key, with the specified value
  • update - updates the dictionary with the specified key-value pairs
  • values - returns a list of all the values in the dictionary
  • dict - converts a list of tuples into a dictionary
  • len - returns the number of items in the dictionary

my_dict = {'name': 'John', 'age': 25, 'city': 'New York'}

# # clear - removes all the elements from the dictionary
# my_dict.clear()
# print(my_dict)  # Output: {}

# copy - returns a copy of the dictionary
new_dict = my_dict.copy()
print(new_dict)  # Output: {'name': 'John', 'age': 25, 'city': 'New York'}

# fromkeys - returns a new dictionary with the specified keys and values
keys = ['name', 'age', 'city']
values = ['John', 25, 'New York']
new_dict = dict.fromkeys(keys, values)
print(new_dict)  # Output: {'name': ['John', 25, 'New York'], 'age': ['John', 25, 'New York'], 'city': ['John', 25, 'New York']}

# get - returns the value of the specified key
name = my_dict.get('name')
print(name)  # Output: John

# items - returns a list containing the dictionary's (key, value) tuple pairs
items = my_dict.items()
print(items)  # Output: dict_items([('name', 'John'), ('age', 25), ('city', 'New York')])

# keys - returns a list containing the dictionary's keys
keys = my_dict.keys()
print(keys)  # Output: dict_keys(['name', 'age', 'city'])

# pop - removes the element with the specified key
age = my_dict.pop('age')
print(age)  # Output: 25
print(my_dict)  # Output: {'name': 'John', 'city': 'New York'}

# popitem - removes the last inserted item
item = my_dict.popitem()
print(item)  # Output: ('city', 'New York')
print(my_dict)  # Output: {'name': 'John'}

# setdefault - returns the value of the specified key. If the key does not exist: insert the key, with the specified value
age = my_dict.setdefault('age', 25)
print(age)  # Output: 25
print(my_dict)  # Output: {'name': 'John', 'age': 25}

# update - updates the dictionary with the specified key-value pairs
my_dict.update({'city': 'London', 'country': 'UK'})
print(my_dict)  # Output: {'name': 'John', 'age': 25, 'city': 'London', 'country': 'UK'}

# values - returns a list of all the values in the dictionary
values = my_dict.values()
print(values)  # Output: dict_values(['John', 25, 'London', 'UK'])

# len - returns the length of the dictionary
length = len(my_dict)
print(length)  # Output: 4

{'name': 'John', 'age': 25, 'city': 'New York'}
{'name': ['John', 25, 'New York'], 'age': ['John', 25, 'New York'], 'city': ['John', 25, 'New York']}
John
dict_items([('name', 'John'), ('age', 25), ('city', 'New York')])
dict_keys(['name', 'age', 'city'])
25
{'name': 'John', 'city': 'New York'}
('city', 'New York')
{'name': 'John'}
25
{'name': 'John', 'age': 25}
{'name': 'John', 'age': 25, 'city': 'London', 'country': 'UK'}
dict_values(['John', 25, 'London', 'UK'])
4

4.3.2 Converting lists and tuples into a dictionary

Lists

• There is a special case when we need to convert two lists into a dictionary. We can use the zip function to iterate through the two lists.
  • syntax: dict(zip(list1, list2))
  • list1 and list2 - lists that we want to convert into a dictionary
  • dict - converts a list of tuples into a dictionary
  • zip - returns an iterator of tuples, where the i-th tuple contains the i-th element from each of the argument sequences or iterables
• if the lists have different lengths, the longer list will be truncated.

# create a dictionary from two lists
keys = ['name', 'age', 'city']
values = ['John', 25, 'New York']
my_dict = dict(zip(keys, values))
print(my_dict)  # Output: {'name': 'John', 'age': 25, 'city': 'New York'}

# convert two lists of different lengths into a dictionary
keys = ['First Name', 'Last Name', 'Date of Birth', 'Country']
values = ['John', 'Doe', '1990-01-01']
my_dict = dict(zip(keys, values))
print(my_dict)  # Output: {'First Name': 'John', 'Last Name': 'Doe', 'Date of Birth': '1990-01-01'}

{'name': 'John', 'age': 25, 'city': 'New York'}
{'First Name': 'John', 'Last Name': 'Doe', 'Date of Birth': '1990-01-01'}

Tuples

• We could also create a dictionary from a list of tuples.
  • syntax: dict(list_of_tuples)
  • the tuples in the list must have the same number of elements (key-value pairs).

# convert a list of tuples into a dictionary
list_of_tuples = [('name', 'John'), ('age', 25), ('city', 'New York')]
new_dict = dict(list_of_tuples)
print(new_dict)  # Output: {'name': 'John', 'age': 25, 'city': 'New York'}

{'name': 'John', 'age': 25, 'city': 'New York'}

4.3.3 Accessing and modifying the dictionary records

• We can access the dictionary records using the keys.
  • syntax: my_dict[key]

# Access elements
my_dict = {'name': 'John', 'age': 25, 'city': 'New York'}
name = my_dict['name']
print(name)  # Output: John

John

• if by any chance the key does not exist, we could receive a KeyError exception.
• to avoid this, we can use the get method. It will return None if the key does not exist.
  • syntax: my_dict.get(key)
• if we want to give a default value when the key doesn’t exist, we can add it to a .get expression.
  • syntax: my_dict.get(key, default_value)

# get method in a dictionary
my_dict = {'name': 'John', 'age': 25}
graduation = my_dict.get('graduated')
print(graduation)  # Output: None
age = my_dict.get('age')
print(age)  # Output: 25
city = my_dict.get('city', 'New York')
print(city)  # Output: New York, because the key 'city' is not present in the dictionary.

None
25
New York

That gives us a temporary solution that doesn’t change the original dictionary.

• if we want to change the original dictionary, we can use the update method.
  • syntax: my_dict.update({key: value})
• or simply my_dict[key] = value

# update dictionary
my_dict = {'name': 'John', 'age': 25}
my_dict.update({'city': 'New York'})
print(my_dict)  # Output: {'name': 'John', 'age': 25, 'city': 'New York'}

my_dict['something'] = 'value'
print(my_dict)  # Output: {'name': 'John', 'age': 25, 'city': 'New York', 'something': 'value'}

{'name': 'John', 'age': 25, 'city': 'New York'}
{'name': 'John', 'age': 25, 'city': 'New York', 'something': 'value'}

• another way to check if the key exists in the dictionary is to use the in operator.
  • syntax: key in my_dict

# check if key exists
my_dict = {'name': 'John', 'age': 25}
name = 'name' in my_dict
print(name)  # Output: True

True

4.3.4 Removing elements from the dictionary

• We can remove elements from the dictionary using the del statement.
  • syntax: del my_dict[key]
• or simply my_dict.pop(key)
• if removing the last element, we can use the popitem method - my_dict.popitem()

# example of removing items
my_dict = {'name': 'John', 'age': 25, 'city': 'New York'}
my_dict.pop('age')
print(my_dict)  # Output: {'name': 'John', 'city': 'New York'}

my_dict.popitem()
print(my_dict)  # Output: {'name': 'John'}

{'name': 'John', 'city': 'New York'}
{'name': 'John'}

• to remove all the items, that is to clear the dictionary, we can use the clear method.
• we can remove all the items by defining the dictionary as empty my_dict = {}.
• to delete the dictionary, we can use the del keyword. Deleting will remove the definition of the variable that contained the dictionary as well.

# to empty a dictionary
my_dict = {'name': 'John', 'age': 25, 'city': 'New York'}
my_dict.clear()
print(my_dict)  # Output: {}

# empty the dictionary
my_dict = {'name': 'John', 'age': 25, 'city': 'New York'}
my_dict = {}
print(my_dict)  # Output: {}

# del method
my_dict = {'name': 'John', 'age': 25, 'city': 'New York'}
del my_dict
print(my_dict)  # NameError: name 'my_dict' is not defined

{}
{}



---------------------------------------------------------------------------

NameError                                 Traceback (most recent call last)

Cell In[13], line 14
     12 my_dict = {'name': 'John', 'age': 25, 'city': 'New York'}
     13 del my_dict
---> 14 print(my_dict)  # NameError: name 'my_dict' is not defined


NameError: name 'my_dict' is not defined

4.3.5 Looping/Iterating through the dictionary

• We can loop through the dictionary using the for loop.
  • syntax: for key, value in my_dict.items(): - this will give us the key and value of the dictionary
• We can also loop only through the keys:
  • syntax: for key in my_dict.keys():
• or through the values:
  • syntax: for value in my_dict.values():

# loop through dictionary
my_dict = {'name': 'John', 'age': 25, 'city': 'New York'}
for key, value in my_dict.items():
    print(key, value)

# loop through the keys
for key in my_dict.keys():
    print(key)

# loop through the values
for value in my_dict.values():
    print(value)

name John
age 25
city New York
name
age
city
John
25
New York

Nesting dictionaries and lists

• We can nest dictionaries and lists in dictionaries and lists.
• there is no limit to the nesting level.
• only limitation we could have is that the keys have to be unique and the key has to be a strings, numbers or tuples.
• syntax:
  • my_dict = {'name': 'John', 'age': 25, 'city': 'New York', 'address': {'street': 'Main Street', 'number': 123}}

In order to print the nested dictionary, we can use the pprint module. It will print the dictionary in a more readable format.

• syntax:

    import pprint
    pprint.pprint(my_dict)
  

import pprint

# nested dictionaries
my_dict = {'name': 'John', 'age': 25, 'city': 'New York'}
nested_dict = {'name': 'Jane', 'age': 30, 'city': 'London'}
my_dict.update({'friend': nested_dict})
# print dictionary with normal print function
print(my_dict)  # Output: {'name': 'John', 'age': 25, 'city': 'New York', 'friend': {'name': 'Jane', 'age': 30, 'city': 'London'}}
# print dictionary with pprint function
pprint.pprint(my_dict)

# access nested dictionary
friend = my_dict['friend']
print(friend)  # Output: {'name': 'Jane', 'age': 30, 'city': 'London'}

# print friend's city
friend_city = my_dict['friend']['city']
print(friend_city)  # Output: London

{'name': 'John', 'age': 25, 'city': 'New York', 'friend': {'name': 'Jane', 'age': 30, 'city': 'London'}}
{'age': 25,
 'city': 'New York',
 'friend': {'age': 30, 'city': 'London', 'name': 'Jane'},
 'name': 'John'}
{'name': 'Jane', 'age': 30, 'city': 'London'}
London

• We can have dictionaries nested in lists.

import pprint

# list of students
students = [
    {'name': 'John', 'age': 25, 'city': 'New York'},
    {'name': 'Jane', 'age': 30, 'city': 'London'},
    {'name': 'Bob', 'age': 35, 'city': 'Paris'}
]
pprint.pprint(students)  # Output: [{'name': 'John', 'age': 25, 'city': 'New York'}, {'name': 'Jane', 'age': 30, 'city': 'London'}, {'name': 'Bob', 'age': 35, 'city': 'Paris'}]

# access student 2
student2 = students[1]
print(student2)  # Output: {'name': 'Jane', 'age': 30, 'city': 'London'}

# loop throught students and print their names
for student in students:
    name = student['name']
    print(name)  # Output: John, Jane, Bob

[{'age': 25, 'city': 'New York', 'name': 'John'},
 {'age': 30, 'city': 'London', 'name': 'Jane'},
 {'age': 35, 'city': 'Paris', 'name': 'Bob'}]
{'name': 'Jane', 'age': 30, 'city': 'London'}
John
Jane
Bob

Important! These combinations of the dictionary and list are very common. This is the cornerstone of the JSON format.