List and Tuples

Updated Oct 28, 2019 ·

Lists

Lists are mutable sequences in Python, which allows you to change their contents.

Common Sequence Operations

Operation	Description
len(sequence)	Returns the number of elements in the sequence.
for element in sequence	Iterates over each element in the sequence.
if element in sequence	Checks whether the element exists in the sequence.
sequence[i]	Accesses the element at index i, starting at 0.
sequence[i:j]	Returns a slice from index i to j-1. If i is omitted, starts at 0; if j is omitted, goes to the end.
enumerate(sequence)	Iterates over both the indexes and elements of the sequence at the same time.

List-Specific Operations

Operation	Description
list[i] = x	Replaces the element at index i with x.
list.append(x)	Adds x to the end of the list.
list.insert(i, x)	Inserts x at position i in the list.
list.pop(i)	Removes and returns the element at index i. If i is omitted, the last element is removed.
list.remove(x)	Removes the first occurrence of x in the list.
list.sort()	Sorts the list in ascending order.
list.reverse()	Reverses the order of the elements in the list.
list.clear()	Removes all elements from the list.
list.copy()	Returns a shallow copy of the list.
list.extend(other_list)	Appends all elements from other_list to the current list.

List Comprehension

• Basic: Creates new list by evaluating the expression for each element in the sequence.

  squares = [x**2 for x in range(5)]  # [0, 1, 4, 9, 16]

• With Condition: Adds elements to the new list only if the condition is met.

• evens = [x for x in range(10) if x % 2 == 0]  # [0, 2, 4, 6, 8]

Sets

A set stores unique elements (no duplicates).

• Sets are unordered, so elements don't have a specific index.
• We can add or remove values, but cannot change them once added.

Sets don’t have an index, so we can't access elements like a list using square brackets.

Creating a Set

Run command below:

attendees = {"Alice", "Bob", "Alice"}
print(attendees)

Output:

{'Bob', 'Alice'}

Even though "Alice" was added twice, the set only keeps one instance.

Convert List to Set

When you convert the list to a set, the duplicates will be removed

my_list = [1, 2, 2, 3, 3, 4]
my_set = set(my_list)
print(my_set)

Output:

{1, 2, 3, 4}

Sorting a Set

Although sets are unordered, we can sort them:

attendees = {"Alice", "Bob", "Charlie"}
sorted_attendees = sorted(attendees)
print(sorted_attendees)

Output:

['Alice', 'Bob', 'Charlie']

Tuples

Tuples are like lists but are immutable and defined with parentheses ( ).

• Useful when element positions must remain fixed/
• Unpacking stores tuple elements into separate variables
• Tuples are ordered, so we can access elements by their index.

Creating a Tuple

locations = ("New York", "Los Angeles", "Chicago")
print(locations)

Output:

('New York', 'Los Angeles', 'Chicago')

Accessing a Tuple

Just like lists, we can access elements in a tuple using an index.

print(locations[1])

Output:

Los Angeles

Dictionaries

Dictionaries are a mutable data structure that organizes data into key-value pairs. Unlike lists, data in dictionaries is accessed using keys, which can be of various data types.

• Defined with curly brackets

  animals = {"bears": 10, "lions": 1, "tigers": 2}  

• Use keys to access values

  animals["bears"]        ## this will return 10  

• Use in to check for a key's existence

  if "bears" in animals  

• Can add, remove, or modify elements

  animals["zebras"] = 2     ## This adds a new pair

Modifying Dictionaries

• Add or modify a value: animals["bears"] = 11 changes the value for "bears".
• Remove a key-value pair: del animals["lions"] deletes the entry for "lions".

Iterating Over Dictionaries

Use a for loop to iterate over keys or access values:

• Iterate keys: for key in animals:.
• Access key-value pairs: for key, value in animals.items():.
• Get only keys: animals.keys().
• Get only values: animals.values().

Dictionary Methods

Method	Description
len(dictionary)	Returns the number of items in the dictionary
dictionary[key]	Accesses the item with key key
dictionary[key] = value	Sets the value associated with key
del dictionary[key]	Removes the item with key key
dictionary.get(key, default)	Returns the element for key, or default if it's not present
dictionary.keys()	Returns a sequence of the keys in the dictionary
dictionary.values()	Returns a sequence of the values in the dictionary
dictionary.update(other_dictionary)	Updates with items from another dictionary
dictionary.clear()	Removes all items from the dictionary

For more information, refer to the official documentation for dictionary operations and methods.