Collections: Lists, Dicts, Sets¶

Try This First: Before reading, try this in Python: fruits = ['apple', 'banana']; fruits.append('cherry'); print(fruits). What does the list look like after append?

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Python has several ways to group multiple values together.

Visualize It¶

See how lists, dicts, and sets store data differently in memory: Open in Python Tutor

Lists — ordered, changeable, allows duplicates¶

fruits = ["apple", "banana", "cherry"]
fruits.append("date")     # Add to end
fruits[0]                 # "apple" (first item)
fruits[-1]                # "date" (last item)
len(fruits)               # 4
"banana" in fruits        # True

Use lists when: you have an ordered collection of similar items (scores, names, files).

Dictionaries — key-value pairs, insertion-ordered (Python 3.7+), changeable¶

person = {"name": "Alice", "age": 30}
person["name"]            # "Alice"
person["city"] = "Denver" # Add new key
person.get("salary")      # None (safe access, no error)

Use dicts when: you have labeled data (a person's details, configuration, lookup table).

Sets — unordered, no duplicates¶

colors = {"red", "blue", "green", "red"}
print(colors)             # {"red", "blue", "green"} — duplicate removed
colors.add("yellow")
"red" in colors           # True

Use sets when: you need unique values or want to check membership quickly.

Set operations — comparing groups¶

Sets really shine when you want to compare two groups. Imagine you are planning a party and have two guest lists:

friday_guests = {"Alice", "Bob", "Charlie", "Diana"}
saturday_guests = {"Charlie", "Diana", "Eve", "Frank"}

# Union — everyone invited to at least one party
all_guests = friday_guests | saturday_guests
print(all_guests)
# {"Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"}

# Intersection — people coming to BOTH parties
both_days = friday_guests & saturday_guests
print(both_days)
# {"Charlie", "Diana"}

# Difference — people only coming Friday (not Saturday)
friday_only = friday_guests - saturday_guests
print(friday_only)
# {"Alice", "Bob"}

# Difference the other way — people only coming Saturday
saturday_only = saturday_guests - friday_guests
print(saturday_only)
# {"Eve", "Frank"}

Think of it this way: - | (union) means "combine everything" - & (intersection) means "only what overlaps" - - (difference) means "what is in the first group but not the second"

These three operations cover most real-world "compare two groups" problems — finding shared friends, overlapping skills on a resume, or items on one shopping list that are missing from another.

Tuples — ordered, unchangeable¶

point = (3, 5)
x = point[0]             # 3
y = point[1]             # 5
# point[0] = 10          # Error! Tuples cannot be changed

Use tuples when: you have a fixed group of values that should not change (coordinates, RGB colors).

Quick comparison¶

Feature	List	Dict	Set	Tuple
Syntax	`[1, 2, 3]`	`{"a": 1}`	`{1, 2, 3}`	`(1, 2, 3)`
Ordered	Yes	Yes*	No	Yes
Changeable	Yes	Yes	Yes	No
Duplicates	Yes	No (keys)	No	Yes
Access by	Index	Key	N/A	Index

*Dicts maintain insertion order in Python 3.7+ but are not indexed by position.

Common mistakes¶

Empty dict vs empty set:

empty_dict = {}     # This is a dict, NOT a set
empty_set = set()   # This is how you make an empty set

Modifying a list while iterating:

# Wrong
for item in items:
    items.remove(item)

# Right — build a new list
items = [item for item in items if item != "remove_me"]

Practice¶

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