Lists have built-in methods — functions that belong to the list itself. The most important one is .append(), which adds an item to the end:

customers = ["Alice", "Bob"]
customers.append("Charlie")
print(customers)  # ["Alice", "Bob", "Charlie"]

Notice that .append() modifies the list in place — it doesn't return a new list. This is a key concept. Most list methods change the original list directly. Think of it like adding a new row at the bottom of your spreadsheet — the spreadsheet itself changes.

Use .insert(index, item). It slides everything after that index to the right:

customers = ["Alice", "Bob", "Charlie"]
customers.insert(1, "VIP_Diana")
print(customers)  # ["Alice", "VIP_Diana", "Bob", "Charlie"]

The first argument is where you want to insert, the second is what you're inserting. Everything at that index and beyond shifts right by one position.

Three ways, depending on what you know:

customers = ["Alice", "Diana", "Bob", "Charlie"]

# Remove by value — removes the first occurrence
customers.remove("Diana")
print(customers)  # ["Alice", "Bob", "Charlie"]

# Remove by index — returns the removed item
popped = customers.pop(1)
print(popped)     # "Bob"
print(customers)  # ["Alice", "Charlie"]

# Remove last item (no index needed)
last = customers.pop()
print(last)       # "Charlie"
print(customers)  # ["Alice"]

.remove() searches by value — if the value isn't found, it raises a ValueError. .pop() removes by index and gives you the removed item back. Called without an index, .pop() removes the last item. Think of it like pulling a card out of a deck — you get the card, and the deck shrinks.

Two approaches. .sort() sorts the list in place (modifies the original). sorted() returns a new sorted list and leaves the original alone:

sales = [850, 920, 670, 740, 550]

# In-place — changes the original
sales.sort()
print(sales)  # [550, 670, 740, 850, 920]

# Reverse sort for your report
sales.sort(reverse=True)
print(sales)  # [920, 850, 740, 670, 550]

original = [3, 1, 4, 1, 5]
new_list = sorted(original)
print(original)  # [3, 1, 4, 1, 5] — unchanged
print(new_list)  # [1, 1, 3, 4, 5] — new sorted list

Use sorted() when you need to keep the original order intact — like when the original order represents the sequence customers signed up.

List comprehensions — the most Pythonic way to build a list from another sequence. Remember the accumulator pattern from Week 2? Loop, .append(), repeat? Comprehensions do the same thing in one expression:

# Without comprehension (the Week 2 way)
squares = []
for x in range(5):
    squares.append(x ** 2)
# squares = [0, 1, 4, 9, 16]

# With comprehension — same result, one line
squares = [x ** 2 for x in range(5)]

This is way better than having square1, square2, square3... You can also add a filter with if:

# Only keep scores above 60
passing = [s for s in scores if s >= 60]

# Get names of high-value customers
vip = [name for name in customers if name.startswith("VIP")]

The pattern is: [expression for item in iterable if condition]. The if part is optional.