Programs rarely deal with one value at a time. A shop has many products, a class has many students, a month has many expenses. A list keeps many values in order inside one variable, and a tuple does the same for values that should not change. Together with the loops from Python Loops, Python lists and tuples let you process any amount of data with a few lines of code.
Creating and reading Python lists
Write a list with square brackets and commas between items. A list can hold any types, even a mix, but in practice most lists hold items of one kind. Lists use the same indexing and slicing as strings: the first item is at index 0, the last at -1, and len() gives the number of items.
snacks = ["murukku", "samosa", "bajji", "vada", "bonda"]
print(snacks[0])
print(snacks[-1])
print(snacks[1:3])
print(len(snacks))
snacks[2] = "paniyaram"
print(snacks)
print("vada" in snacks)murukku bonda ['samosa', 'bajji'] 5 ['murukku', 'samosa', 'paniyaram', 'vada', 'bonda'] True
Unlike strings, Python lists are mutable: you can change an item in place, as the line snacks[2] = "paniyaram" does.
Adding and removing items
| Method | What it does |
|---|---|
items.append(x) | Adds x to the end |
items.insert(i, x) | Inserts x before index i |
items.extend(other) | Adds every item of another list |
items.remove(x) | Removes the first item equal to x (error if missing) |
items.pop() / items.pop(i) | Removes and returns the last item, or the item at i |
del items[i] | Deletes the item at index i |
items.index(x), items.count(x) | Position of x; how many times it appears |
cart = ["rice", "dal"]
cart.append("ghee")
cart.insert(0, "jaggery")
cart.extend(["cardamom", "cashews"])
print(cart)
cart.remove("dal")
last = cart.pop()
print("Removed:", last)
print(cart)
print(cart.index("ghee"))['jaggery', 'rice', 'dal', 'ghee', 'cardamom', 'cashews'] Removed: cashews ['jaggery', 'rice', 'ghee', 'cardamom'] 2
Sorting and summarising
items.sort() sorts the list itself and returns None. sorted(items) leaves the original alone and returns a new sorted list. Both accept reverse=True, and a key function that decides what to sort by, such as key=len to sort words by length. For lists of numbers, the built-ins sum(), min() and max() do the common maths in one call.
expenses = [1200, 350, 4800, 999, 150]
print(sorted(expenses))
print(expenses)
expenses.sort(reverse=True)
print(expenses)
print("Total:", sum(expenses))
print("Biggest:", max(expenses), "Smallest:", min(expenses))
print(f"Average: {sum(expenses) / len(expenses):.2f}")
cities = ["Ooty", "Coimbatore", "Salem", "Erode"]
print(sorted(cities, key=len))[150, 350, 999, 1200, 4800] [1200, 350, 4800, 999, 150] [4800, 1200, 999, 350, 150] Total: 7499 Biggest: 4800 Smallest: 150 Average: 1499.80 ['Ooty', 'Salem', 'Erode', 'Coimbatore']
Look at the last line of output. "Salem" and "Erode" both have five letters, and they stay in their original order. Python’s sorting is stable: items that compare equal never swap places, which matters when you sort data by one column after another.
Watch out: A common bug is expenses = expenses.sort(). Because sort() returns None, the list is lost. Either call expenses.sort() on its own line or use expenses = sorted(expenses).
List comprehensions
A list comprehension builds a new list from another in a single line: [expression for item in items if condition]. It does the same job as a for loop with append(), but it is shorter and, once you are used to it, easier to read. For example, [p * 1.18 for p in prices] adds 18% GST to every price, and [p for p in prices if p < 500] keeps only the cheaper items. Keep comprehensions simple; if one needs several conditions or nested loops, a normal loop is clearer.
Copying lists safely
Assigning a list to a second name does not copy it. backup = cart makes both names point to the same list, so changing one changes the other. To make an independent copy, use cart.copy(), list(cart) or cart[:].
Tuples
A tuple is like a list that cannot be changed after it is created. Write it with round brackets: location = (11.93, 79.83). A tuple with one item needs a trailing comma: ("Chennai",). Use tuples for fixed groups of values, such as coordinates, a date or a record of (name, price). Because they cannot change, tuples protect data from accidental edits, and they can be used as dictionary keys, which lists cannot.
Unpacking assigns the items of a tuple (or list) to several variables at once, as you did for swapping in Python Syntax and Variables. A star collects “the rest” into a list.
prices = [120, 450, 899, 60]
with_gst = [round(p * 1.18, 2) for p in prices]
cheap = [p for p in prices if p < 500]
print(with_gst)
print(cheap)
cart = ["rice", "dal"]
alias = cart
snapshot = cart.copy()
cart.append("salt")
print(alias, snapshot)
puducherry = (11.93, 79.83)
lat, lon = puducherry
print(f"Latitude {lat}, longitude {lon}")
first, *others = ["Gold", "Silver", "Bronze", "Fourth"]
print(first, others)[141.6, 531.0, 1060.82, 70.8] [120, 450, 60] ['rice', 'dal', 'salt'] ['rice', 'dal'] Latitude 11.93, longitude 79.83 Gold ['Silver', 'Bronze', 'Fourth']
Tip: Trying to change a tuple, as in puducherry[0] = 12, raises TypeError: 'tuple' object does not support item assignment. If you need to change the values, convert it with list() or simply create a new tuple.