Python Lists in 15 Minutes: Add, Remove, Loop, Done

A list is an ordered collection of values. That's it. You need four moves on day one: make one, add to it, remove from it, and loop over it. Everything else, like slicing tricks, sorting with keys and comprehensions, can wait until you have a real reason to learn it.

Make a list

Square brackets, values separated by commas:

groceries = ["rice", "eggs", "milk"]
empty = []

print(groceries)
print(len(groceries))
print(groceries[0])
print(groceries[-1])

['rice', 'eggs', 'milk']
3
rice
milk

Positions start at 0, not 1. groceries[0] is the first item, groceries[-1] is the last. You will get this wrong at least once. Everybody does. Ask for a position that doesn't exist, like groceries[3] here, and you get IndexError: list index out of range.

Add: append and insert

groceries = ["rice", "eggs", "milk"]

groceries.append("bread")
print(groceries)

groceries.insert(0, "coffee")
print(groceries)

['rice', 'eggs', 'milk', 'bread']
['coffee', 'rice', 'eggs', 'milk', 'bread']

append adds to the end. That's what you want 90% of the time. insert(position, value) puts the value at that position and moves everything after it one step to the right.

Remove: remove, pop and del

Three ways, for three different situations:

groceries = ["coffee", "rice", "eggs", "milk", "bread"]

groceries.remove("eggs")
print(groceries)

last = groceries.pop()
print(last, groceries)

first = groceries.pop(0)
print(first, groceries)

del groceries[0]
print(groceries)

['coffee', 'rice', 'milk', 'bread']
bread ['coffee', 'rice', 'milk']
coffee ['rice', 'milk']
['milk']

• remove(value): you know what to remove. Only the first match is removed. If the value isn't there, you get ValueError: list.remove(x): x not in list.
• pop() or pop(position): you know where, and you want the removed value back.
• del list[position]: you know where, and you don't need the value.

If you're not sure the value is in the list, check first:

groceries = ["rice", "milk"]

if "eggs" in groceries:
    groceries.remove("eggs")
else:
    print("No eggs on the list")

No eggs on the list

Loop: for

prices = [45.0, 120.0, 89.5]

for price in prices:
    print(price)

total = 0
for price in prices:
    total += price
print("Total:", total)

for number, price in enumerate(prices, start=1):
    print(number, price)

45.0
120.0
89.5
Total: 254.5
1 45.0
2 120.0
3 89.5

Loop over the items directly: for price in prices. Don't write for i in range(len(prices)) and then prices[i]. It works, but it's harder to read and easier to get wrong. If you need the position too, use enumerate. And for a plain total, sum(prices) does the same as that loop in one line.

The one mistake that really hurts: removing while looping

This looks right. It isn't:

scores = [90, 40, 35, 80, 20]

for score in scores:
    if score < 50:
        scores.remove(score)

print(scores)

[90, 35, 80]

35 should be gone, and it's still there. When you remove 40, everything after it moves one step left. 35 moves into the spot the loop just checked, so the loop skips it. No error. Just a wrong result. That's the dangerous kind.

The fix: build a new list with only what you want to keep.

scores = [90, 40, 35, 80, 20]

passed = []
for score in scores:
    if score >= 50:
        passed.append(score)

print(passed)

[90, 80]

Common mistakes

• Counting from 1. The first item is [0].
• x = x.append(...). You just lost your list.
• Removing inside a for loop over the same list. Items get skipped silently.
• Using a list to look things up by name. If you keep searching a list for "the item called X", you probably want a dictionary.

What happens if you skip this

You'll still write programs, but you'll fight your data instead of using it. Lists are in almost everything: rows from a file, items in an order, results from a database. Learn these four moves properly and the rest of Python gets easier. Skip them, and every bug about "missing" or "extra" items will look like magic.