Common Lists Mistakes

You've covered a lot of ground with lists. Here are the mistakes that show up most often — and how to fix them.

1. Capturing None from a mutating method

squad = ["Raven", "Wolf"]
squad = squad.append("Ghost")
print(squad)

Output → None

append(), sort(), reverse(), clear() — they all modify the list in place and return None. If you assign the result, you overwrite your list with nothing.

squad = ["Raven", "Wolf"]
squad.append("Ghost")
print(squad)

Output → ['Raven', 'Wolf', 'Ghost']

Call the method. Don't assign it.

2. IndexError — going out of bounds

squad = ["Raven", "Wolf", "Ghost"]
print(squad[3])

Output → IndexError: list index out of range

A list with 3 elements has indexes 0, 1, 2. Index 3 doesn't exist.

print(squad[2])     # last element — index is len(squad) - 1
print(squad[-1])    # or just use -1

When in doubt, check len() before indexing.

3. remove() on a value that isn't there

squad = ["Raven", "Wolf", "Ghost"]
squad.remove("Fox")

Output → ValueError: list.remove(x): x not in list

Always check with in first:

if "Fox" in squad:
    squad.remove("Fox")

4. Confusing append() and extend()

squad = ["Raven", "Wolf"]
squad.append(["Ghost", "Viper"])
print(squad)

Output → ['Raven', 'Wolf', ['Ghost', 'Viper']]

append() adds the list as a single element — a nested list you didn't want.

squad = ["Raven", "Wolf"]
squad.extend(["Ghost", "Viper"])
print(squad)

Output → ['Raven', 'Wolf', 'Ghost', 'Viper']

Use extend() when you want to add multiple elements from another list.

5. Modifying a list while looping over it

numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        numbers.remove(num)
print(numbers)

Output → [1, 3, 5] — sometimes. Other times elements get skipped silently.

When you remove an element during a loop, Python shifts the remaining elements and the loop loses its place.

Loop over a copy instead, or build a new list:

numbers = [1, 2, 3, 4, 5]
odd = []
for num in numbers:
    if num % 2 != 0:
        odd.append(num)
print(odd)

Output → [1, 3, 5]

6. Assignment is not a copy

original = ["Raven", "Wolf", "Ghost"]
backup = original
backup.append("Viper")
print(original)

Output → ['Raven', 'Wolf', 'Ghost', 'Viper']

backup = original doesn't create a new list. Both variables point to the same one. Any change through either variable affects both.

backup = original.copy()
backup.append("Viper")
print(original)     # ['Raven', 'Wolf', 'Ghost']
print(backup)       # ['Raven', 'Wolf', 'Ghost', 'Viper']

Use copy() when you need an independent list.

7. Using index() without checking first

squad = ["Raven", "Wolf", "Ghost"]
print(squad.index("Fox"))

Output → ValueError: 'Fox' is not in list

if "Fox" in squad:
    print(squad.index("Fox"))
else:
    print("Not found.")

count() is safer for existence checks — it returns 0 instead of raising an error.

8. Forgetting that slice stop is exclusive

squad = ["Raven", "Wolf", "Ghost", "Viper", "Bull"]
print(squad[0:3])

Output → ['Raven', 'Wolf', 'Ghost']

You asked for indexes 0 to 3 — but index 3 is not included. You get 0, 1, 2.

If you want the first 3 elements: squad[:3]. If you want up to and including index 3: squad[:4].

What you should take away

• Mutating methods return None — don't assign them
• Check bounds before indexing, check in before removing
• Use extend() to add multiple elements, not append()
• Never modify a list while looping over it
• Use copy() when you need an independent list
• Slice stop is exclusive — list[0:3] gives you indexes 0, 1, 2