List Basics

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≈ 15 min · python-lists python-loops · Open in WolfCode

The most important habit in Think Python ch.10: don't mutate your caller's list. Copy first.

The two functions

Python · runnable

def take_first_n(t, n):
    return t[:n]                # slicing handles overshoot & zero gracefully

def with_appended(t, x):
    result = t[:]               # copy first
    result.append(x)            # modify the copy
    return result

a = [1, 2, 3]
print(with_appended(a, 99))     # [1, 2, 3, 99]
print(a)                        # [1, 2, 3]  ← caller's list unchanged

print(take_first_n([1, 2, 3, 4, 5], 3))    # [1, 2, 3]
print(take_first_n([1, 2], 10))            # [1, 2]   (overshoot is OK)
print(take_first_n([1, 2, 3], 0))          # []       (zero is OK)

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Check yourself

Multiple right answers — but the t.append(t)... one is the trap.

Which of these returns [1, 2, 3, 99] AND leaves the caller's list intact?