Python output questions: 10 tricky snippets, answered

Python output questions give you a short program and ask you to guess the output. They look easy because Python reads almost like English, and that is exactly why they catch people out. A few of the language's rules surprise even experienced developers.

Here are 10 tricky Python output questions with answers and clear explanations. A lot of the "guess the output" sheets passed around online contain mistakes. Every output below was produced by running the snippet on CPython 3.12, as part of the puzzle bank for our multiplayer game Gotcha Rush.

How to use these questions

Cover the output, read the code, and write down exactly what you think Python prints, including brackets, quotes and spacing. Then check. Half right is still wrong: [1, 2] and [1] are different answers. Keep a count and read the explanation for every miss.

Functions and default arguments

1. The mutable default

def f(x, acc=[]):
    acc.append(x)
    return acc

print(f(1))
print(f(2))
print(f(3, []))

Output:

[1]
[1, 2]
[3]

Why: default values are created once, when the function is defined, not each time it is called. Every call that leaves out acc shares the same list, so it keeps growing. The third call passes a fresh list and is not affected. The fix is acc=None and then acc = [] if acc is None else acc. The official Python FAQ explains this one in detail.

2. finally always runs

def f():
    try:
        return 1
    finally:
        return 2

def g():
    try:
        raise ValueError
    except ValueError:
        return "e"
    finally:
        print("f")

print(f())
print(g())

Output:

2
f
e

Why: a return inside finally replaces the return from try, so f() gives 2. In g() the except block's return value is kept, but finally still runs before the function actually returns, so f prints before e.

Closures and scope

3. Late binding

fs = [lambda: i for i in range(3)]
print([f() for f in fs])
gs = [lambda i=i: i for i in range(3)]
print([g() for g in gs])

Output:

[2, 2, 2]
[0, 1, 2]

Why: a closure looks up i when it is called, not when it is created, in accordance with the Python Execution Model for scopes and binding. By the time the lambdas run, the loop has finished and i is 2. Writing i=i as a default argument captures the value at the moment each lambda is defined.

Identity and equality

4. is versus ==

a = 256
b = 256
print(a is b)
c = [1]
d = [1]
print(c == d, c is d)

Output:

True
True False

Why: == compares values and is compares identity, meaning whether two names point at the very same object. CPython keeps small integers cached, so both names refer to one 256 object. The two lists have equal values but are two separate objects. Use is only for None, True and False.

5. Three keys, one slot

print(1 == 1.0, hash(1) == hash(1.0), {1: "a", 1.0: "b", True: "c"})

Output:

True True {1: 'c'}

Why: 1, 1.0 and True are all equal and have the same hash, so a dictionary treats them as one key. The first key written, 1, keeps its spelling. Each later write only replaces the value, which ends up as 'c'.

Lists and tuples

6. += on a list

a = [1]
b = a
a += [2]
print(a is b)
a = a + [3]
print(a is b, b)

Output:

True
False [1, 2]

Why: a += [2] changes the existing list in place, so b, which is the same object, sees the change. a = a + [3] builds a brand new list and points a at it, leaving b behind at [1, 2].

7. The tuple that changed anyway

t = (1, [2])
try:
    t[1] += [3]
except TypeError:
    print("error")
print(t)

Output:

error
(1, [2, 3])

Why: t[1] += [3] does two things. First it extends the list inside the tuple, which works. Then it tries to store the result back into the tuple, which raises TypeError because tuples cannot be assigned to. So the error prints, and the list has still grown. This is one of the hardest questions in our bank.

Numbers

8. Banker's rounding

print(round(0.5), round(1.5), round(2.5), round(2.675, 2))

Output:

0 2 2 2.67

Why: Python's round() sends exact halves to the nearest even number, so 0.5 becomes 0, 1.5 becomes 2 and 2.5 also becomes 2. round(2.675, 2) gives 2.67 because 2.675 is stored in binary as a number very slightly below it. See round() in the Python docs.

Loops and generators

9. for ... else

for i in range(3):
    if i == 5:
        break
else:
    print("no break")

for i in range(3):
    if i == 1:
        break
else:
    print("broke")

Output:

no break

Why: the else on a loop runs only when the loop finishes without hitting break. The first loop never breaks, so no break prints. The second breaks at i == 1, so its else is skipped and nothing else prints. Many people expect broke here because of the name.

10. Used up

g = (x * x for x in range(3))
print(sum(g), sum(g), list(g))

Output:

5 0 []

Why: a generator can only be consumed once. The first sum drains it (0 + 1 + 4 = 5), so the second sum sees nothing and gives 0, and list gets an empty result. If you need the values twice, store them in a list first.

How did you score?

• 9 or 10: excellent. You know Python's object model and scoping rules well.
• 6 to 8: good. Go back over mutability (questions 1, 6 and 7), which is where most real bugs come from.
• 5 or fewer: normal for questions this tricky. Working through our free Python development course will fill in the gaps, and the data science with Python course puts the same rules to work on real data.

Practise them against other people

These 10 come from a bank of 42 verified Python puzzles in Gotcha Rush. In the game you type the exact output against a clock while up to four other players do the same, then the real output prints line by line against yours, with the explanation underneath. Solo Rush mode gives you three lives and a timer that keeps shrinking.

Prefer JavaScript? Try our 10 tricky JavaScript output questions. For more ways to practise, see our free coding games.