Syntax Medium

What does this code print?

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

add(1)
print(add(2))

  • [2]

  • [1, 2]

  • [1]

  • [2, 1]

Answer

A default argument is evaluated once, when the function is defined, and the same list object is reused on every call. The first call appends 1 and the second appends 2 to that same list, so it prints [1, 2].

What is the output?

funcs = [lambda: i for i in range(3)]
print([f() for f in funcs])

  • [0, 1, 2]

  • [2, 2, 2]

  • [0, 0, 0]

  • [3, 3, 3]

Answer

Closures capture the variable i, not its value at creation time (late binding). All three lambdas share the same i, which is 2 once the comprehension finishes, so every call returns 2.

What does this print?

print(False == False in [False])

  • True

  • False

  • [False]

  • TypeError

Answer

This is a chained comparison, so it expands to (False == False) and (False in [False]). Both parts are true, so the whole expression is True.

What does this code print?

a = [1, 2, 3]
i = 0
i, a[i] = 1, 5
print(a)

  • [5, 2, 3]

  • [1, 5, 3]

  • [1, 2, 5]

  • [5, 5, 3]

Answer

The right-hand side (1, 5) is evaluated first, then targets are assigned left to right. i becomes 1, and only then is a[i] = 5 applied using the new i, so a[1] is set, giving [1, 5, 3].

What is the output?

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

  • [1, 2]

  • [1, 2, 3]

  • [3, 1, 2]

  • [1, 2, [3]]

Answer

For lists, += calls list.extend and mutates the object in place rather than creating a new one. b still refers to that same list, so it also sees the appended 3: [1, 2, 3].

What does this print?

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

  • [1, 2, 3]

  • [1, 2]

  • [3]

  • [1, 2, 3, 3]

Answer

Unlike +=, the expression a + [3] builds a brand-new list and rebinds a to it. b still points at the original, unmodified list, so it prints [1, 2].

What does this code print?

i = 5

def f(x=i):
    return x

i = 10
print(f())

  • 5

  • 10

  • None

  • NameError

Answer

The default value is captured when the def statement runs, at which point i is 5. Reassigning i afterward does not change the stored default, so f() returns 5.

What is the output?

a, *b, c = [1, 2, 3, 4, 5]
print(b)

  • [2, 3, 4]

  • 2

  • [2, 3, 4, 5]

  • (2, 3, 4)

Answer

A starred target in unpacking soaks up all the leftover items as a list. a takes 1, c takes 5, and b gets everything between them: [2, 3, 4].

What does this print?

print(0 and 1 or 2)

  • 0

  • 1

  • 2

  • True

Answer

and binds tighter than or. 0 and 1 short-circuits to 0 (falsy), then 0 or 2 returns the second operand 2.

What does this code print?

grid = [[0] * 2] * 2
grid[0][0] = 1
print(grid)

  • [[1, 0], [0, 0]]

  • [[1, 0], [1, 0]]

  • [[1, 1], [0, 0]]

  • [[1], [0]]

Answer

The outer * 2 copies the reference to the inner list, not the list itself, so both rows are the same object. Mutating one row is visible in the other, giving [[1, 0], [1, 0]].

What is the output?

x = [1, 2]
y = [1, 2]
print(x == y, x is y)

  • True True

  • True False

  • False False

  • False True

Answer

== compares values, so equal contents make it True. is compares identity, and two separate list literals are distinct objects, so it is False.

What does this print?

print(-7 // 2)

  • -3

  • -4

  • -3.5

  • 3

Answer

Floor division rounds toward negative infinity, not toward zero. -7 / 2 is -3.5, and the floor of that is -4.

What does this code print?

print(-7 % 3)

  • -1

  • 2

  • -2

  • 1

Answer

In Python the result of % takes the sign of the divisor. Since -7 // 3 is -3, the remainder is -7 - (-3 * 3) = 2.

What is the output?

def f(lst):
    lst = lst + [4]

def g(lst):
    lst.append(4)

x = [1]
f(x)
g(x)
print(x)

  • [1]

  • [1, 4]

  • [1, 4, 4]

  • [1, 4, 4, 4]

Answer

f rebinds its local name lst to a new list, leaving the caller's list untouched. g mutates the list in place via append, so only g's change is visible: [1, 4].

What does this print?

x = "outer"
result = [x for x in range(3)]
print(x)

  • outer

  • 2

  • [0, 1, 2]

  • NameError

Answer

In Python 3 a comprehension has its own scope, so its loop variable x does not leak out and overwrite the outer x. The outer name still holds "outer".

What does this code print?

x = 1 if True else 2 if False else 3
print(x)

  • 1

  • 2

  • 3

  • SyntaxError

Answer

Conditional expressions group right to left, reading as 1 if True else (2 if False else 3). The first condition is true, so the whole thing is 1 and the nested part is never evaluated.

What is the output?

def f(a, b=10, *, c):
    return a + b + c

print(f(1, c=2))

  • 13

  • 3

  • TypeError

  • 11

Answer

The bare * makes c keyword-only, so it must be passed by name. With a=1, the default b=10 and c=2, the sum is 13.

What does this print?

data = [y := x + 1 for x in range(3)]
print(y)

  • NameError

  • 3

  • 2

  • [1, 2, 3]

Answer

Unlike the comprehension's own loop variable, a walrus (:=) target binds in the enclosing scope. After the loop it holds the last assigned value, 2 + 1, so y is 3.

What does this code print?

print("5" + 3 * "2")

  • 522

  • 5222

  • 5 222

  • TypeError

Answer

* runs before +, so 3 * "2" is "222", then string concatenation prepends "5", producing 5222.

What is the output?

def f():
    return 1, 2, 3

print(f()[1])

  • 1

  • 2

  • (1, 2, 3)

  • TypeError

Answer

A comma-separated return builds a tuple, so f() returns (1, 2, 3). Indexing it with [1] gives the second element, 2.