Predict the Output - Python Scoping

Problem

Read the program below. For each numbered part, say what is printed or which exception is raised, and state the scoping rule responsible. Do this before running it.

x = "module"


def outer():                       # part 1
    x = "outer"
    def show():
        print("show:", x)
    x = "rebound"
    show()


def bump():                        # part 2
    total = 0
    def add(n):
        total += n
    add(5)
    return total


def make_names():                  # part 3
    class Config:
        prefix = "svc-"
        names = [prefix + str(i) for i in range(2)]
    return Config.names


def leak():                        # part 4
    for i in range(3):
        pass
    squares = [j * j for j in range(3)]
    return i, "j" in dir()


outer()
try:
    print("bump:", bump())
except Exception as e:
    print("bump:", type(e).__name__)
try:
    print("names:", make_names())
except Exception as e:
    print("names:", type(e).__name__)
print("leak:", leak())

Examples

The program prints:

show: rebound
bump: UnboundLocalError
names: NameError
leak: (2, False)

• Part 1 prints rebound, not outer: the inner function reads the variable at call time.
• Part 2 raises before add ever reads total: the assignment makes total local to add for the whole function body.
• Part 3 raises NameError for prefix: a class body is not an enclosing scope for the comprehension.
• Part 4 returns (2, False): a for loop variable leaks into the function scope, a comprehension variable does not.

Hints

1. Python decides whether a name is local to a function at compile time by scanning the whole body for assignments. What does that imply for a name that is assigned on line 3 but read on line 2?
2. Closures capture variables, not values. When show() runs, which binding of x does it see?
3. The rule is LEGB: Local, Enclosing function, Global, Builtins. Where does a class body fit? (It does not.)