Lab 11 — Scoping and mutable state

Problem 1

This problem is about scope: where in the program directory a name is looked up in, and whether an assignment creates a new local name or updates an existing one.

Part A

balance = 50

def can_afford(price):
    return price <= balance

def buy(price):
    if price <= balance:
        balance = balance - price
        return balance
    return "too expensive"

print(can_afford(30))
print(buy(30))
print(balance)

1. What happens when this program runs? (Consider each print separately: some may never run.)
2. Does it do what you expect / think is intended? If not, fix it.

Part B

title = "global"

def story():
    def current():
        return title
    print(current())
    title = "local"
    return current()

print(story())
print(title)

1. What happens when this program runs? Explain in terms of which title each function refers to.
2. Does it do what you expect / think is intended? If not, fix it.

Part C

var score = 0

fun points(n :: Number) block:
  score := score + n
  score
end

fun reset() block:
  score := 0
  score
end

points(10)
points(5)
reset()
score

1. What is the final value of score? Trace through all changes.
2. Rewrite points and reset in Python so they work the same way.

Problem 2

Part A

Draw the program directory and heap for this scenario in Python:

@dataclass
class Student:
   name: str
   age: int
obj1 = Student("Alice", 25)
obj2 = Student("Alice", 25)
obj3 = obj1
obj1.age = 26

Show which variables are affected and why.

Part B

Analyze this Python code. What do the comparisons return?

list1 = [1, 2, 3]
list2 = [1, 2, 3]
list3 = list1
print(list1 == list2)
print(list1 is list2)
print(list1 is list3)

Part C

Trace through this Python function. What happens after each call?

def add_item(my_list, item):
    my_list.append(item)
    return my_list

original = [1, 2]
result1 = add_item(original, 3)
result2 = add_item(original, 4)
print(original)
print(result1)
print(result2)

Part D

Analyze this Python code. What gets printed?

list1 = [10, 20, 30]
list2 = list1
list1[1] = 99
print(list1)
print(list2)

Part E

Trace this Python function through multiple calls:

def make_counter():
    count = 0
    count = count + 1
    return count

result1 = make_counter()
result2 = make_counter()
print(result1 == result2)