042. Else Statement

RAM Manager: Process Status

Aryan’s RAM manager needs to respond to process status checks. When a process is alive and consuming RAM, report it. When it’s gone, report that too.

The silent None return bug is one of the most common Python mistakes:

def check_process(pid, processes):
    if pid in processes:
        return f"PID {pid}: {processes[pid]}MB"
    # ← if pid is NOT in processes, returns None silently
    # caller does: status = check_process(999, procs)
    # then: print(status)  →  "None"  (confusing!)

Aryan’s dispense function has this exact bug — it returns None when the item is out of stock instead of the error message. The tests catch the missing else.

💡 Fun fact: Implicit None returns are one of the most common sources of AttributeError: 'NoneType' object has no attribute '...' errors in Python. This error is so frequent that a popular saying in the Python community is: “Explicit is better than implicit” — the very first line of The Zen of Python (import this), written by Tim Peters in 1999.

⚠️ Watch out: A function missing an else branch doesn’t raise an error — it silently returns None. Your code will appear to work until something tries to use the return value, at which point you get a confusing TypeError or AttributeError far from the actual bug. Always ask: “What does this function return when the if condition is False?”

🤔 Think about it: Every `if` without an `else` has an implicit “do nothing and return `None`” branch. Is there ever a situation where returning `None` from a function is the correct, intentional design — and how would you communicate to callers that `None` is a valid return value rather than a bug?

Learning objectives

Add else to handle the default/fallback case
Avoid implicit None returns from missing else branches
Use else to make all code paths explicit

Key concepts

else
default case
explicit branching

Try it

Concept detail

Python functions that don’t explicitly return a value return None. This is legal but dangerous — callers may not check for None.

Without else: def dispense(item, stock): if item in stock: return “Dispensing “ + item # falls off the end → returns None implicitly

With else: def dispense(item, stock): if item in stock: return “Dispensing “ + item else: return “Out of stock” # explicit, can’t be accidentally None

Every if/elif chain should ask: “what happens when NONE of my conditions are True?”

If None is acceptable, document it with a comment
If it’s a bug, add an else with an explicit return or raise an exception

else on loops (unusual but valid): for item in items: if found_it(item): break else: # runs only if loop completed WITHOUT hitting break print(“not found”)

This for/else pattern is rare but useful for search loops.

Solution

def dispense(item, stock):
    if item in stock and stock[item] > 0:
        stock[item] -= 1
        return "Dispensing " + item
    else:
        return "Sorry, " + item + " is out of stock"

def max_of_two(a, b):
    if a >= b:
        return a
    else:
        return b

Tests

def test_dispense_available():
    stock = {"cola": 3, "water": 1}
    assert dispense("cola", stock) == "Dispensing cola"

def test_dispense_reduces_stock():
    stock = {"cola": 3}
    dispense("cola", stock)
    assert stock["cola"] == 2

def test_dispense_out_of_stock():
    stock = {"cola": 0}
    result = dispense("cola", stock)
    assert result == "Sorry, cola is out of stock", f"Got {repr(result)} — missing else branch returns None"

def test_dispense_not_in_stock():
    stock = {"cola": 3}
    result = dispense("juice", stock)
    assert result == "Sorry, juice is out of stock", f"Got {repr(result)} — missing else branch returns None"

def test_dispense_none_check():
    stock = {"cola": 0}
    result = dispense("cola", stock)
    assert result is not None, "Function returned None — add an else branch"

def test_max_of_two():
    assert max_of_two(5, 3) == 5
    assert max_of_two(3, 5) == 5
    assert max_of_two(5, 5) == 5