Excellent.
DAY 12 is where you stop writing control flow by habit and start writing it by understanding how Python actually executes it.

This day directly improves:

• Code clarity
• Performance intuition
• Interview answers (“why for over while?”)

🧠 DAY 12 — Control Flow Deep Dive

(if / for / while / bytecode & execution cost)

🔑 CORE IDEA OF DAY 12

Python control flow is not syntax magic — it is bytecode + stack manipulation + iterator protocol.

Once you see that, everything becomes predictable.

1️⃣ Control Flow in Python = Conditional Jumping

At runtime, Python:

• Evaluates expressions
• Pushes results on stack
• Uses jump instructions to move execution

There is no special CPU instruction for if or for.
It’s all bytecode + jumps.

2️⃣ if Statement — What REALLY Happens

if x > 10:
    y = 1
else:
    y = 2

Execution steps:

1. Evaluate x > 10
2. Result is True or False
3. Conditional jump based on truthiness
4. Execute one branch

Bytecode View (Simplified)

import dis

def f(x):
    if x > 10:
        return 1
    else:
        return 2

dis.dis(f)

You’ll see instructions like:

COMPARE_OP
POP_JUMP_IF_FALSE
RETURN_VALUE

🧠 if is just a conditional jump based on truthiness

3️⃣ Truthiness Revisited (Control Flow Depends on It)

In if x: Python does:

1. Call x.__bool__() if exists
2. Else call x.__len__()
3. Else assume True

This means:

if []:
    ...

does NOT check equality with False.
It checks truthiness protocol.

4️⃣ for Loop — NOT What You Think

❌ Common misconception

for is index-based looping

✅ Reality

Python for is iterator-based looping

for x in iterable:
    ...

Internally:

iter_obj = iter(iterable)
while True:
    try:
        x = next(iter_obj)
    except StopIteration:
        break

5️⃣ Why for Is Preferred Over while

for x in lst:
    ...

vs

i = 0
while i < len(lst):
    x = lst[i]
    i += 1

Reasons for is better:

• Fewer bytecode instructions
• No repeated len() calls
• No index errors
• Cleaner semantics
• Faster in practice

6️⃣ Bytecode Comparison: for vs while

for loop uses:

GET_ITER
FOR_ITER
STORE_FAST

while loop uses:

COMPARE_OP
POP_JUMP_IF_FALSE

👉 for delegates complexity to C-level iterator → faster.

7️⃣ break, continue, pass (Not the Same)

break

• Exits loop immediately
• Skips else block (important!)

continue

• Skips current iteration
• Goes to next next()

pass

• Does nothing
• Placeholder, syntactic

8️⃣ Loop else (INTERVIEW FAVORITE)

for x in lst:
    if x == 5:
        break
else:
    print("not found")

else runs ONLY IF:

• Loop finishes naturally
• No break executed

🧠 This is not a bug — it’s intentional.

9️⃣ range() Is NOT a List

r = range(10)

• Lazy
• Immutable
• Constant memory
• Produces numbers on demand

list(range(10))  # creates list

Use range directly whenever possible.

🔟 Nested Loops & Time Complexity (Silent Killer)

for i in range(n):
    for j in range(n):
        ...

Time complexity:

O(n²)

Python won’t warn you.
You must think algorithmically.

11️⃣ Short-Circuit Evaluation (VERY IMPORTANT)

if a and b:

Python evaluates:

1. a
2. If a is falsy → stop
3. Else evaluate b

This allows:

if obj is not None and obj.value > 10:

Without crashing.

12️⃣ Control Flow Cost Model (Mental Model)

🔥 INTERVIEW TRAPS (DAY 12)

Q1

if []:
    print("yes")
else:
    print("no")

✔ Output: no

Q2

for i in range(3):
    print(i)
else:
    print("done")

✔ Output:

0
1
2
done

Q3

i = 0
while i < 3:
    print(i)
    i += 1
else:
    print("done")

✔ else still runs
(Only skipped if break)

Q4

Why is for preferred in Python?

✔ Iterator protocol
✔ Fewer errors
✔ Faster
✔ Cleaner bytecode

🧠 DAY 12 MENTAL CHECKLIST

Before writing loops:

1. Is this iterable?
2. Can I avoid indexing?
3. Is nesting necessary?
4. Can comprehension replace this?
5. Will this terminate safely?

📝 DAY 12 ASSIGNMENT (MANDATORY)

1️⃣ Predict output (NO RUNNING):

for i in range(5):
    if i == 3:
        break
else:
    print("finished")

print("end")

2️⃣ Explain clearly:

• Why Python for is not index-based
• Why for-else exists

3️⃣ Think like interviewer:

Why does Python avoid do-while loops?

🔜 DAY 13 PREVIEW

DAY 13 — Functions Internals (Stack Frames, LEGB & Call Mechanics)

You’ll learn:

• How function calls work internally
• Stack frames & local scopes
• Why recursion is expensive in Python
• How Python resolves variable names

When ready, say 👉 “START DAY 13”