Control flow

if / elif / else

python

marks = 72

if marks >= 90:

    grade = "A+"

elif marks >= 60:

    grade = "First class"

else:

    grade = "Keep going"

print(grade)

Python checks each condition from top to bottom and runs only the first block whose condition is true. Order matters: if you checked marks >= 60 first, a score of 95 would get "First class".

Indentation is syntax. Python uses 4 spaces, not {} braces, to decide which lines belong inside the if. A missing or extra space is an IndentationError. VS Code inserts the spaces for you when you press Tab.

Truthiness. Empty values count as false: "", 0, [], {}, None. So if expenses: means "if the list isn't empty".

for loops: repeat for each item

python

for city in ["Pune", "Delhi", "Chennai"]:

    print(city)

for i in range(5):          # 0, 1, 2, 3, 4   (the stop value is excluded)

    print(i)

for i in range(1, 11, 2):   # 1, 3, 5, 7, 9   (start, stop, step)

    print(i)

for i, city in enumerate(["Pune", "Delhi"], start=1):   # get a counter as well

    print(i, city)          # 1 Pune / 2 Delhi

while loops: repeat until something changes

python

while True:

    choice = input("Type q to quit: ").strip().lower()

    if choice == "q":

        break               # leave the loop immediately

    if choice == "":

        continue            # skip the rest of this round and ask again

    print("You typed", choice)

while True + break is the standard pattern for menus, and it's exactly how your expense tracker's main loop works. If you create an infinite loop by mistake, press Ctrl+C.

Worked example: FizzBuzz

This is the most famous screening problem. Print 1 to 15, but print "Fizz" for multiples of 3, "Buzz" for multiples of 5, and "FizzBuzz" for multiples of both.

python

for n in range(1, 16):

    if n % 15 == 0:          # check the most specific case FIRST

        print("FizzBuzz")

    elif n % 3 == 0:

        print("Fizz")

    elif n % 5 == 0:

        print("Buzz")

    else:

        print(n)

Why check 15 first? Because 15 is also divisible by 3, so the n % 3 branch would catch it and print "Fizz".

Control flow

if / elif / else

for loops: repeat for each item

while loops: repeat until something changes

Worked example: FizzBuzz

Try it