Control flow: if, for, while

Programs become useful when they make decisions and repeat work. Python gives you three tools for that: if statements, for loops over sequences, and while loops that run until a condition changes.

Conditions

marks = 72
if marks >= 90:
    grade = "A"
elif marks >= 60:
    grade = "B"
else:
    grade = "C"
print(grade)     # B

Branches are tested in order and only the first match runs, so write the strictest test first.

Operator	Meaning	Example
==, !=	equal, not equal	city == "Pune"
< <= > >=	comparison	amount > 50000
and, or, not	combine conditions	paid and not cancelled
in	membership	"Delhi" in cities
is None	identity with None	discount is None

Chained comparisons are allowed: if 1000 <= amount <= 5000: reads exactly as it means.

Truthiness

Empty values are false: 0, 0.0, "", [], {}, set() and None. Everything else is true, so if orders: is the idiomatic way to ask "is the list non-empty?".

for loops

A for loop walks through any sequence.

for fruit in ["apple", "mango", "banana"]:
    print(fruit.upper())

for i in range(1, 6):      # 1..5 - the stop value is excluded
    print(i * i)

for position, city in enumerate(["Surat", "Kochi"], start=1):
    print(position, city)

range(start, stop, step) produces numbers lazily, and range(10, 0, -2) counts down.

while loops

Use while when you do not know in advance how many repetitions are needed.

balance = 1000
while balance > 0:
    balance -= 300
    print("Remaining:", balance)

Something inside the loop must move the condition towards False, otherwise it never ends (Ctrl+C stops a runaway loop).

break, continue and the loop else

stock = {"laddu": 12, "barfi": 0, "peda": 7}
for item, qty in stock.items():
    if qty == 0:
        continue                 # skip out-of-stock items
    if qty < 10:
        print("Reorder:", item)
        break                    # stop at the first low item
else:
    print("All items comfortably stocked")

A loop's else block runs only when the loop finished without hitting break — handy for search loops.

Comprehensions

A compact way to build a list or dict from a loop:

squares = [n * n for n in range(10) if n % 2 == 0]
lengths = {w: len(w) for w in ["ai", "python", "data"]}

A worked example

sales = [12000, 4500, 78000, 300, 26000]
large = small = 0
for amount in sales:
    if amount >= 10000:
        large += 1
    else:
        small += 1
print(f"{large} large orders, {small} small ones, total Rs. {sum(sales):,}")

Common mistakes

• Using = (assignment) where == (comparison) belongs inside a condition.
• Forgetting the colon, or mis-indenting the block under if / for.
• Modifying a list while looping over it; loop over a copy with for x in items[:] instead.
• Writing if x == True: when if x: says the same thing.
• An endless while loop because the counter is never updated.