Python for Loops in Depth: range, enumerate, zip, Dictionaries and for-else

The Loops lesson introduced for and while. This lesson goes deeper into the for loop, Python's workhorse for repeating work over collections, and the built-in tools that make it powerful.

The basic pattern#

for fruit in ["apple", "banana", "cherry"]:
    print(fruit)

apple
banana
cherry

A for loop works with any iterable: lists, strings, tuples, sets, dictionaries, files, and range().

range() in detail#

range(stop), range(start, stop) and range(start, stop, step) produce numbers on demand. The stop value is never included:

for i in range(1, 6):
    print(i, i * i)

1 1
2 4
3 9
4 16
5 25

A negative step counts down:

for i in range(10, 0, -3):
    print(i, end=" ")

10 7 4 1

enumerate(): index and value together#

Instead of tracking a counter yourself, let enumerate() do it:

names = ["Asha", "Ravi", "Meera"]
for index, name in enumerate(names, start=1):
    print(f"{index}. {name}")

1. Asha
2. Ravi
3. Meera

Avoid for i in range(len(names)): followed by names[i]; enumerate is clearer and less error-prone.

zip(): loop over two lists in step#

names = ["Asha", "Ravi", "Meera"]
scores = [92, 85, 78]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

Asha: 92
Ravi: 85
Meera: 78

zip stops at the shortest list. Use zip(a, b, strict=True) (Python 3.10+) if the lists must be the same length.

Looping over a dictionary#

Use .items() to get each key and value:

prices = {"tea": 20, "coffee": 35, "juice": 50}
for item, price in prices.items():
    print(f"{item:<8} ₹{price}")

tea      ₹20
coffee   ₹35
juice    ₹50

:<8 pads the name to 8 characters so the prices line up. Loop over .keys() or .values() when you need only one of them. More in Dictionaries.

Nested loops#

A loop inside a loop runs the inner loop completely for each turn of the outer one. A small multiplication grid:

for row in range(1, 4):
    for col in range(1, 4):
        print(row * col, end="\t")
    print()

1	2	3
2	4	6
3	6	9

The inner print(..., end="\t") stays on one line; the outer print() ends each row.

Accumulating results#

Common loop patterns build up an answer step by step:

total = 0
for n in range(1, 101):
    total += n
print(total, sum(range(1, 101)))

5050 5050

word = "banana"
count = 0
for ch in word:
    if ch == "a":
        count += 1
print(count, word.count("a"))

3 3

Python often has a built-in (sum, count, max) that does the job in one call, but knowing the loop version helps you solve problems that have no built-in.

The for-else clause#

A for loop can have an else block, which runs only if the loop finished without hitting break. It is perfect for "search, and report if nothing was found":

for n in [3, 8, 11]:
    if n % 2 == 0:
        print("Found an even number:", n)
        break
else:
    print("No even numbers")

for n in [3, 7, 11]:
    if n % 2 == 0:
        print("Found an even number:", n)
        break
else:
    print("No even numbers")

Found an even number: 8
No even numbers

A preview: list comprehensions#

Building a list with a loop is so common that Python has a one-line form:

squares = [n * n for n in range(1, 6)]
print(squares)

[1, 4, 9, 16, 25]

See Comprehensions.

Common mistakes#

Practice#

for n in range(1, 16):
    if n % 15 == 0:
        print("FizzBuzz")
    elif n % 3 == 0:
        print("Fizz")
    elif n % 5 == 0:
        print("Buzz")
    else:
        print(n)

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz

Key takeaway#