Loops

A loop repeats a block of code, so you don't have to write out the same steps by hand for every item or every pass. Python has two: a for loop, which runs once per item in a sequence, and a while loop, which keeps running for as long as a condition stays True.

Loop	Syntax	Use it for
for	for item in sequence:	When you know (or can generate) what you're looping over ahead of time: A range() — loop a fixed number of times A list — an ordered, changeable collection of items A tuple — an ordered collection that can't be changed A dict — looping gives you the keys (or use .items() for key-value pairs) A str — loops over the string one character at a time
while	while condition:	Repeating until something changes — not tied to a fixed collection When you don't know in advance how many passes you'll need

For Loops

A for loop steps through a sequence — a list, tuple, string, or other iterable — running its body once per item, in order.

species = ["burmese", "rock", "ball", "blood"]

for snake in species:
    print(snake)

Loop a Fixed Number of Times

range(5)          # range(0, 5) — produces 0, 1, 2, 3, 4
range(2, 6)       # 2, 3, 4, 5
range(2, 10, 3)   # 2, 5, 8  (step of 3)

range() generates a sequence of numbers without building a whole list in memory. One argument counts up from 0 to (but not including) that number; a second sets a custom start; a third sets a step size other than 1. It's the standard way to loop a fixed number of times, or to loop by index instead of by value.

for i in range(5):
    print(i)

for i in range(2, 6):
    print(i)

for i in range(2, 10, 3):
    print(i)

Loop Over a List

for s in species: 
    print(s)    # burmese  rock  ball  blood

Each pass hands you the item itself, not its position — this is what sets a Python for loop apart from the index-counting loops in some other languages. A list is the most common thing to loop over, since it's Python's all-purpose ordered collection.

species = ["burmese", "rock", "ball", "blood"]

for s in species:
    print(s)

Loop Over a Tuple

for s in constrictors: 
    print(s)    # ball  burmese  boa

Looping over a tuple works exactly like looping over a list — the only difference is that a tuple can't be changed once it's created.

constrictors = ("ball", "burmese", "boa")

for s in constrictors:
    print(s)

Loop Over a Dictionary

for key, value in snake.items(): 
    print(key, value)          # species ball  length_ft 5  venomous False
for key in snake: 
    print(key)                 # species  length_ft  venomous
for value in snake.values(): 
    print(value)               # ball  5  False

Looping directly over a dictionary gives you its keys, one at a time. Use .values() to get just the values instead, or .items() to get the key and value together — usually the most useful of the three.

snake = {"species": "ball", "length_ft": 5, "venomous": False}

# 1. Loop through both keys and values (recommended)
for key, value in snake.items():
    print(f"{key}: {value}")

# 2. Loop through keys only
for key in snake:
    print(key)

# 3. Loop through values only
for value in snake.values():
    print(value)

Loop Over a String

for letter in name: print(letter)    # b  u  r  m  e  s  e     p  y  t  h  o  n

A string is just a sequence of characters, so looping over one hands you each character in turn, in order — including spaces.

name = "burmese python"

for letter in name:
    print(letter)

Loop With Index and Value

for i, s in enumerate(species): print(i, s)    # 0 burmese  1 rock  2 ball  3 blood

enumerate() hands you both the index and the value on every pass — the usual alternative to looping over range(len(species)) when you need the position but still want direct access to each item.

species = ["burmese", "rock", "ball", "blood"]

for i, s in enumerate(species):
    print(i, s)

Nested Loops

for s in species:
    for letter in s[:2]:
        print(s, letter)
# burmese b  burmese u  rock r  rock o  ball b  ball a  blood b  blood l

A loop can contain another loop — the inner loop runs all the way through for every single pass of the outer one. Useful for combinations, grids, or looping over a collection nested inside another collection.

species = ["ball", "burmese"]

for s in species:
    for letter in s[:2]:
        print(s, letter)

While Loops

A while loop repeats its body for as long as a condition stays True — checked again before every pass — so it's the right choice when you don't know ahead of time how many times you'll need to loop.

count = 0

while count < 3:
    print(count)
    count += 1

Avoiding Infinite Loops

count = 0
while count < 3:
    print(count)
    count += 1    # forgetting this line means count < 3 is always True

Nothing stops a while loop automatically — if its condition never becomes False, it repeats forever and freezes whatever's running it. Always make sure something inside the loop body moves it toward ending, like incrementing a counter or updating the value being checked.

count = 0
while count < 3:
    print(count)
    count += 1

Loop Control

A for loop and a while loop can both be redirected mid-run — cut short, skipped ahead by one pass, or wrapped up with a bit of code that only runs if nothing interrupted them. These keywords work identically in either loop type.

species = ["burmese", "rock", "ball", "blood"]

for s in species:
    if s == "ball":
        break
    print(s)

Break

for s in species:
    if s == "ball":
        break
    print(s)                     # burmese  rock

count = 0
while count < 5:
    if count == 3:
        break
    print(count)
    count += 1                   # 0  1  2

break exits the loop immediately — nothing after it runs, and anything left in the sequence (or any remaining passes of the condition) is skipped entirely.

species = ["burmese", "rock", "ball", "blood"]

for s in species:
    if s == "ball":
        break
    print(s)

count = 0
while count < 5:
    if count == 3:
        break
    print(count)
    count += 1

Continue

for s in species:
    if s == "ball":
        continue
    print(s)                     # burmese  rock  blood

count = 0
while count < 5:
    count += 1
    if count == 3:
        continue
    print(count)                 # 1  2  4  5

continue skips just the current pass — the rest of the loop body doesn't run for that item — but the loop itself keeps going from the next item or the next check of the condition.

species = ["burmese", "rock", "ball", "blood"]

for s in species:
    if s == "ball":
        continue
    print(s)

count = 0
while count < 5:
    count += 1
    if count == 3:
        continue
    print(count)

The Else Clause

for s in species:
    print(s)
else:
    print("done")                # burmese  rock  ball  blood  done

count = 0
while count < 3:
    print(count)
    count += 1
else:
    print("done")                # 0  1  2  done

Both loop types can end with an else block, which runs once the loop finishes all its passes on its own — it's skipped entirely if the loop was cut short with break.

species = ["burmese", "rock", "ball", "blood"]

for s in species:
    print(s)
else:
    print("done")

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

The pass Statement

for s in species:
    pass    # does nothing, but keeps the loop syntactically valid

An empty loop body is a syntax error — pass is a no-op you can use as a placeholder while you're still deciding what a loop (or if, function, etc.) should do.

species = ["burmese", "rock", "ball", "blood"]

for s in species:
    pass

print("loop finished without doing anything each pass")