CODESARRAY

Lesson 3, by Codesarray, , 5 min read

Python if else and loops: if, elif, for, while, break, continue

How a program decides with if, elif and else, repeats with for and while, and stops or skips a round with break and continue.

Python if else and loops are how a program decides what to do and does it again. By the end of this page your programs will make decisions with if, elif and else, repeat work with for and while, and stop a loop early with break or skip one round with continue.

Decide with if, elif and else

Write if, a condition, and a colon. The indented lines run only when the condition is True. elif checks another condition if the first one was False, and else catches everything that is left:

temperature = 23
if temperature > 30:
    print("hot")
elif 20 <= temperature <= 30:
    print("warm")
else:
    print("cold")
warm

Python tries the conditions in order, runs the first block that matches, and skips the rest. temperature is 23, so the first test fails, the second passes, and only warm is printed.

Chained comparisons

That middle condition, 20 <= temperature <= 30, is a chained comparison. In most languages you would write two comparisons joined by and:

20 <= temperature and temperature <= 30

Python lets you write it the way you would on paper, and it means exactly the same thing. Any comparisons chain this way:

>>> 0 < 5 < 10
True
>>> 0 < 15 < 10
False

Repeat with for

A for loop repeats a block once for each item in a sequence. range(1, 6) gives the numbers from 1 up to, but not including, 6:

for n in range(1, 6):
    print(n)
1
2
3
4
5

So n takes 1, 2, 3, 4, 5 and the block runs five times. A for loop works on any collection: for word in ["red", "green", "blue"]: visits each word, and for letter in "abc": visits each letter.

Repeat with while

A while loop repeats as long as a condition stays True:

count = 3
while count > 0:
    print(count)
    count -= 1
3
2
1

count starts at 3. Each time round, the loop prints count and subtracts one. When count reaches 0, the condition count > 0 is False and the loop ends. Make sure something inside the loop changes the condition, or it runs forever.

break and continue

Two keywords change a loop from inside. continue skips the rest of this round and goes straight to the next item:

for n in range(1, 6):
    if n == 3:
        continue
    print(n)
1
2
4
5

break stops the whole loop at once:

for word in ["red", "green", "blue"]:
    if word == "green":
        print("found", word)
        break
found green

The loop ends as soon as it finds green, so blue is never visited.

The else on a loop

A loop can have an else too. Its block runs only when the loop finished without a break:

for word in ["red", "green", "blue"]:
    if word == "green":
        print("found", word)
        break
else:
    print("not found")
found green

Searching for green, break fires, so the else is skipped. If green were not in the list, say ["red", "blue"], the loop would run out of words and the else would print not found. It is Python's way of saying the search failed.

The whole program

temperature = 23
if temperature > 30:
    print("hot")
elif 20 <= temperature <= 30:
    print("warm")
else:
    print("cold")

for n in range(1, 6):
    if n == 3:
        continue
    print(n)

count = 3
while count > 0:
    print(count)
    count -= 1

Lines 2 to 7 decide between hot, warm and cold, and line 4 is the chained comparison. Lines 9 to 12 loop over 1 to 5, and line 11 skips the 3. Lines 14 to 17 count down from 3 with while, and line 17 is what makes the loop end.

warm
1
2
4
5
3
2
1

What you learned

  • if, elif and else choose one block: the first condition that is True wins.
  • A chained comparison like 0 < x < 10 reads like maths and means the same as two tests joined by and.
  • for repeats once per item; while repeats while a condition holds.
  • break stops a loop, continue skips to the next round.
  • A loop's else runs only when there was no break.

Common questions

**Why does range(5) stop at 4?** range counts from the start up to, but not including, the end, and the start is 0 when you give only one number, so range(5) is 0, 1, 2, 3, 4. A third number is the step: range(1, 10, 2) gives 1, 3, 5, 7, 9.

**What happens if I write if x = 3:?** Python refuses to run it. A single equals assigns, a double equals compares, and in Python 3.14 the error message even suggests the fix: SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

When should I use while instead of for? Use for when you know what you are looping over: a range, a list, a string. Use while when you do not know in advance how many rounds it will take, such as reading input until the user types quit or halving a number until it drops below 1.

Does break leave every loop I am inside? No, only the innermost one. The outer loop carries on with its next item. To leave both, put the inner loop in a function and return from it.

Can a while loop have an else as well? Yes, with the same rule: the else runs when the condition became False on its own, and is skipped when a break ended the loop.

This lesson follows on from Variables and types. Next in the series: strings.

This article goes with the video Control flow. Watch it on YouTube.