for loops and range

A while loop waits for a condition. A for loop is for the other common case: do this for each item. In Python it works on anything iterable: a string, a range of numbers, the lines of a file, a list, and many more. The collections module introduces lists, and a for loop treats them exactly as it treats the strings and ranges in this lesson.

for over a range

for i in range(3):
    print(i)

0
1
2

for i in range(3): binds the name i to each number in turn and runs the indented block once per number. There is no counter to manage and no condition to get wrong.

range

range produces numbers on demand. It takes the same arguments as a slice:

• range(stop) counts from 0 up to, but not including, stop.
• range(start, stop) begins at start.
• range(start, stop, step) moves by step, which may be negative.

range(3) gives 0, 1, 2: three numbers, which is why it suits “do this 3 times”. A range does not build the whole sequence in memory, so range(1_000_000) is cheap. To see the numbers in one go, wrap it in list(...), which turns any sequence into a list:

print(list(range(4)))
print(list(range(2, 10, 3)))
print(list(range(5, 0, -2)))

[0, 1, 2, 3]
[2, 5, 8]
[5, 3, 1]

for over a string

A string is a sequence of characters, so for can visit one character at a time.

for letter in "abc":
    print(letter.upper())

A
B
C

Nested loops

The inner loop runs completely for every pass of the outer one. This prints a small multiplication table, building each row as a string:

for row in range(1, 4):
    line = ""
    for col in range(1, 6):
        line += f"{row * col:3}"
    print(line)

  1  2  3  4  5
  2  4  6  8 10
  3  6  9 12 15

Going backwards: reversed

reversed(sequence) walks a sequence from the end to the start without changing it. It works on ranges, strings, and the lists you will meet soon.

for n in reversed(range(1, 4)):
    print(n)
print("Liftoff")

for letter in reversed("abc"):
    print(letter, end=" ")
print()

3
2
1
Liftoff
c b a

reversed(range(1, 4)) is easier to read than range(3, 0, -1), although they give the same numbers.

enumerate

If you need the position as well as the item, do not write range(len(items)). Use enumerate, which yields (position, item) pairs. The optional start=1 begins counting at 1, which suits numbered lists shown to people.

for position, letter in enumerate("abc", start=1):
    print(position, letter)

1 a
2 b
3 c

The two names before in receive the two halves of each pair. The tuples lesson explains this unpacking properly.

zip

zip walks several sequences in step and yields one pair per position:

for letter, digit in zip("abc", "123"):
    print(letter, digit)

a 1
b 2
c 3

By default zip stops silently at the shortest input. The c simply vanishes below, with no warning:

for letter, digit in zip("abc", "12"):
    print(letter, digit)

a 1
b 2

3.10+Requires version 3.10 or later Passing strict=True makes zip raise a ValueError instead when the lengths differ (PEP 618). Use it whenever two sequences are supposed to line up, so a mismatch becomes a loud error rather than lost data. The error is raised when the shorter input runs out, so the pairs before it are still processed:

for letter, digit in zip("abc", "12", strict=True):
    print(letter, digit)

a 1
b 2
ValueError: zip() argument 2 is shorter than argument 1

Common pitfalls

• Off-by-one with range. range(1, 10) ends at 9. To include 10, write range(1, 11).
• Changing a sequence while looping over it. Adding or removing items from the list you are iterating makes the loop skip items. Loop over a copy or build a new list.
• for i in range(len(items)) just to read items. Loop over the items themselves, or use enumerate when you need positions.
• Reusing the loop variable. After the loop, the name still holds the last item. It does not disappear, and it can clash with a name you meant to use elsewhere.
• Silent truncation with zip. Without strict=True, mismatched lengths lose data without a message.
• Expecting range to include the stop value. It never does, just like slices.

total = 0
for n in range(1, 6):
    total += n * n
print(total)

55

for n in reversed(range(1, 6)):
    print(n)
print("Liftoff")

5
4
3
2
1
Liftoff

      1   2   3   4
 1    1   2   3   4
 2    2   4   6   8
 3    3   6   9  12
 4    4   8  12  16

size = 4

print("   ", end="")
for col in range(1, size + 1):
    print(f"{col:4}", end="")
print()

for row in range(1, size + 1):
    print(f"{row:2} ", end="")
    for col in range(1, size + 1):
        print(f"{row * col:4}", end="")
    print()

      1   2   3   4
 1    1   2   3   4
 2    2   4   6   8
 3    3   6   9  12
 4    4   8  12  16

Summary

• for item in iterable: visits each item in turn; range(start, stop, step) generates numbers and never includes stop.
• Use reversed to walk backwards, enumerate for positions and zip for several sequences at once.
• 3.10+Requires version 3.10 or later zip(..., strict=True) raises ValueError when lengths differ.

Next: loop control and loop else.