Pythonoid

CurriculumPython Variables for KidsSwapping and Tracing

Swapping Two Python Variables in One Line

left, right = right, left swaps two boxes in one line — and why the obvious two-line swap actually fails.

Lesson 5 of 5FreeAges 10+
Try this lesson interactively →

Swapping and Tracing

You know how to change ONE box at a time. Here's how to trade the values in TWO boxes at once.

left, right = right, left swaps them. Python builds the whole right side first — (right, left) — THEN hands it back to left and right.

left = 1
right = 2
left, right = right, left
print(left, right)

Output

2 1

Why not just do it the obvious way — left = right, then right = left? Watch closely.

left = right overwrites left's original 1 immediately. By the time right = left runs, left's old value is already gone.

left = 1
right = 2
left = right
right = left

So right = left doesn't copy the OLD left — it copies the NEW left, which is already 2. Both boxes end up the same.

left = 1
right = 2
left = right
right = left
print(left, right)

Output

2 2

left, right = right, left never has this problem, because Python reads BOTH old values — building the pair (right, left) — before changing either box.

left = 1
right = 2
left, right = right, left
print(left, right)

Output

2 1

One more trick: left = right = 0 sets BOTH boxes to 0 at once. It's the same right-side-first rule, just with one value instead of two.

left = right = 0 gives them the same starting value — but they're still two separate boxes from then on.

left = right = 0
left = left + 1
print(left, right)

Output

1 0

Check your understanding

left = 1, right = 2, then left = right, then right = left. What does print(left, right) show?

left = 1
right = 2
left = right
right = left
print(left, right)
  • 2 2
  • 2 1 — they swapped
  • 1 2 — nothing changed

Why: left = right overwrites left's 1 with right's 2 — that original 1 is gone for good. Then right = left copies the CURRENT left, which is already 2. Both boxes end up holding 2, the value right started with. This is exactly why a real swap needs left, right = right, left (or a temporary third box) instead.

left = 5, right = 9, then left, right = right, left. What does print(left, right) show?

left = 5
right = 9
left, right = right, left
print(left, right)
  • 9 5 — a real swap
  • 9 9 — same problem as the naive swap
  • 5 9 — nothing changed

Why: left, right = right, left builds the pair (right, left) — (9, 5) — using both OLD values first, before touching left or right at all. Only then does it assign 9 to left and 5 to right. That's the whole trick: read everything first, assign everything after, so neither box's old value can be lost mid-swap the way the naive version loses it.

What you'll practice

left = 5 and right = 9. Print both, labelled, before anything happens. Then swap them in one line — left, right = right, left — and print both again, labelled. left should now be 9, and right should now be 5.

Start this lesson for real →

More from this topic

Swapping and Tracing is one lesson inside Python Variables for Kids — see the full lesson order and what the whole topic covers.

View the full topic →