How can I swap two Python values without a temporary variable?
Quick answer
Write a, b = b, a. Python evaluates the values on the right before assigning them to the names on the left, so the names receive each other’s original values. For example, if a is 1 and b is 2, afterward a is 2 and b is 1. Two separate assignments like a = b and then b = a instead overwrite the original value of a. [S1]
The question, the details, the next step.
What’s the straightforward way to swap two values in Python without making a temporary variable?
Use tuple unpacking:
a, b = b, aFor a concrete example:
a = 1
b = 2
a, b = b, a
print(a, b) # 2 1The values on the right are evaluated before assignment to the names on the left, so each name gets the other name’s value from before the statement. [S1] You don’t need a separate temporary name to hold one value while you move the other.
Why doesn’t a = b followed by b = a do the same thing?
Because the first statement changes what a refers to before the second statement runs. Suppose a is 1 and b is 2:
a = b # a is now 2; b is still 2
b = a # reads the new value of a, so b is 2The original 1 is no longer available through either name. The two-line version therefore leaves both names with the value 2, rather than exchanging their values. In the unpacking version, Python evaluates the right-hand values before assigning to the left-hand targets, which is why both original values are available for the swap. [S1]
Can I use the same pattern with different variable names, and how do I check the result?
Yes. Put the two names you want to exchange on the left, then put them in the opposite order on the right. For example, assuming left and right already hold the values you want to swap:
left, right = right, leftYou can check the pattern with distinct starting values:
left = "east"
right = "west"
left, right = right, left
print(left, right) # west eastAfter the assignment, left has the former value of right, and right has the former value of left. This small check helps catch a reversed or mistyped name; it does not tell you whether other parts of a program depend on those names.
What’s a quick way to remember the order when editing an existing example?
Keep the left-hand names in the order you want to refer to them afterward, and write their values in the order they should receive them on the right. For a two-name swap, the right-hand names are reversed:
first, second = second, firstBefore using it, check that both sides have the two intended names and that they appear in opposite order. If you accidentally write first, second = first, second, each name gets its own current value, so nothing is exchanged. The swap behavior described here is Python assignment behavior; don’t assume another language handles the same-looking syntax the same way. [S1]
A practical checklist
Identify the two names whose current values should exchange places.
Put both names on the left side of one assignment, separated by a comma.
Put the same names in reverse order on the right:
a, b = b, a.If unsure, try two distinct starting values and inspect both names afterward.
Avoid two sequential assignments that overwrite an original value.
A few more useful answers
Does `a, b = a, b` swap the values?
No. Each name receives its own current value, so that statement leaves them unchanged.
Does the same swap syntax apply in every programming language?
Not necessarily. This describes Python’s assignment behavior; check the rules for another language separately.
Why does tuple unpacking avoid losing either original value?
Python evaluates the right-hand values before assigning them to the left-hand targets, so both original values are available for the assignment. [S1]
Sources
References used for this guide.
- [S1] Python Language Reference: Assignment statements and unpacking docs.python.org · Checked