Problem: Given two variables a and b. Use a single line of Python code to swap the variables to assign the value of a to b and the value of b to a.
Example: Say, you have to integers a=21 and b=42. You want to swap the variables so that a=42 and b=21.
a = 21 b = 42 # SWAP MAGIC HERE print(a, b) # 42 21
How to swap to variables as a Python one-liner?
Let’s dive into the best ways to swap integers!
Swap Integers
To swap two variables a and b, use the multiple assignment expression a, b = b, a that assigns the value of a to b and the value of b to a.
a = 21 b = 42 # Swap One-Liner a, b = b, a # Print the result to the shell print(a, b) # 42 21
Explanation
The Python interpreter evaluates every expression from left to right. Python realizes that in order to assign the value to the variables, it must first evaluate the right-hand-side of the equation. So, here are the steps it takes to determine the result of the line a, b = b, a:
- The right side
b, ais short for(b, a)and, thus, it creates a tuple in memory with two values. The first tuple value is the object to which the variablebpoints in memory (here:42). The second tuple value is the object to which the variableapoints in memory (here:21). In our case, the memory now contains a new tuple object(42, 21). - Python now assigns this tuple to the list of variables defined on the left-hand side of the equation (here:
a, b = ...). The variable nameanow points to the first tuple value (here:42). The variable namebnow points to the second tuple value (here:21). - This way, the tuple is unpacked to the variables on the left-hand side. Variables
aandbhave been swapped.
Do It Yourself: You can see this code and the memory in action in the interactive memory visualizer:
Click “Next” to view how the memory objects unfold.
Does this also work to swap other objects like lists or strings?
How to Swap Lists, Tuples, and Other Objects
Let’s try out in our interactive code shell:
Exercise: Print the result of the variables a and b before and after swapping. Can you swap any object using Python’s multiple assignment feature?
You can see this code and the memory in action in the interactive memory visualizer:
Where to Go From Here?
Enough theory. Let’s get some practice!
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