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Unpacking Python lists vs. destructuring Javascript arrays

ari_o profile image Arika O ・2 min read

If you are already using ES6 you must be pretty familiar with destructuring by now. As a refresher, destructuring allows us to assign the properties of an array/ object to individual variables, without accessing those properties using the . notation.

So instead of doing this:

const someNumbersJs = [3, 6, 9];
const oneJs  = someNumbers[0];
const twoJs = someNumbers[1];
const thereJs = someNumbers[2];
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... we could do the same thing using a much shorter code:

const [oneJs, twoJs, threeJs] = someNumbersJs;
console.log(oneJs); // prints 3
console.log(twoJs); // prints 6
console.log(threeJs); // prints 9
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In Python we can also destructure (unpack) items from a list (Python doesn't have a native array data structure but a list looks identical to a Javascript array). The syntax would look like so:

someNumbersPy = [3, 6, 9]
[onePy, twoPy, threePy] = someNumbersPy 
print(onePy) #prints 3
print(twoPy) #prints 6
print(threePy) #prints 9
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We can skip items in the list, just like in Javascript. In Python we skip them using an _ and not a comma (,).

Javascript

const [oneJs, , threeJs] = someNumbersJs
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Python

[onePy, _, threePy] = someNumbersPy 
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We can also use the rest operator - in Python is represented by an *, while in Javascript by an ellipsis (...):

Javascript

const [oneJs, ...restJs] = someNumbersJs;
console.log(restPy); // prints [6, 9]
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Python

[onePy, *restPy] = someNumbersPy 
print(restPy) #prints [6, 9]
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VERY NICE FEATURE: Compared to Javascript where the rest operator must be the last element in the array, in Python we can use it wherever we want, so we could do something like this:

otherNumbers = [528, 79, 2456, 1203, 74, 1]
[first, *restPy, last] = otherNumbers
print(first) #prints 528
print(rest) #prints [79, 2456, 1203, 74]
print(last) #prints 1
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Trying to do the same thing in Javascript will throw an error. Pretty neat, right?

Image source: Christina Morillo/ @divinetechygirl on Pexels

Discussion

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andrewbaisden profile image
Andrew Baisden

Nice to know that you can do stuff like this so useful.

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amirdarx profile image
amir

wonderful topic, really thanks 💛

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ari_o profile image
Arika O Author

You're welcome 🙌

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hanpari profile image
Pavel Morava

In Python, this is superfluous

[onePy, *restPy] = someNumbersPy

one can use

onePy, *restPy = someNumbersPy
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ari_o profile image
Arika O Author

Thank you for mentioning.