Spread operator in Python

By Thomas Taylor | | 2 minutes
Python

The JavaScript spread operator (...) is a useful and convenient syntax for expanding iterable objects into function arguments, array literals, or other object literals.

Python contains a similar “spread” operator that allows for iterable unpacking. Each of the examples below will demonstrate the comparison between the two languages.

Function Arguments

JavaScript:

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function multiply(a, b) {
    return a * b;
}
const numbers = [3, 5];
console.log(multiply(...numbers));
// Output: 15

Python:

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def multiply(a, b):
    return a * b
numbers = [3, 5]
print(multiply(*numbers))
# Output: 15

Array Literals

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const numbers = [1, 2, 3];
const newNumbers = [0, ...numbers, 4]
console.log(newNumbers);
// Output: [ 0, 1, 2, 3, 4 ]

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numbers = [1, 2, 3]
new_numbers = [0, *numbers, 4]
print(new_numbers)
# Output: [0, 1, 2, 3, 4]

Object Literals

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const testObj = { foo: 'bar' };
console.log({ ...testObj, foo2: 'bar2' });
// Output: { foo: 'bar', foo2: 'bar2' }

A very similar technique can be applied with Python dictionaries. Notice the double asterisk operator (**).

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test_obj = { 'foo': 'bar' }
print({ **test_obj, 'foo2': 'bar2' })
# Output: {'foo': 'bar', 'foo2': 'bar2'}

To unpack keyword arguments, the double asterisk operator (**) is used. In contrast, the single asterisk operator (*) is used for iterable objects.