Question

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In Python, what is the difference between “.append()” and “+= []”?

In Python, I’ve recently noticed that you can append list items in two ways:

a.append(1)
a += [1]

I like using the second approach because it is more readable for me. Are there any downsides to using it?

Answer 1

Those two methods aren’t quite equivalent. The += method:

a += [1]

requires that you first create a new list containing the single element 1, tack it on to the list a, then discard the single-element list. This would be more equivalent to:

a.extend([1])

You will likely find that a.append(1) does less work, since it does not need to create a single-element list which it’s just going to throw away in the next step.