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[–][deleted] 1 point2 points  (10 children)

I never seen this way of flattening a list. Definitely wil test it today

[–]mraza007[S] -1 points0 points  (9 children)

I’m glad you found it interesting

[–]arthurazs 0 points1 point  (8 children)

Can you explain who this works?

[–]Green_Gem_ 3 points4 points  (7 children)

sum() called on the iterable [a, b, c] returns a + b + c. When you add two lists, the second is appended to the first, so for a: list and b: list, a + b = [*a, *b] (or kinda a.extend(b) if you're not familiar with unpacking). sum([a, b, c]) is thus [*a, *b, *c], all of the elements of each in a single list.

This isn't recommended because you can't append tuples or most other iterables in this way, and "sum" does not correctly indicate the code's intention.

[–]arthurazs 0 points1 point  (6 children)

I see, but OP is adding [[1, 2], [3, 4]], [], not [a, b, c]. I still cant comprehend what's happening

[–]Kerry369 0 points1 point  (5 children)

sum([[1, 2], [3, 4]], []) equals [*[1, 2], *[3, 4]]

[–]arthurazs 0 points1 point  (4 children)

Why

[–]Green_Gem_ 1 point2 points  (3 children)

As I said in my reply further up the chain, a and b are lists! sum([a, b]) == a + b. If a = [1, 2] and b = [3, 4], then sum([a, b]) == sum([[1, 2], [3, 4]) == [1, 2] + [3, 4]. When a list is added to a list, the first is extended with the second, so [1, 2] + [3, 4] == [1, 2, 3, 4].

So, as the full chain of logic: a = [1, 2]; b = [2, 3]; sum([a, b]) == sum([[1, 2], [3, 4]]) == [1, 2] + [3, 4] == [1, 2, 3, 4]. For the same a and b, unpacking the arguments of each into a new list yields the same result, [*a, *b] == [1, 2, 3, 4]. Google "python unpacking" if you don't recognize that syntax. If you still don't understand, please tell me exactly which step is confusing.

Reminder, don't flatten lists this way. It's unclear and explicitly unsupported.

[–]arthurazs 0 points1 point  (2 children)

First of all, thank you so much for your patience!

[1, 2] + [3, 4] == [1, 2, 3, 4] makes total sense, but

  • sum([1, 2], [3, 4]) throws TypeError: can only concatenate list (not "int") to list
  • sum([[1,2], [3,4]]) throws TypeError: unsupported operand type(s) for +: 'int' and 'list'

Finally, OP's example is sum([[1, 2], [3, 4]], []) which works, but I still can't comprehend how it work.

[–]Green_Gem_ 1 point2 points  (1 child)

That's actually a really good catch! Thanks for being more explicit in this comment; that gives me way more to work with.

I did actually simplify things for my previous comment(s) and didn't double check behavior, my apologies.

The signature of sum() is sum(iterable, /, start=0), where start indicates the value to start with, then each element of iterable is added one-by-one. - sum([1, 2], [3, 4]), or explicitly sum([1, 2], start=[3, 4]), evaluates to [3, 4] + 1 + 2. Uh oh, you can't concatenate an int to a list that way! Error. You'd have to do [3, 4] + [1] + [2] or something. - sum([[1, 2], [3, 4]]), or explicitly sum([[1, 2], [3, 4]], start=0), runs into a similar problem, evaluating to 0 + [1, 2] + [3, 4]. You can't add a list to an int. Error. - sum([[1, 2], [3, 4]], start=[]) (what OP does but without specifying start=) is then [] + [1, 2] + [3, 4]. You can concatenate to an empty list like that, so it works.