CODEWITHSUNDEEP
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SoraHeart
SoraHeart

Reputation: 430

What is the logic behind Start=list in sum()

a = sum([[1, 4], [2, 3]],[])

#output = [1, 4, 2, 3]

Can please explain how does the start = [] unpack both list? Thanks

Upvotes: 0

Views: 39

Answers (1)

Snild Dolkow
Snild Dolkow

Reputation: 6866

sum() adds all elements in the input collection, as if they were joined by the + operator. One possible implementation could be something like this:

def sum(vals, start=0):
    result = start
    for v in vals:
        result = result + v
    return result

Since the default start value is 0, and int + list gives a type error, supplying [] as the start value instead will give the expected result.

In the end, a = sum([[1, 4], [2, 3]],[]) will behave equivalently to:

a = []
a = a + [1, 4]
a = a + [2, 3]

Upvotes: 3

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