Python lambda returns 0 [closed]

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Is there a simple way to return 0 with a lambda function using and and or operators? For example, consider this function to sum elements in an array:

sum = lambda tab: tab == [] and 0 or tab[0] + sum(tab[1:])

Neutral element for number addition is 0 so it produces an IndexError: 0 is a falsy value.

Thus with multiplication it works:

>>> prod = lambda tab: tab == [] and 1 or tab[0] * prod(tab[1:])
>>> prod([1, 2, 3])
6

Other numbers than 0, for example 1, are truthy.

I would 100% just make this a full function. lambdas lose their readability pretty quickly. — Carcigenicate
– Carcigenicate, Commented Jul 29 at 13:45
I have no idea what your lambda is doing and when it should return 0. Maybe first write it as normal function - so you could use print() to test values - and later try to reduce it to one line. — furas
– furas, Commented Jul 29 at 13:46
Shouldn't it be as simple as if/else? Why do you need to juggle and/ors? — VLAZ
– VLAZ, Commented Jul 29 at 13:46
@trincot It's just an example... to illustrate the problem with 0 in this situation of using and and or. — david
– david, Commented Jul 29 at 14:05

no comment · Accepted Answer · 2025-07-29 14:08:45Z

1

I usually do it without or:

sum = lambda tab: len(tab) and tab[0] + sum(tab[1:])

print(sum([1, 2, 3]))

But if you insist on it:

sum = lambda tab: tab and tab[0] + sum(tab[1:]) or 0

print(sum([1, 2, 3]))

answered Jul 29 at 14:08

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2 Comments

Stef Jul 30 at 8:25

I know the op used sum as the name of their function, but your answer might be used as a reference by many people in the future, and using sum as a name is bad practice in python since it shadows a builtin, so I recommend changing the name to anything else in your answer.

no comment Jul 30 at 14:58

@Stef Yeah I'm not a fan of that, but I dislike mismatches between answers and questions more. Besides, who would use this instead of the built-in? And it even takes quadratic time and memory.

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