What's the param (`_`) in a function?

Question

I saw a function like bellow, I don't know the _ meaning.

def child_handler(signum, _):
    logging.warn('received SIGQUIT, doing graceful shutting down..')

what's the _ there?

but, however, if we ignore the _, why we need there an ignored param?

means to ignore the value, similar to the way you would use it in a loop — gold_cy, May 03 '19 at 11:25
[Possible Duplicate](https://stackoverflow.com/questions/5893163/what-is-the-purpose-of-the-single-underscore-variable-in-python) — Smart Manoj, May 03 '19 at 11:26
@SmartManoj you can flag for duplicate, not just leave a comment. — sanyassh, May 03 '19 at 11:29
while this can look a duplicate, I still can't get the point of using `_` as a function param. Can anyone explain? — Lante Dellarovere, May 03 '19 at 11:30
To answer as to why put a unused argument : this function is probably meant to be used in a context where it will be passed two arguments. — Olivier Melançon, May 03 '19 at 12:32

score 1 · Answer 1 · answered May 03 '19 at 11:25

1

the _ variable is just a way to say that it's not going to be used further down the line.

Basically, you don't care what the name is because it'll never be referenced.

answered May 03 '19 at 11:25

Nordle

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Devesh Kumar Singh · Accepted Answer · 2019-05-06T08:04:08.407

0

It essentially is a way of ignoring the value of the variable, and we don't want to use it down the line.

Another way of thinking is that it is a placeholder for the value that will be ignored

def func(a, b):

    return a,b

#I care about both return values
c,d = func(2,3)

#I don't care about the second returned value, so I put a _ to ignore it
c, _ = func(2, 3)

Another good use case of this is when you are running a for loop, but don't care about the index.

for _ in range(10):
    #do stuff

Even for functions, it acts like a don't care variable

def func(a, _):
    print(a)

func(1, 5)

The output will be 1

edited May 06 '19 at 08:04

answered May 03 '19 at 11:31

Devesh Kumar Singh

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why we need there an ignored param in a function? – user7693832 May 03 '19 at 12:07
Updated, please check! Actually I am also unsure why we need a ignored param, but could you point out where did you get `def child_handler(signum, _):` from ? – Devesh Kumar Singh May 03 '19 at 12:14
Why will the output of `func(1, 5)` be `5`, when the second parameter is ignored? It prints `1` for me... – funie200 May 06 '19 at 08:02

score -1 · Answer 3 · answered May 03 '19 at 11:27

-1

The underscore _ in Python can be used for different cases. In this case it means that the second argument of the child_handler function is ignored (so-called "Don't care").

answered May 03 '19 at 11:27

funie200

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What's the param (`_`) in a function?

3 Answers3