Expanding tuples into arguments

Question

Suppose I have a function like:

def myfun(a, b, c):
    return (a * 2, b + c, c + b)

Given a tuple some_tuple = (1, "foo", "bar"), how would I use some_tuple to call myfun? This should output the result (2, "foobar", "barfoo").

I know could define myfun so that it accepts the tuple directly, but I want to call the existing myfun.

_{See also: What do ** (double star/asterisk) and * (star/asterisk) mean in a function call?.}

score 963 · Accepted Answer · edited May 27 '20 at 13:31

963

myfun(*some_tuple) does exactly what you request. The * operator simply unpacks the tuple (or any iterable) and passes them as the positional arguments to the function. Read more about unpacking arguments.

edited May 27 '20 at 13:31

Nicolas Gervais

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answered Jan 03 '10 at 02:24

Alex Martelli

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The * operator simply unpacks the tuple and passes them as the positional arguments to the function. See more here: https://docs.python.org/3/tutorial/controlflow.html#tut-unpacking-arguments – john_mc Jun 30 '17 at 21:09
4

Note that the same syntax can be used for lists as well as tuples. – brendon-ai Aug 17 '17 at 13:22
I've found that you can do the same with lists, (in fact, any iterable, including strings), not sure how their mutability affects things. That would be interesting to look into. – wcyn Nov 26 '17 at 11:47

score 76 · Answer 2 · answered Mar 19 '15 at 06:31

76

Note that you can also expand part of argument list:

myfun(1, *("foo", "bar"))

answered Mar 19 '15 at 06:31

Valentas

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It appears you can only do this if the expanded tuple is after the normally-provided arguments - the interpreter doesn't like it when I do this: `some_func(*tuple_of_stuff, another_argument)` – Tom Galvin Apr 19 '15 at 20:46
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@Quackmatic Having the expanded tuple in any location seems to work fine in Python 3.5.1 – River Jun 13 '16 at 13:00
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@Quackmatic seconding @River, this works fine in Python 3.5.4: `def func(a,b,c,d): print(a,b,c,d)` with `args = ('fee', 'fi', 'fo'); func(*args, 'fum')` – R. Navega Sep 06 '18 at 13:25

score 16 · Answer 3 · edited Oct 25 '17 at 10:02

16

Take a look at the Python tutorial section 4.7.3 and 4.7.4. It talks about passing tuples as arguments.

I would also consider using named parameters (and passing a dictionary) instead of using a tuple and passing a sequence. I find the use of positional arguments to be a bad practice when the positions are not intuitive or there are multiple parameters.

edited Oct 25 '17 at 10:02

Kellen Donohue

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answered Jan 03 '10 at 02:28

Uri

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score 10 · Answer 4 · edited Nov 20 '20 at 01:12

10

This is the functional programming method. It lifts the tuple expansion feature out of syntax sugar:

apply_tuple = lambda f, t: f(*t)

Redefine apply_tuple via curry to save a lot of partial calls in the long run:

from toolz import curry
apply_tuple = curry(apply_tuple)

Example usage:

from operator import add, eq
from toolz import thread_last

thread_last(
    [(1,2), (3,4)],
    (map, apply_tuple(add)),
    list,
    (eq, [3, 7])
)
# Prints 'True'

edited Nov 20 '20 at 01:12

Mateen Ulhaq

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answered Jan 07 '17 at 21:55

Dominykas Mostauskis

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This is not helpful for a beginner. It uses third-party modules and does other confusing stuff... – gberger May 26 '17 at 14:09
gberger, `lambda f, t: f(*t)` does not use third-party modules and I am a Python beginner and this is helpful to me. This is a pure functional approach. If you don't use this style then this answer is not for you. – Dominykas Mostauskis May 28 '17 at 07:32
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toolz is third-party, is what I meant – gberger May 30 '17 at 08:54
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Not every answer has to be for a beginner – Jordan Sep 02 '17 at 04:30
@gberger I'm here because I was curious about a functional approach. :D – Mateen Ulhaq Nov 14 '18 at 22:43
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For the example you gave, one can use `starmap` https://docs.python.org/3.7/library/itertools.html#itertools.starmap – Bo. Sep 04 '19 at 11:31
2

This is a terrible answer – Nicolas Gervais May 27 '20 at 13:33
1

An alternative definition with currying done by hand: `apply_tuple = lambda f: lambda args: f(*args)` – Mateen Ulhaq Nov 20 '20 at 01:08

score 6 · Answer 5 · answered Nov 20 '20 at 00:53

6

Similar to @Dominykas's answer, this is a decorator that converts multiargument-accepting functions into tuple-accepting functions:

apply_tuple = lambda f: lambda args: f(*args)

Example 1:

def add(a, b):
    return a + b

three = apply_tuple(add)((1, 2))

Example 2:

@apply_tuple
def add(a, b):
    return a + b

three = add((1, 2))

answered Nov 20 '20 at 00:53

Mateen Ulhaq

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You can also just define : def add(some_tyuple: tuple) and then access with an index – MissSergeivna Mar 24 '21 at 13:02

Expanding tuples into arguments

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