Unpack positional arguments from data structure in Python

Question

given:

def psum(a,b,c):
    return a**b+c

and x = [1,2] and y = 3 how can I do psum(*x,3) --> an equivalent.

I do not want to do x[0], x[1] because a function returns x and calling it twice would be inefficient. Could one do z = function(a). where z = x. and then do z[0], z[1].

But I'm wondering if one can do this otherwise and use positional arguments in such a way.

Also, without using a wrapper.

Edit: One cannot use names because I didn't implement the function and the function writers did not use named arguments :/

Also, note that if you have x -- x = func(...), then you can index x naturally. It won't call the function again. x = func(); psum(x[0],x[1],3) only calls func once. — mgilson
– mgilson, Commented Oct 12, 2012 at 18:25

Janne Karila · Accepted Answer · 2012-10-12 18:22:47Z

2

You can do this:

psum(*x + [y])

answered Oct 12, 2012 at 18:22

Janne Karila

25.3k6 gold badges59 silver badges97 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Eiyrioü von Kauyf Over a year ago

it works! and he winss :). you can do any order of *x + [2], [2] + x... etc it seems. now what about interspersed keyword and positional e.g. func(a=2,b,c,d=7) ;)

mgilson · Accepted Answer · 2012-10-12 18:26:43Z

1

In this case, the order of your arguments doesn't matter, so psum(3,*x) will work.

In the slightly-more-general case, if you know the names of the arguments,

psum(*x,c=3)

seems to work for me as well, but it won't work if sum is defined so that it takes varargs ...

I don't think there is a way to do this in the completely general case without modifying the object you pass to psum:

x.append(3)
psum(*x)

edited Oct 12, 2012 at 18:26

answered Oct 12, 2012 at 18:15

mgilson

312k70 gold badges656 silver badges722 bronze badges

9 Comments

Eiyrioü von Kauyf Over a year ago

sorry. fixed. order matters. i'm thinking ideally plot(x,y,z) is my problem and i'm using meshgrid(x,y) to get x,y

ely Over a year ago

I think you can do something like x_out,y_out = meshgrid(x,y) so that the result is two separate objects.

ely Over a year ago

The problem is that named arguments cannot follow a * pointer type expression. So either name all of the arguments, or write the function to handle an iterable, or use *args and **kwargs in the definition. I don't think this is very satisfactory on Python's part, but it is what it is.

ely Over a year ago

@Eiyrioü von Kauyf, check out meshgrid's docs: link. The main use case is such that you get two separate objects back, rather than a tuple of them.

jfs Over a year ago

@EMS: wrong. 1. Named arguments may follow *expression: f(*[1,2],c=3) is legal 2.A function always returns a single object in Python (it may be a tuple).

|

Collectives™ on Stack Overflow

Unpack positional arguments from data structure in Python

2 Answers 2

1 Comment

9 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

9 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related