Returning multiple objects and using them as arguments

Question

I have a function which goes something like this:

def do_something(lis):
    do something
    return lis[0], lis[1]

and another function which needs to take those two return objects as arguments:

def other_function(arg1, arg2):
    pass

I have tried:

other_function(do_something(lis))

but incurred this error:

TypeError: other_function() missing 1 required positional argument: 'arg2'

Answer 1

You need to unpack those arguments when calling other_function.

other_function(*do_something(lis))

Based on the error message, it looks like your other function is defined (and should be defined as)

def other_function(arg1, arg2):
    pass

So, when you return from do_something, you are actually returning a tuple containing (lis[0], lis[1]). So when you originally called other_function, you passing a single tuple, when your other_function was still expecting a second argument.

You can see this if you break it down a bit further. Below is a breakdown of how the returns look like when handled differently, a reproduction of your error and a demo of the solution:

Returning in to a single variable will return a tuple of the result:

>>> def foo():
...     lis = range(10)
...     return lis[1], lis[2]
...
>>> result = foo()
>>> result
(1, 2)

Returning in to two variables, unpacks in to each var:

>>> res1, res2 = foo()
>>> res1
1
>>> res2
2

Trying to call other_function with result which now only holds a tuple of your result:

>>> def other_function(arg1, arg2):
...     print(arg1, arg2)
...
>>> other_function(result)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: other_function() missing 1 required positional argument: 'arg2'

Calling other_function with res1, res2 that holds each value of the return from foo:

>>> other_function(res1, res2)
1 2

Using result (your tuple result) and unpacking in your function call to other_function:

>>> other_function(*result)
1 2