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Python Functions, Packing, and Unpacking Packing and Unpacking Unpacking

Posted by aaron roberts
aaron roberts
2,004 Points

unpacking question.

can anyone tell me why this code doesn't run interms of unpacking?

def league(*teams):
    team1, team2, team3 = teams
    print(team1)

prem = ["arsenal", "chelsea", "liverpool"]
league(prem)

'ValueError: not enough values to unpack (expected 3, got 1)' is what is return but considering there is 3 strings in the list i don't understand what the problem is?

2 Answers

AJ Tran
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STAFF
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AJ Tran
Treehouse Teacher
AJ Tran
.a{fill-rule:evenodd;}techdegree
AJ Tran
Treehouse Teacher

Hi aaron roberts !

prem = ["arsenal", "chelsea", "liverpool"]
league(prem)

# => ValueError: not enough values to unpack (expected 3, got 1)

prem counts as one value, since it is one list!

The notation you'll want is:

league('arsenal', 'chelsea', 'liverpool')

# => arsenal

Try this out and see :)

aaron roberts
aaron roberts
2,004 Points

Thanks!

Alexis Othonos
Alexis Othonos
3,171 Points

Or you can unpack the list before passing it as an argument:

prem = ["arsenal", "chelsea", "liverpool"]
league(*prem)
aaron roberts
aaron roberts
2,004 Points
aaron roberts
aaron roberts
2,004 Points

thanks! i think i just covered this in one of the courses, so the * in the argument counts towards there being more values?

Alexis Othonos
Alexis Othonos
3,171 Points

The asterisk (*) works only on iterables.

So if you know that what you are passing is a list, tuple, etc. then you can safely add the asterisk to unpack the values.

Try with print function

prem = ["arsenal", "chelsea", "liverpool"]
teams = 3

# pass list as a single element, therefore the list as an element will be printed:
print(prem)
# ['arsenal', 'chelsea', 'liverpool']

# pass list arguments individually by unpacking. The print function can automatically handle multiple arguments
print(*prem)
# arsenal chelsea liverpool

# unpack non iterable value in function:
print(*teams)
# TypeError

Hope this helps