Unpack value(s) into variable(s) or None (ValueError: not enough values to unpack)

Question

How should an iterable be unpacked into a mismatching number of variable(s)?

Too many values:

>>> one,two = [1,2,3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack (expected 2)

can be ignored with

>>> one,two,*_ = [1,2,3,4]  
>>>

Note: "extended iterable unpacking" since Python 3 only. About the underscore.

How can the opposite case of too few data / more variables:

>>> one,two,three = [1,2]   
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: not enough values to unpack (expected 3, got 2)
>>>

be handled, specifically so that the remaining variables are assigned None (or some other value)?

Something like this:

>>> one,two,three = [1,2] or None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: not enough values to unpack (expected 3, got 2)
>>>

https://stackoverflow.com/a/8857846/1619432 suggests expanding the list:

>>> one,two,three = [1,2] + [None]*(3-2)
>>> one   
1
>>> two
2
>>> three
>>>

Are you looking for a more general answer where you can have more than three variables on the left as well? — Devesh Kumar Singh, Dec 12 '19 at 11:16
@DeveshKumarSingh Yes. The number variables is constant, though. — handle, Dec 12 '19 at 11:18

score 15 · Answer 1 · answered Dec 12 '19 at 11:15

15

You can unpack a sequence to three variable using:

one, two, *three = [1,2]

At this point, three will be an empty list. You can then assign three to None using an or check if three is empty.

three = three or None

answered Dec 12 '19 at 11:15

James

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I think he is looking for a more general answer, where the number of variable on the left can be any quantity greater than or equal to 3 – Devesh Kumar Singh Dec 12 '19 at 11:16

score 5 · Accepted Answer · edited Feb 23 '21 at 18:03

Use the * operator and fill an intermediate iterable with that which you're unpacking and fill the remainder with your default value of choice.

x = [1, 2]
default_value= None
one, two, three = [*x, *([default_value] * (3 - len(x)))]

And a bonus function to handle both cases:

def unpack(source, target, default_value=None):
    n = len(source)
    if n < target:
        return [*source, *([default_value] * (target - len(source)))]
    elif n > target:
        return source[0:target]
    else:
        return source

Amend to handle non-iterable input as needed.

score 5 · Answer 3 · answered Dec 12 '19 at 11:16

5

Does this work for you? You can make the nones array as big as you need.

nones = [None]*100 # or however many you think you might need
one, two, three, four, five, *_ = [1,2] + nones

answered Dec 12 '19 at 11:16

igg

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Unpack value(s) into variable(s) or None (ValueError: not enough values to unpack)

3 Answers3