Unpacking a Sequence into Separate Variables

Problem:

You have an N-element tuple or sequence that you would like to unpack into a collection of N variables.

Solution:

Any sequence (or iterable) can be unpacked into variables using a simple assignment operation. The only requirement is that the number of variables and structure match the sequence. For example:

>>> p = (4, 5)

>>> x, y = p

>>> x

4

>>> y

5

>>> 

>>> data = [ 'ACME', 50, 91.1, (2012, 12, 21) ]

>>> name, shares, price, date = data

>>> name

1

'ACME'

>>> date

(2012, 12, 21)

>>> name, shares, price, (year, mon, day) = data

>>> name

'ACME'

>>> year

2012

>>> mon

12

>>> day

21

>>>        

If there is a mismatch in the number of elements, you’ll get an error. For example:

>>> p = (4, 5)

>>> x, y, z = p

Traceback (most recent call last): File "<stdin>", line 1, in <module>

ValueError: need more than 2 values to unpack

>>>        

Discussion:

Unpacking actually works with any object that happens to be iterable, not just tuples or lists. This includes strings, files, iterators, and generators. For example:

>>> s = 'Hello'

>>> a, b, c, d, e = s

>>> a

'H'

>>> b

'e'

>>> e

'o'

>>>        

When unpacking, you may sometimes want to discard certain values. Python has no special syntax for

this, but you can often just pick a throwaway variable name for it. For example:

>>> data = [ 'ACME', 50, 91.1, (2012, 12, 21) ]

>>> _, shares, price, _ = data

>>> shares

50

>>> price

91.1

>>>        

However, make sure that the variable name you pick isn’t being used for something else already.

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