Sequences can be explicitly/implicitly unpacked into discrete, ordered units of some type. Unpacking happens automatically using the , (comma). (The syntax is also used for Tuple construction.)
Unpacking works only in instances where the number of variables used exactly matches the number of items in the sequence.
Note: In the below examples parenthesis are not required, and are included for clarity.
The most common application of unpacking is to (shallow) copy items into named variables.
Unpacking a List.
>>> seq = [1, 2, 3] >>> (a, b, c) = seq # a=1, b=2, c=3
Unpacking a Tuple.
>>> tup = ('a', 'b') >>> (x, y) = tup # x='a', y='b'
Unpacking the characters of a String.
>>> (first, second, third) = "abc" # first='a', second='b', third='c'
Since input() is done a line at a time (and returns string data), unpacking is idiomatic Python3 for many input operations.
The below examples use the string split() method and the functional tool map().
>>> (x, y) = input().split() # x='a', y='b' on input "a b"
>>> (x, y) = map(int, input().split()) # x=1, y=2 on input "1 2"
Swapping objects.
>>> x = 1 >>> y = 2 >>> (x, y) = (y, x) # x=2, y=1
The unary * (star) operator is used to explicitly indicate unpacking, often outside the context of assignment. Use is common when elements of a sequence are intended to be used as the parameters to a function.
E.g., (Assume that seq contains 3 elements) fun(seq[0], seq[1], seq[2]) could be rephrased more tersely as fun(*seq).
Consider printing values in a List.
>>> print([1, 2, 3]) # legal, but performs a cast str([1, 2, 3]) [1, 2, 3]
In many instances we may wish to print a list's elements, but not formatted as a Python3 list.
>>> print(*[1, 2, 3]) # as though we indexed each individually 1 2 3
Extended Iterable Unpacking 1) extends unpacking to allow sequences on the left hand side of an assignment.
Like regular unpacking for member assignment, extended iterable unpacking is only allowed in instances where the Python3 interpreter can infer the number and position of arguments.
Note that the following examples make use of Python3 range() objects.
>>> (first, *rest) = list(range(1, 10)) # first=0, rest=[1, 2, ..., 9]
Note that the *rest as a second element in the previous example is somewhat arbitrary depending on need.
It is also valid to start with the iterable.
>>> (*rest, last) = list(range(1, 10)) # rest=[0, 1, ..., 8], last=9
One or more additional named variables can be specified before or after the rest.
>>> (first, *rest, last) = list(range(1, 10)) # first=0, rest=[2, 3, ..., 8], last=9
Note that the catch-all nature of a starred member does not mean that these values are required. In the event where additional elements are omitted they are treated as an empty sequence.
>>> (first, second, *rest) = [1, 2] # first=1, second=2, rest=[]
The number of named elements before and after *rest is arbitrary as long as those elements do not include additional starred iterables.2)