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--- python3_unpacking [2018/08/08 10:14]
jguerin Added the word "Note".
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-====== Unpacking Sequences ======
-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 [[python3_tuples|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.
-=====Implicit Unpacking: Member Assignment=====
-The most common application of unpacking is to (shallow) copy items into named variables.
-====Lists====
-Unpacking a [[python3_lists|List]].
-<code python>
->>> seq = [1, 2, 3]
->>> (a, b, c) = seq  # a=1, b=2, c=3
-</code>
-====Tuples====
-Unpacking a [[python3_tuples|Tuple]].
-<code python>
->>> tup = ('a', 'b')
->>> (x, y) = tup # x='a', y='b'
-</code>
-====Strings====
-Unpacking the characters of a [[python3_strings|String]].
-<code python>
->>> (first, second, third) = "abc" # first='a', second='b', third='c'
-</code>
-=====Input (Console, File)=====
-Since ''input()'' is done a line at a time (and returns [[python3_strings|string]] data), unpacking is idiomatic Python3 for many input operations.
-The below examples use the [[python3_strings|string]] ''split()'' method and the [[python3_functional|functional]] tool ''map()''.
-<code python>
->>> (x, y) = input().split() # x='a', y='b' on input "a b"
-</code>
-<code python>
->>> (x, y) = map(int, input().split()) # x=1, y=2 on input "1 2"
-</code>
-=====In-Place Swap/Mutual Replacement=====
-Swapping objects.
-<code python>
->>> x = 1
->>> y = 2
->>> (x, y) = (y, x) # x=2, y=1
-</code>
-=====Explicit Unpacking: The * (Star) Operator=====
-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)''.
-====Unpacking Arguments====
-Consider printing values in a [[python3_lists|List]].
-<code python>
->>> print([1, 2, 3]) # legal, but performs a cast str([1, 2, 3])
-[1, 2, 3]
-</code>
-In many instances we may wish to print a list's elements, but not formatted as a Python3 ''list''.
-<code python>
->>> print(*[1, 2, 3]) # as though we indexed each individually
-2 3
-</code>
-====Extended Iterable Unpacking====
-Extended Iterable Unpacking (([[https://www.python.org/dev/peps/pep-3132/|PEP 3132]] - Extended Iterable Unpacking)) 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 [[python3_ranges|range()]] objects.
-===Unpacking First and Rest===
-<code python>
->>> (first, *rest) = list(range(1, 10)) # first=0, rest=[1, 2, ..., 9]
-</code>
-===Unpacking Rest and Last===
-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.
-<code python>
->>> (*rest, last) = list(range(1, 10)) # rest=[0, 1, ..., 8], last=9
-</code>
-===Unpacking First, Rest, and Last===
-One or more additional named variables can be specified //before// or //after// the ''rest''.
-<code python>
->>> (first, *rest, last) = list(range(1, 10)) # first=0, rest=[2, 3, ..., 8], last=9
-</code>
-===Empty Iterables===
-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.
-<code python>
->>> (first, second, *rest) = [1, 2] # first=1, second=2, rest=[]
-</code>
-The number of named elements before and after ''*rest'' is arbitrary as long as those elements do not include additional starred iterables.(((fst, *rest1, mid, *rest2, last) = list(range(1, 10%%))%% would result in ''SyntaxError: two starred expressions in assignment'' because it is unknown how many items would be in ''rest1'' and '''rest2'''. In this instance prefer list slicing.))

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