Unpacking in Python (1)

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*Memo:

• My post explains the unpacking with an assignment statement (2).
• My post explains the unpacking with a for statement (1).
• My post explains the unpacking with a for statement (2).
• My post explains the iterable unpacking with * and a function (1).
• My post explains the iterable unpacking with * and a function (2).
• My post explains the dictionary unpacking with ** within a dictionary and function.
• My post explains *args.
• My post explains **kwargs.
• My post explains a variable assignment.
• My post explains a function (1).

Iterable unpacking can be done without * and with an assignment statement as shown below:

*Memo:

• The syntax of the one or more variables and commas(,), tuples(()) and/or lists([]) with = iterable is an iterable unpacking:
  • v, = [0] and v = [0] are different.
  • One or more variables and commas are a tuple even if () isn't used.
  • Lists are converted to tuples if lists are explicitly used on the left side of =:
• The number of variables must match the number of the elements in an iterable unless a *variable is used on the left side of =:
  • Only one *variable can be used within each tuple and list on the left side of = to flexibly accept the zero or more elements of an iterable:
  • A *variable is the list whose default value is [] so the type is list.
  • A *variable cannot be used outside any list and tuple.
• One or more *iterables can be used within each list, tuple and set on the right side of = to unpack iterables:
  • A *iterable cannot be used outside any list, tuple and set.
• One or more *iterables can be used as the arguments within a function call including print() to unpack them into the one or more parameters including args but excluding *kwargs within a function definition:
  • One or more *iterables can be used with print() and *args but not with **kwargs.
• One or more **dictionaries can be used within each dictionary on the right side of = to unpack dictionaries:
  • A **dictionary cannot be used outside any dictionary.
• A * is called an iterable unpacking operator to unpack an iterable as the name suggests so the one or more *iterables used within each list, tuple and set on the right side of = and used as the arguments within a function call including print() to unpack iterables are iterable unpacking operators but the only one *variable used within each tuple and list on the left side of = to flexibly accept the zero or more elements of an iterable isn't an iterable unpacking operator:
  • The iterable unpacking operator * can unpack(flat) only the most outer dimension of an iterable.
• A ** is called a dictionary unpacking operator to unpack a dictionary as the name suggests so the one or more **dictionaries used within each dictionary on the right side of = to unpack dictionaries are dictionary unpacking operators:
  • The dictionary unpacking operator ** can unpack(flat) only the most outer dimension of a dictionary.
• A string(str) can be unpacked infinitely because even the single character unpacked is also the string(str) which is an iterable.
• The doc explains an assignment statement.

v1, v2, v3 = [0, 1, 2] # list v1, v2, v3 = (0, 1, 2) # tuple v1, v2, v3 = {0, 1, 2} # set v1, v2, v3 = frozenset({0, 1, 2}) # frozenset v1, v2, v3 = {0:1, 2:3, 4:5} # dict v1, v2, v3 = {0:1, 2:3, 4:5}.keys() # dict.keys() v1, v2, v3 = iter([0, 1, 2]) # iterator v1, v2, v3 = '012' # str v1, v2, v3 = range(3) # range  print(v1, v2, v3) # 0 1 2  v4, v5, v6 = v1, v2, v3 v1, v2, v3 = {0:1, 2:3, 4:5}.values() # dict(int:int).values()  print(v1, v2, v3) # 1 3 5  v1, v2, v3 = {0:1, 2:3, 4:5}.items() # dict(int:int).items()  print(v1, v2, v3) # (0, 1) (2, 3) (4, 5)  v1, v2, v3 = b'012' # bytes v1, v2, v3 = bytearray(b'012') # bytearray  print(v1, v2, v3) # 48 49 50  v1, = '0' # str v2, = v1 v3, = v2 print(v1, v2, v3) # 0 0 0  v1, v2, v3 = [0, 1, 2] (v1, v2, v3) = [0, 1, 2] [v1, v2, v3] = [0, 1, 2] print(v1, v2, v3) # 0 1 2  v1, v2 = [0, [1, [2]]] (v1, v2) = [0, [1, [2]]] [v1, v2] = [0, [1, [2]]] print(v1, v2) # 0 [1, [2]]  v1, (v2, v3) = [0, [1, [2]]] v1, [v2, v3] = [0, [1, [2]]] (v1, (v2, v3)) = [0, [1, [2]]] [v1, [v2, v3]] = [0, [1, [2]]] print(v1, v2, v3) # 0 1 [2]  v1, (v2, (v3,)) = [0, [1, [2]]] v1, [v2, [v3]] = [0, [1, [2]]] (v1, (v2, (v3,))) = [0, [1, [2]]] [v1, [v2, [v3]]] = [0, [1, [2]]] print(v1, v2, v3) # 0 1 2  v, = [0] (v,) = [0] [v] = [0] print(v) # 0  # It's not an iterable unpacking. v = [0, 1, 2] print(v) # [0, 1, 2]  # It's not an iterable unpacking. v = [0] print(v) # [0]  # It's not an iterable unpacking. v1 = v2 = v3 = [0, 1, 2] print(v1, v2, v3) # [0, 1, 2] [0, 1, 2] [0, 1, 2]  v1, v2, v3 = [0, 1] # ValueError: not enough values to unpack (expected 3, got 2)  v1, v2, v3 = [0, 1, 2, 3] # ValueError: too many values to unpack (expected 3)  v, = [] # ValueError: not enough values to unpack (expected 1, got 0)  v, = 0 # TypeError: cannot unpack non-iterable int object