for statement unpacking in Python (1)

Buy Me a Coffee☕

*Memo:

• My post explains for statement unpacking (2).
• My post explains for statement unpacking (3).
• My post explains assignment statement unpacking (1).
• My post explains function iterable unpacking (1).
• My post explains dictionary unpacking.

Iterable unpacking can be done without * and with a for statement as shown below:

*Memo:

• The syntax of the one or more variables and commas(,), tuples(()) and/or lists([]) with in iterable(iterable) is an iterable unpacking:
  • for v, in [[0]]: and for v in [[0]]: are different.
  • On the left side of in:
    • One or more variables and commas are a tuple even if () isn't used.
    • Lists are converted to tuples if lists are explicitly used.
    • Unless a *variable is used, the number of variables must match the number of the elements in each iterable in an iterable:
      • Only one *variable can be used within each tuple and list 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.
      • A **variable cannot be used.
  • On the right side of in:
    • For a dictionary, the keys are used by default.
    • One or more *iterables can be used within each list, tuple and set literal within the most outer list, tuple and set literal to unpack iterables:
    • The one or more *iterables of str and dict.items() can be used outside any list, tuple and set literal within the most outer list, tuple and set literal.
    • A *iterable cannot be used outside any list, tuple and set literal.
    • A *iterable cannot be used within a dictionary(dict) literal.
    • One or more \*\*dictionaries can be used within each dictionary literal within the most outer list, tuple and set literal to unpack dictionaries.
    • A **dictionary cannot be used outside any dictioanry(dict) literal.
    • ** cannot be used with an iterable except a dictionary.
• 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 within print() and with *args but not with **kwargs.
• 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 literal within the most outer list, tuple and set literal on the right side of in 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 in 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(dict) literal within the most outer list, tuple and set literal on the right side of in 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 a for statement.

for v1, v2, v3 in [[0, 1, 2], [3, 4, 5]]:              # list(list)
# for v1, v2, v3 in ((0, 1, 2), (3, 4, 5)):            # tuple(tuple)
# for v1, v2, v3 in iter([iter([0, 1, 2]),             # iterator(iterator)
#                         iter([3, 4, 5])]):
# for v1, v2, v3 in ['012', '345']:                    # list(str)
# for v1, v2, v3 in [range(3), range(3, 6)]:           # list(range)
    print(v1, v2, v3)
# 0 1 2
# 3 4 5

for v1, v2, v3 in {frozenset([0, 1, 2]),               # set(frozenset)
                   frozenset([3, 4, 5])}:
# for v1, v2, v3 in frozenset([frozenset([0, 1, 2]),   # frozenset(frozenset)
#                              frozenset([3, 4, 5])]):
    print(v1, v2, v3)
# 3 4 5
# 0 1 2

for v1, v2, v3 in [{0:1, 2:3, 4:5},                    # list(dict)
                   {6:7, 8:9, 10:11}]:
# for v1, v2, v3 in [{0:1, 2:3, 4:5}.keys(),           # list(dict.keys())
#                    {6:7, 8:9, 10:11}.keys()]:
    print(v1, v2, v3)
# 0 2 4
# 6 8 10

for v1, v2, v3 in [{0:1, 2:3, 4:5}.values(),           # list(dict.values())
                   {6:7, 8:9, 10:11}.values()]:
    print(v1, v2, v3)
# 1 3 5
# 7 9 11

for v1, v2, v3 in [{0:1, 2:3, 4:5}.items(),            # list(dict.items())
                   {6:7, 8:9, 10:11}.items()]:
    print(v1, v2, v3)
# (0, 1) (2, 3) (4, 5)
# (6, 7) (8, 9) (10, 11)

for v1, in '012345':                                   # str
    for v2, in v1:
        for v3, in v2:
            print(v1, v2, v3)
# 0 0 0
# 1 1 1
# 2 2 2
# 3 3 3
# 4 4 4
# 5 5 5

for v1, v2, v3 in [b'012', b'345']:                    # list(bytes)
# for v1, v2, v3 in [bytearray(b'012'),                # list(bytearray)
#                    bytearray(b'345')]:
    print(v1, v2, v3)
# 48 49 50
# 51 52 53

for v1, v2, v3 in [[0, 1, 2], [3, 4, 5]]:
# for (v1, v2, v3) in [[0, 1, 2], [3, 4, 5]]:
# for [v1, v2, v3] in [[0, 1, 2], [3, 4, 5]]:
    print(v1, v2, v3)
# 0 1 2
# 3 4 5

for v1, v2 in [[0, [1, [2]]], [3, [4, [5]]]]:
# for (v1, v2) in [[0, [1, [2]]], [3, [4, [5]]]]:
# for [v1, v2] in [[0, [1, [2]]], [3, [4, [5]]]]:
    print(v1, v2)
# 0 [1, [2]]
# 3 [4, [5]]

for v1, (v2, v3) in [[0, [1, [2]]], [3, [4, [5]]]]:
# for v1, [v2, v3] in [[0, [1, [2]]], [3, [4, [5]]]]:
# for (v1, (v2, v3)) in [[0, [1, [2]]], [3, [4, [5]]]]:
# for [v1, [v2, v3]] in [[0, [1, [2]]], [3, [4, [5]]]]:
    print(v1, v2, v3)
# 0 1 [2]
# 3 4 [5]

for v, in [[0]]:
# for (v,) in [[0]]:
# for [v] in [[0]]:
    print(v)
# 0

# It's not an iterable unpacking.
for v in [[0, 1, 2], [3, 4, 5]]:
    print(v)
# [0, 1, 2]
# [3, 4, 5]

# It's not an iterable unpacking.
for v in [[0], [1]]:
    print(v)
# [0]
# [1]

for v1, v2, v3 in [[0], [1]]:
    pass
# ValueError: not enough values to unpack (expected 3, got 2)

for v1, v2, v3 in [[0, 1, 2, 3], [4, 5, 6, 7]]:
    pass
# ValueError: too many values to unpack (expected 3)

for v, in [[], []]:
    pass
# ValueError: not enough values to unpack (expected 1, got 0)

for v, in [0, 1]:
    pass
# TypeError: cannot unpack non-iterable int object