Yanz mini Shell

>>> class Seq:
    ...     def __getitem__(self, i):
    ...         if i >= 0 and i < 3: return i
    ...         raise IndexError
    ...
    >>> a, b, c = Seq()
    >>> a == 0 and b == 1 and c == 2
    True

Single element unpacking, with extra syntax

>>> st = (99,)
    >>> sl = [100]
    >>> a, = st
    >>> a
    99
    >>> b, = sl
    >>> b
    100

Now for some failures

Unpacking non-sequence

>>> a, b, c = 7
    Traceback (most recent call last):
      ...
    TypeError: cannot unpack non-iterable int object

Unpacking tuple of wrong size

>>> a, b = t
    Traceback (most recent call last):
      ...
    ValueError: too many values to unpack (expected 2)

Unpacking tuple of wrong size

>>> a, b = l
    Traceback (most recent call last):
      ...
    ValueError: too many values to unpack (expected 2)

Unpacking sequence too short

>>> a, b, c, d = Seq()
    Traceback (most recent call last):
      ...
    ValueError: not enough values to unpack (expected 4, got 3)

Unpacking sequence too long

>>> a, b = Seq()
    Traceback (most recent call last):
      ...
    ValueError: too many values to unpack (expected 2)

Unpacking a sequence where the test for too long raises a different kind of
error

>>> class BozoError(Exception):
    ...     pass
    ...
    >>> class BadSeq:
    ...     def __getitem__(self, i):
    ...         if i >= 0 and i < 3:
    ...             return i
    ...         elif i == 3:
    ...             raise BozoError
    ...         else:
    ...             raise IndexError
    ...

Trigger code while not expecting an IndexError (unpack sequence too long, wrong
error)

>>> a, b, c, d, e = BadSeq()
    Traceback (most recent call last):
      ...
    test.test_unpack.BozoError

Trigger code while expecting an IndexError (unpack sequence too short, wrong
error)

>>> a, b, c = BadSeq()
    Traceback (most recent call last):
      ...
    test.test_unpack.BozoError

Allow unpacking empty iterables

>>> () = []
    >>> [] = ()
    >>> [] = []
    >>> () = ()

Unpacking non-iterables should raise TypeError

>>> () = 42
    Traceback (most recent call last):
      ...
    TypeError: cannot unpack non-iterable int object

Unpacking to an empty iterable should raise ValueError

>>> () = [42]
    Traceback (most recent call last):
      ...
    ValueError: too many values to unpack (expected 0)

"""

__test__ = {'doctests' : doctests}

def test_main(verbose=False):
    from test import support
    from test import test_unpack
    support.run_doctest(test_unpack, verbose)

if __name__ == "__main__":
    test_main(verbose=True)

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