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39 lines
1.2 KiB
Python
39 lines
1.2 KiB
Python
# from more_itertools 9.0
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def only(iterable, default=None, too_long=None):
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"""If *iterable* has only one item, return it.
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If it has zero items, return *default*.
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If it has more than one item, raise the exception given by *too_long*,
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which is ``ValueError`` by default.
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>>> only([], default='missing')
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'missing'
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>>> only([1])
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1
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>>> only([1, 2]) # doctest: +IGNORE_EXCEPTION_DETAIL
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Traceback (most recent call last):
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...
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ValueError: Expected exactly one item in iterable, but got 1, 2,
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and perhaps more.'
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>>> only([1, 2], too_long=TypeError) # doctest: +IGNORE_EXCEPTION_DETAIL
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Traceback (most recent call last):
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...
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TypeError
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Note that :func:`only` attempts to advance *iterable* twice to ensure there
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is only one item. See :func:`spy` or :func:`peekable` to check
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iterable contents less destructively.
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"""
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it = iter(iterable)
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first_value = next(it, default)
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try:
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second_value = next(it)
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except StopIteration:
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pass
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else:
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msg = (
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'Expected exactly one item in iterable, but got {!r}, {!r}, '
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'and perhaps more.'.format(first_value, second_value)
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)
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raise too_long or ValueError(msg)
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return first_value
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