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A converter that allows an attribute to be optional. An optional attribute
is one which can be set to ``None``.
Type annotations will be inferred from the wrapped converter's, if it
has any.
:param callable converter: the converter that is used for non-``None``
values.
.. versionadded:: 17.1.0
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A converter that allows to replace ``None`` values by *default* or the
result of *factory*.
:param default: Value to be used if ``None`` is passed. Passing an instance
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is *not*.
:param callable factory: A callable that takes no parameters whose result
is used if ``None`` is passed.
:raises TypeError: If **neither** *default* or *factory* is passed.
:raises TypeError: If **both** *default* and *factory* are passed.
:raises ValueError: If an instance of `attr.Factory` is passed with
``takes_self=True``.
.. versionadded:: 18.2.0
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