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- """
- Commonly useful converters.
- """
- from __future__ import absolute_import, division, print_function
- def optional(converter):
- """
- A converter that allows an attribute to be optional. An optional attribute
- is one which can be set to ``None``.
- :param callable converter: the converter that is used for non-``None``
- values.
- .. versionadded:: 17.1.0
- """
- def optional_converter(val):
- if val is None:
- return None
- return converter(val)
- return optional_converter
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