view mercurial/thirdparty/attr/converters.py @ 42813:268662aac075

interfaces: create a new folder for interfaces and move repository.py in it

I was trying to understand current interfaces and write new ones and I realized
we need to improve how current interfaces are organised. This creates a
dedicated folder for defining interfaces and move `repository.py` which defines
all the current interfaces inside it.

Differential Revision: https://phab.mercurial-scm.org/D6741

"""
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