view mercurial/thirdparty/attr/converters.py @ 46291:e2713c68b477

shelve: replace repo instance in Shelf class by vfs instance

I'd like to be able to teach the `Shelf` class to read shelves from
`.hg/shelve-backup/` in addition to `.hg/shelved/`. That means that
we'll have to pass in a vfs instead of the repo, so this patch does
that preparation.

Differential Revision: https://phab.mercurial-scm.org/D9738

"""
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