IPython/utils/contexts.py

# encoding: utf-8
"""
Context managers for temporarily updating dictionaries.

Authors:

* Bradley Froehle
"""

#-----------------------------------------------------------------------------
#  Copyright (C) 2012  The IPython Development Team
#
#  Distributed under the terms of the BSD License.  The full license is in
#  the file COPYING, distributed as part of this software.
#-----------------------------------------------------------------------------

#-----------------------------------------------------------------------------
# Code
#-----------------------------------------------------------------------------

class preserve_keys(object):
    """Preserve a set of keys in a dictionary.

    Upon entering the context manager the current values of the keys
    will be saved. Upon exiting, the dictionary will be updated to
    restore the original value of the preserved keys. Preserved keys
    which did not exist when entering the context manager will be
    deleted.

    Example
    -------

    >>> d = {'a': 1, 'b': 2, 'c': 3}
    >>> with preserve_keys(d, 'b', 'c', 'd'):
    ...     del d['a']
    ...     del d['b']      # will be reset to 2
    ...     d['c'] = None   # will be reset to 3
    ...     d['d'] = 4      # will be deleted
    ...     d['e'] = 5
    ...     print(sorted(d.items()))
    ...
    [('c', None), ('d', 4), ('e', 5)]
    >>> print(sorted(d.items()))
    [('b', 2), ('c', 3), ('e', 5)]
    """

    def __init__(self, dictionary, *keys):
        self.dictionary = dictionary
        self.keys = keys

    def __enter__(self):
        # Actions to perform upon exiting.
        to_delete = []
        to_update = {}

        d = self.dictionary
        for k in self.keys:
            if k in d:
                to_update[k] = d[k]
            else:
                to_delete.append(k)

        self.to_delete = to_delete
        self.to_update = to_update

    def __exit__(self, *exc_info):
        d = self.dictionary

        for k in self.to_delete:
            d.pop(k, None)
        d.update(self.to_update)