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# encoding: utf-8
"""Miscellaneous context managers.
"""
import warnings
# Copyright (c) IPython Development Team.
# Distributed under the terms of the Modified BSD License.
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.
Examples
--------
>>> 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)
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