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# encoding: utf-8

"""Miscellaneous context managers.

"""

# 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)

class NoOpContext(object):

"""Context manager that does nothing."""

def __enter__(self): pass

def __exit__(self, type, value, traceback): pass