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# encoding: utf-8
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"""Miscellaneous context managers.
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"""
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# Copyright (c) IPython Development Team.
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# Distributed under the terms of the Modified BSD License.
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class preserve_keys(object):
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"""Preserve a set of keys in a dictionary.
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Upon entering the context manager the current values of the keys
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will be saved. Upon exiting, the dictionary will be updated to
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restore the original value of the preserved keys. Preserved keys
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which did not exist when entering the context manager will be
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deleted.
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Examples
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--------
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>>> d = {'a': 1, 'b': 2, 'c': 3}
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>>> with preserve_keys(d, 'b', 'c', 'd'):
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... del d['a']
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... del d['b'] # will be reset to 2
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... d['c'] = None # will be reset to 3
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... d['d'] = 4 # will be deleted
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... d['e'] = 5
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... print(sorted(d.items()))
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...
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[('c', None), ('d', 4), ('e', 5)]
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>>> print(sorted(d.items()))
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[('b', 2), ('c', 3), ('e', 5)]
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"""
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def __init__(self, dictionary, *keys):
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self.dictionary = dictionary
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self.keys = keys
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def __enter__(self):
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# Actions to perform upon exiting.
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to_delete = []
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to_update = {}
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d = self.dictionary
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for k in self.keys:
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if k in d:
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to_update[k] = d[k]
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else:
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to_delete.append(k)
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self.to_delete = to_delete
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self.to_update = to_update
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def __exit__(self, *exc_info):
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d = self.dictionary
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for k in self.to_delete:
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d.pop(k, None)
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d.update(self.to_update)
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class NoOpContext(object):
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"""Context manager that does nothing."""
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def __enter__(self): pass
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def __exit__(self, type, value, traceback): pass
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