1# Copyright (c) 2010 Google Inc. All rights reserved.
2#
3# Redistribution and use in source and binary forms, with or without
4# modification, are permitted provided that the following conditions are
5# met:
6#
7#     * Redistributions of source code must retain the above copyright
8# notice, this list of conditions and the following disclaimer.
9#     * Redistributions in binary form must reproduce the above
10# copyright notice, this list of conditions and the following disclaimer
11# in the documentation and/or other materials provided with the
12# distribution.
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15# this software without specific prior written permission.
16#
17# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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24# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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27# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
28
29# Python does not (yet) seem to provide automatic memoization.  So we've
30# written a small decorator to do so.
31
32import functools
33
34
35class memoized(object):
36    def __init__(self, function):
37        self._function = function
38        self._results_cache = {}
39
40    def __call__(self, *args):
41        try:
42            return self._results_cache[args]
43        except KeyError:
44            # If we didn't find the args in our cache, call and save the results.
45            result = self._function(*args)
46            self._results_cache[args] = result
47            return result
48        # FIXME: We may need to handle TypeError here in the case
49        # that "args" is not a valid dictionary key.
50
51    # Use python "descriptor" protocol __get__ to appear
52    # invisible during property access.
53    def __get__(self, instance, owner):
54        # Return a function partial with obj already bound as self.
55        return functools.partial(self.__call__, instance)
56