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diff --git a/src/leap/common/decorators.py b/src/leap/common/decorators.py
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+++ b/src/leap/common/decorators.py
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+# -*- coding: utf-8 -*-
+# decorators.py
+# Copyright (C) 2013 LEAP
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+"""
+Useful decorators.
+"""
+import collections
+import datetime
+import functools
+import logging
+
+logger = logging.getLogger(__name__)
+
+
+class _memoized(object):
+    """
+    Decorator.
+
+    Caches a function's return value each time it is called.
+    If called later with the same arguments, the cached value is returned
+    (not reevaluated).
+    """
+
+    # cache invalidation time, in seconds
+    CACHE_INVALIDATION_DELTA = 1800
+
+    def __init__(self, func, ignore_kwargs=None, is_method=False,
+                 invalidation=None):
+
+        """
+        :param ignore_kwargs: If True, ignore all kwargs.
+                              If tuple, ignore those kwargs.
+        :type ignore_kwargs: bool, tuple or None
+        :param is_method: whether the decorated function is a method.
+                          (ignores the self argument if so).
+        :type is_method: True
+        """
+        self.ignore_kwargs = ignore_kwargs if ignore_kwargs else []
+        self.is_method = is_method
+        self.func = func
+
+        if invalidation:
+            self.CACHE_INVALIDATION_DELTA = invalidation
+
+        # TODO should put bounds to the cache dict so we do not
+        # consume a huge amount of memory.
+        self.cache = {}
+        self.cache_ts = {}
+
+    def __call__(self, *args, **kwargs):
+        """
+        Executes the call.
+
+        :tyoe args: tuple
+        :type kwargs: dict
+        """
+        def ret_or_raise(value):
+            """
+            Returns the value except if it is an exception,
+            in which case it's raised.
+            """
+            if isinstance(value, Exception):
+                raise value
+            return value
+
+        if self.is_method:
+            # forget about `self` as key
+            key_args = args[1:]
+        else:
+            key_args = args
+
+        if self.ignore_kwargs is True:
+            key = key_args
+        else:
+            key = (key_args, frozenset(
+                [(k, v) for k, v in kwargs.items()
+                 if k not in self.ignore_kwargs]))
+
+        if not isinstance(key, collections.Hashable):
+            # uncacheable. a list, for instance.
+            # better to not cache than blow up.
+            logger.warning("Key is not hashable, bailing out!")
+            return self.func(*args, **kwargs)
+
+        if key in self.cache:
+            if self._is_cache_still_valid(key):
+                value = self.cache[key]
+                logger.debug("Got value from cache...")
+                return ret_or_raise(value)
+            else:
+                logger.debug("Cache is invalid, evaluating again...")
+
+        # no cache, or cache invalid
+        try:
+            value = self.func(*args, **kwargs)
+        except Exception as exc:
+            logger.error("Exception while calling function: %r" % (exc,))
+            value = exc
+        self.cache[key] = value
+        self.cache_ts[key] = datetime.datetime.now()
+        return ret_or_raise(value)
+
+    def _is_cache_still_valid(self, key, now=datetime.datetime.now):
+        """
+        Returns True if the cache value is still valid, False otherwise.
+
+        For now, this happen if less than CACHE_INVALIDATION_DELTA seconds
+        have passed from the time in which we recorded the cached value.
+
+        :param key: the key to lookup in the cache
+        :type key: hashable
+        :param now: a callable that returns a datetime object. override
+                    for dependency injection during testing.
+        :type now: callable
+        :rtype: bool
+        """
+        cached_ts = self.cache_ts[key]
+        delta = datetime.timedelta(seconds=self.CACHE_INVALIDATION_DELTA)
+        return (now() - cached_ts) < delta
+
+    def __repr__(self):
+        """
+        Return the function's docstring.
+        """
+        return self.func.__doc__
+
+    def __get__(self, obj, objtype):
+        """
+        Support instance methods.
+        """
+        return functools.partial(self.__call__, obj)
+
+
+def memoized_method(function=None, ignore_kwargs=None):
+    """
+    Wrap _memoized to allow for deferred calling
+
+    :type function: callable, or None.
+    :type ignore_kwargs: None, True or tuple.
+    """
+    if function:
+        return _memoized(function, is_method=True)
+    else:
+        def wrapper(function):
+            return _memoized(
+                function, ignore_kwargs=ignore_kwargs, is_method=True)
+        return wrapper