Memoization in Python

”’

This code snippet is taken from

http://code.activestate.com/recipes/577452-a-memoize-decorator-for-instance-methods/

”’

from functools import partial

class memoize(object):
“””cache the return value of a method

This class is meant to be used as a decorator of methods. The return value
from a given method invocation will be cached on the instance whose method
was invoked. All arguments passed to a method decorated with memoize must
be hashable.

If a memoized method is invoked directly on its class the result will not
be cached. Instead the method will be invoked like a static method:
class Obj(object):
@memoize
def add_to(self, arg):
return self + arg
Obj.add_to(1) # not enough arguments
Obj.add_to(1, 2) # returns 3, result is not cached
“””
def __init__(self, func):
self.func = func
def __get__(self, obj, objtype=None):
if obj is None:
return self.func
return partial(self, obj)
def __call__(self, *args, **kw):
obj = args[0]
try:
cache = obj.__cache
except AttributeError:
cache = obj.__cache = {}
key = (self.func, args[1:], frozenset(kw.items()))
try:
res = cache[key]
except KeyError:
res = cache[key] = self.func(*args, **kw)
return res
if __name__ == “__main__”:
# example usage
class Test(object):
v = 0
@memoize
def inc_add(self, arg):
self.v += 1
return self.v + arg

t = Test()
assert t.inc_add(2) == t.inc_add(2)
assert Test.inc_add(t, 2) != Test.inc_add(t, 2)

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