Class Decorator decorating method in python

Question

I'm trying to memoize using a decorator with the decorator being a class not a function, but I'm getting the error

TypeError: seqLength() takes exactly 2 arguments (1 given)

I'm guessing this has something to do with the classes, but not sure what's wrong from there.

The code:

import sys

class memoize(object):
    '''memoize decorator'''
    def __init__(self, func):
        self.func = func
        self.cache = {}
    def __call__(self, *args):
        try:
            return self.cache[args]
        except KeyError:
            value = self.func(self, *args)
            self.cache[args] = value
            return value

class collatz(object):
    def __init__(self, n):
        self.max = 1
        self.n = n
    @memoize
    def seqLength(self, n):
        if n>1:
            if n%2 == 0:
                return 1+self.seqLength(n/2)
            else:
                return 1+self.seqLength(3*n+1)
        else:
            return 1
    def maxLength(self):
        for n in xrange(1, self.n):
            l = self.seqLength(n)
            if l > self.max:
                self.max = n
        return self.max

n = int(sys.argv[1])
c = collatz(n)
print c.maxLength()

score 2 · Answer 1 · answered Jun 04 '13 at 11:45

Using a class as a decorator is tricky, because you have to implement the descriptor protocol correctly (the currently accepted answer doesn't.) A much, much easier solution is to use a wrapper function, because they automatically implement the descriptor protocol correctly. The wrapper equivalent of your class would be:

import functools

def memoize(func):
    cache = {}

    @functools.wraps(func)
    def wrapper(*args):
        try:
            return cache[args]
        except KeyError:
            value = func(*args)
            cache[args] = value
            return value
    return wrapper

When have so much state you want to encapsulate it in a class anyway, you can still use a wrapper function, for example like so:

import functools

class _Memoize(object):
    '''memoize decorator helper class'''
    def __init__(self, func):
        self.func = func
        self.cache = {}

    def __call__(self, *args):
        try:
            return self.cache[args]
        except KeyError:
            value = self.func(*args)
            self.cache[args] = value
            return value

def memoize(func):
    o = _Memoize(func)
    @functools.wraps(func)
    def wrapper(*args):
        return o(*args)
    return wrapper

score 2 · Answer 2 · answered Jan 13 '12 at 19:40

This is confusing, syntactically. It's not clear if self.func is part of your memoize or a separate function that's part of some other object of some other class. (You mean the latter, BTW)

        value = self.func(self, *args)

Do this to make it clear that the_func is just a function, not a member of the memoize class.

        the_func= self.func
        value= the_func( *args )

That kind of thing prevents confusion over the class to which self. is bound.

Also, please spell it Memoize. With a leading capital letter. It is a class definition, after all.

AdamKG · Accepted Answer · 2013-07-21T18:00:27.173

A decorator is just syntactic sugar for foo = decorator(foo), so in this case you're ending up making the self of seqLength be memoize instead of collatz. You need to use descriptors. This code works for me:

class memoize(object):
    '''memoize descriptor'''
    def __init__(self, func):
        self.func = func

    def __get__(self, obj, type=None):
        return self.memoize_inst(obj, self.func)

    class memoize_inst(object):
        def __init__(self, inst, fget):
            self.inst = inst
            self.fget = fget

            self.cache = {}

        def __call__(self, *args):
            # if cache hit, done
            if args in self.cache:
                return self.cache[args]
            # otherwise populate cache and return
            self.cache[args] = self.fget(self.inst, *args)
            return self.cache[args]

Class Decorator decorating method in python

3 Answers3