Keyword arguments and user-defined dictionaries

G. S. Hayes sjdevnull at yahoo.com
Thu Jun 24 02:29:39 CEST 2004


Hi,

I'm implementing a lazy-load dictionary; my implementation basically
stores keys that are to be loaded lazily internally (in a member
dictionary) and then in __getitem__ I look to see if the key needs to
be loaded and if so I load it.  My class inherits from dict and has
keys(), __contains__, __get/setitem__, items(), etc all properly
implemented--seems to work fine in most cases.

My question is, what methods do I need to implement to make ** work on
a custom dictionary-like object, or is it not possible?

Here's how you can see the problem.

Simplified LazyDict (the real one actually stores callbacks to
generate values, but this is enough to show the problem--my real one
implements a lot more dict methods, but still doesn't work):

class LazyDict(dict):
    def __init__(self):
        dict.__init__(self)
        self.special_keys={}
    def addLazy(self, key, val):
        if dict.has_key(self, key):
            dict.__delitem__(self, key)
        self.special_keys[key]=val
    def has_key(self, key):
        return self.__contains__(key)
    def __contains__(self, key):
        if dict.has_key(self, key):
            return 1
        elif dict.has_key(self.special_keys, key):
            return 1
        else:
    def __getitem__(self, key):
        if dict.has_key(self, key):
           return dict.__getitem__(self, key)
        elif key in self.special_keys.keys():
           self[key]=self.special_keys[key]
           del self.special_keys[key]
        return dict.__getitem__(self, key)
           return 0

e.g. put the above in LSimple.py and:

>>> def g(foo):
...   print foo
...
>>> a=LSimple.LazyDict()
>>> a.addLazy("foo", 2)
>>> g(**a)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: g() takes exactly 1 argument (0 given)
>>> a["foo"]
2
>>> g(**a)
2
>>>

Thanks for your time.



More information about the Python-list mailing list