用于存储键值对的Python中的类与字典

Question

I need to define some key-value parameters to configure the behaviour of a class. My requirements are two:

• To be able to process them programatically
• To be able to make more specific configurations that overwrite some of the values.

The natural approach for the first requirement is to use a dict (so I can loop over keys, values or items with a for loop), but for the second requirement it seems more appropriate to use a class hierarchy and let the attribute lookup mechanism to do the work (plus I get other goodies like being able to use properties for some values).

Is there a way to get best of both worlds?

---

After reading the comments, It does not need to be mutable (I won't be changing the values of an already created instance), it's just for static configuration. But I might need to add new values in a more specific instance.

I think in the end I could just use a dict.copy + dict.update for the specific instances (I can live without the properties).

Answer 1

Somehow along the lines of what suggested in the commment by Fabio, I would remark that the two are not mutually exclusive, given that in Python [nearly] everything is an object (including classes!).

One obvious solution is - again quoting Fabio - subclassing a dict . The other would be make your own class starting from the collections.abc containers (using an abstract base class ( abc ) is not a requirement, but would probably the most pythonic way to implement your own container from scratch).

HTH!

Answer 2

If you can use python 3.3 ChainMap is exactly what you want.

http://docs.python.org/3.3/library/collections#collections.ChainMap

It's basically a list of dictionaries and will look for a given key in each one sequentially until a match is found.

Actually, here's the code to use it in 2.7:

http://code.activestate.com/recipes/305268-chained-map-lookups/

Answer 3

is this what you need?

class my_dict(dict):
    def __getattr__(self, name):
        return self[name]
    def __setattr__(self, name, value):
        self[name] = value

d = my_dict()
d.key = "value"
print d.key       #>>>value

for k,v in d.items():
    print k,v     #>>>key value

Answer 4

You could just use named tuples . See here and the accepted answer on this question for a nice introduction to them. Some important points from that answer are:

• They were added in Python 2.6 and Python 3.0, although there is a recipe for implementation in Python 2.4.

• Named tuples are backwards compatible with normal tuples

Also, named tuples have a number of useful methods such as _fields to get the names of the used and an _asdict method which returns an (ordered) dict of the named tuple contents.

Answer 5

Here's my take:

class Config(object):
    def _keys(self):
        return (name for name in dir(self) if not name.startswith('_'))
    __iter__ = _keys

    def _values(self):
        return (getattr(self, name) for name in self)

    def _items(self):
        return ((name, getattr(self, name)) for name in self)

>>> class Foo(Config):
...     bar = 3
...     baz = 'spam'
...
>>> class Sub(Foo):
...     bar = 4
...
>>> x = Sub()
>>> list(x._items())
[('bar', 4), ('baz', 'spam')]
>>>