4

I'm trying to implement a dict-like object which can be accessed/modified with __getattr__ and __setattr__ for ease of use for my users. The class also implements some other simple functionality.

Using this answer as a template, my implementation is currently as follows:

from collections import MutableMapping

class Dictish (MutableMapping):
    """
    A dict-like mapping object. vals are always coerced to str.
    Should provide __getattr__ and __setattr__ as aliases for
    __getitem__ and __setitem__.
    """
    def __init__ ( self, *args, **kwargs ):
        self.store = dict()
        self.update(dict(*args,**kwargs))

    def __getitem__ ( self, key : str ) -> str:
        return self.store[key]

    def __setitem__ ( self, key : str, val : str ) -> None:
        self.store[key] = str(val)

    def __delitem__ ( self, key : str ) -> None:
        del self.store[key]

    def __iter__ ( self ):
        return iter(self.store)

    def __len__ ( self ) -> int:
        return len(self.store)

    def __repr__ ( self ) -> str:
        return repr(self.store)

    # works fine by itself, but goes into infinite recursion
    # when __setattr__ is defined
    def __getattr__ ( self, attr : str ) -> str:
        return self.__getitem__(attr)

#    def __setattr__ ( self, attr : str, val : str ) -> None:
#        self.__setitem__(attr,val)
nivk
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1 Answers1

4

As I was writing and formalizing the question, I found the answer (happens to me a lot). Maybe this can help someone else.

The solution for me was the following:

def __getattr__ ( self, attr : str ) -> str:
    return self.__getitem__(attr)

def __setattr__ ( self, attr : str, val : str ) -> None:
    if attr == 'store':
        super().__setattr__(attr,val)
    else:
        self.__setitem__(attr,val)

The key is that the store attribute must be separated out and called from the base class to avoid recursion. Pretty simple but was easy for me to miss!

UPDATE:

I added functionality for adding attributes that you do not want to keep in store (ie. the usual meaning of attributes). I also implemented store as an OrderedDict, but this is just for my use-case. Obviously the set_inst_attr exception is temporary/a placeholder.

from collections import MutableMapping, OrderedDict

class ODictish (MutableMapping):
    """
    An OrderedDict-like mapping object.
    Provides __getattr__ and __setattr__ as aliases for __getitem__
    and __setitem__.
    Attributes which you do not want to keep in 'store' can be set with
    self.set_inst_attr.
    """
    def __init__ ( self , od=None):
        if od is None: od = OrderedDict()
        super().__setattr__('store', OrderedDict(od))

    def __getitem__ ( self, key ):
        return self.store[key]

    def __setitem__ ( self, key, val ):
        self.store[key] = val

    def __delitem__ ( self, key ):
        del self.store[key]

    def __iter__ ( self ):
        return iter(self.store)

    def __len__ ( self ):
        return len(self.store)

    def __repr__ ( self ):
        return repr(self.store)

    def __getattr__ ( self, attr ):
        if attr in vars(self):
            return vars(self)[attr]
        return self.__getitem__(attr)

    def __setattr__ ( self, attr, val ):
        if attr in vars(self):
            self.set_inst_attr(attr,val)
        else:
            self.__setitem__(attr,val)

    def set_inst_attr ( self, attr, val ):
        if attr == 'store':
            raise Exception("Don't do that.")
        super().__setattr__(attr,val)

    def move_to_end ( self, key, last=True ):
        self.store.move_to_end(key,last)
nivk
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