[Python-Dev] dict.supplement() (was Re: list.shift())

[Peter Funk]

I wrote:
>  > Note the similarities to {}.update(dict), but update replaces existing
>  > entries in self, which is sometimes not desired.  I know, that supplement
>  > can also simulated with:
> 
Fred L. Drake, Jr.:
> Peter,
>   I like this!
> 
>  > 	tmp = dict.copy()
>  > 	tmp.update(self)
>  > 	self.data = d
> 
>   I presume you mean "self.data = tmp"; "self.data.update(tmp)" would
> be just a little more robust, at the cost of an additional update.

Ouppss... I should have tested this before posting.  But currently I use 
the more explicit (and probably slower version) in my code:

class ConfigDict(UserDict.UserDict):
    def supplement(self, defaults):
    	for k, v in defaults.items():
	    if not self.data.has_key(k):
		self.data[k] = v

Works fine so far, although it requires usually an additional copy operation.
Consider another example, where arbitrary instance attributes should be
specified as keyword arguments to the constructor:

  >>> class Example:
  ...     _defaults = {'a': 1, 'b': 2}
  ...     _config = _defaults
  ...     def __init__(self, **kw):
  ...         if kw:
  ...             self._config = self._defaults.copy()
  ...             self._config.update(kw)
  ... 
  >>> A = Example(a=12345)
  >>> A._config
  {'b': 2, 'a': 12345}
  >>> B = Example(c=3)
  >>> B._config
  {'b': 2, 'c': 3, 'a': 1}

If 'supplement' were a dictionary builtin method, this would become simply:
	kw.supplement(self._defaults)
	self._config = kw

Unfortunately this can't be achieved using a wrapper class like UserDict,
since the **kw argument is always a builtin dictionary object.

Regards, Peter
-- 
Peter Funk, Oldenburger Str.86, 27777 Ganderkesee, Tel: 04222 9502 70, Fax: -60