[Python-ideas] Dict-like object with property access (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Mon Jan 30 21:56:13 CET 2012

On Tue, Jan 31, 2012 at 4:49 AM, Massimo Di Pierro <massimo.dipierro at gmail.com> wrote:

Trying to make sure I understand where we disagree and perhaps explain my problem better. For me this has very little to do with dictionaries.

One general challenge with "arbitrary keys as attributes" data stores in Python is that they can get into messy namespace conflicts, because you want to allow arbitrary keys, but you also want access to ordinary methods. collections.namedtuple gets around this by leaving the attribute namespace largely free for end users and prefixing method names and class information with underscores.

Ultimately, what ends up happening as far as the standard library goes is that "explicit is better than implicit" wins. If you don't know what keys you're going to get, then the appropriate data structure is a dict, not an object. If you do know which keys you're going to get, then the appropriate data structure is a predefined class (perhaps generated programmatically once the full set of permitted attributes is determined - e.g. when creating an appropriate namedtuple definition from a set of column headers).

Outside the standard library, it's "practicality beats purity" that wins. People like being able to type ".attr" instead of "['attr']" when a field name happens to be a legal identifier. The core problem is that it's so easy to write a "good enough" version of such a class for yourself that nobody has ever bothered to standardise on a particular way of handling this that is suitable for stdlib inclusion (particularly when there are so many people that object to the idea in principle, regardless of the details of how it is implemented)

Cheers, Nick.

STEP 4) class Dummy(object): def getitem(self,key): return getattr(self,key) def setitem(self,key,value): return setattr(self,key,value) def getattr(self,key): return object.getattr(self,key) if hasattr(self,key) else Dummy()

P.S. Tip for fast autovivification in Python:

from collections import defaultdict

def autodict():
    return defaultdict(autodict)

>>> store['a']['b']['c']
defaultdict(<function autodict at 0x7f5adfe349b0>, {})

If you want this behaviour in instances of a particular class, set the instance dict attributes to one of those in new instead of overriding getattr.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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