Guido van Rossum wrote:

> I've seen this pattern a lot at a past employer, and despite the obvious convenience I've come to see it as an anti-pattern: for people expecting Python semantics it's quite surprising to read code that writes foo.bar and then reads back foo['bar'].

Would it be significantly less surprising if it were to be exclusively allowed for the JSON module rather than anything that accepts custom dicts? To me, that seems like the most useful and intuitive location for the dot notation, and it would keep the Python semantics intact everywhere else. I think it's fairly unlikely that most users of JSON would be surprised by it, but I can see that it could be surprising elsewhere.

On Tue, Apr 14, 2020 at 11:33 PM Guido van Rossum <guido@python.org> wrote:

I've seen this pattern a lot at a past employer, and despite the obvious convenience I've come to see it as an anti-pattern: for people expecting Python semantics it's quite surprising to read code that writes foo.bar and then reads back foo['bar']. We should not try to import JavaScript's object model into Python.

On Tue, Apr 14, 2020 at 8:06 PM Raymond Hettinger <raymond.hettinger@gmail.com> wrote:

SimpleNamespace() is really good at giving attribute style-access. I would like to make that functionality available to the JSON module (or just about anything else that accepts a custom dict) by adding the magic methods for mappings so that this works:

     catalog = json.load(f, object_hook=SimpleNamespace)
     print(catalog['clothing']['mens']['shoes']['extra_wide']['quantity'])      # currently possible with dict()
     print(catalog.clothing.mens.shoes.extra_wide.quantity])                 # proposed with SimpleNamespace()
     print(catalog.clothing.boys['3t'].tops.quantity                                   # would also be supported

I've already seen something like this in production; however, people are having to write custom subclasses to do it.  This is kind of bummer because the custom subclasses are a pain to write, are non-standard, and are generally somewhat slow.  I would like to see a high-quality version this made more broadly available.

The core idea is keep the simple attribute access but make it easier to load data programmatically:

    >>> ns = SimpleNamespace(roses='red', violets='blue')
    >>> thing = input()
    sugar
    >>> quality = input()
    sweet
    >>> setattr(ns, thing, quality)            # current
    >>> ns['sugar'] = 'sweet'                   # proposed

If the PEP 584 __ior__ method were supported, updating a SimpleNamespace would be much cleaner:

      ns |= some_dict

I posted an issue on the tracker: https://bugs.python.org/issue40284 .  There was a suggestion to create a different type for this, but I don't see the point in substantially duplicating everything SimpleNamespace already does just so we can add some supporting dunder methods.   Please add more commentary so we can figure-out the best way to offer this powerful functionality.


Raymond


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--

--Guido van Rossum (python.org/~guido)

Pronouns: he/him (why is my pronoun here?)

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