[Python-ideas] JavaScript-Style Object Creation in Python (using a constructor function instead of a class to create objects)

Guido van Rossum guido at python.org
Mon May 15 13:31:46 EDT 2017

On Mon, May 15, 2017 at 9:50 AM, Brett Cannon <brett at python.org> wrote:

> On Mon, 15 May 2017 at 08:30 Guido van Rossum <guido at python.org> wrote:
>> This should be worked into a PEP, instead of living on as a bunch of
>> python-ideas posts and blogs.
>> I find the attrs documentation (and Glyph's blog post about it) almost
>> unreadable because of the exalted language -- half the doc seems to be
>> *selling* the library more than *explaining* it. If this style were to
>> become common I would find it a disturbing trend.
>> But having something alongside NamedTuple that helps you declare classes
>> with mutable attributes using the new PEP 526 syntax (and maybe a few
>> variants) would definitely be useful. Will someone please write a PEP? Very
>> few of the specifics of attrs need be retained (its punny naming choices
>> are too much for the stdlib).
> In case someone decides to take this on, I wrote a blog post back in March
> that shows how to use __init_subclass__() to do a rough approximation of
> what Guido is suggesting: https://snarky.ca/customizing-class-creation-in-
> python/ .
> Based on my thinking on the topic while writing my blog post, the tricky
> bit is going to be deciding how to handle default values (i.e. if you set a
> default value like `attr: int = 42` on the class definition then you have
> `cls.attr` exist which might not be what you want if you would rather have
> the default value explicitly set on every instance but not fall through to
> the class (e.g. `del ins.attr; ins.attr` raises an AttributeError instead
> of falling through to `cls.attr`). You could remove the default from the
> class in your __init_subclass__(), but then you have to decide if that's
> too unexpected/magical for someone looking at the code.

I would personally prefer the initializer to stay in the class in cases
like this. If the initializer needs to be a default instance of a mutable
class (e.g. an empty list or dict) there could be a special marker to
indicate that, e.g.

  attacks: List[int] = MAKE_NEW  # Creates a new [] for each instance

while if the default needs to be something more custom it could be a
similar marker with a callable argument, e.g.

  fleet: Dict[str, str] = MAKE_NEW(lambda: {'flagship': 'Enterprise'})

I would prefer not to have cleverness like initialization with a callable
automatically does something different.

> And I too would be interested in seeing something like this, if for any
> other reason than to help people not to misuse NamedTuple for
> quick-and-dirty data objects in new APIs (NamedTuple is meant to help move
> old-style tuple-based APIs to a class-based one).

Not sure I agree that is its only purpose.

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