[echoangle]

> I can't quite picture how operator overloading would look like, could you give an example?

Instead of this:

self.filter(end__gt=self._midnight(today))

You could write a "Field" class that implements __getattr__ and __gt__ so you could do

self.filter(Field.end > self._midnight(today))

The "Field.end > self._midnight(today)" would evaluate to an object that would just store "my field name is end and my value needs to be larger than xyz".

filter() can then look into its argument list and construct the filter criteria from the passed Field objects instead of the key value pairs as it does now.

[mmclar]

SQLalchemy does that. One advantage of the Django syntax is that it can be (pretty much) directly dropped into a query string on any admin page or DRF query and filter the results. E.g. the admin page for all the events after noon today:

  admin/event/?end__gt=2026-07-26T12:00:00

Being able to do ad-hoc queries using the same paradigm your app queries are written in, and then pass urls around with those queries included (e.g., quick one-off reports or answers to client questions) is so helpful.

[ptx]

This convenience sounds a little dangerous. Would this allow the user to specify e.g. joins or SQL procedure calls using the query parameter?

[zbentley]

If you mean “would it allow arbitrary queries in the admin interface?”, then yes, it would. That’s why it’s the admin interface.

If you mean “would it allow arbitrary queries from normal view code”, then no it would not—not unless you string eval on the backend or something else dangerous.

[pbalau]

if self._midnight(today) returns a datetime object, than:

   self.filter(end__gt=self._midnight(today))

will evaluate to:

   self.filter(end__gt=<some_datetime_object>)

While

    self.filter(Field.end > self._midnight(today))

will evaluate to:

   self.filter(<True/False>)

[echoangle]

Not if you do the magic with getattr and comparison overrides. You actually need to do it on the metaclass because the Field as I wrote it isn't an instance but this works:

    from datetime import datetime

    class Filter():
        def __init__(self, name):
            self.name = name
        def __gt__(self, value):
            return {
                "field": self.name,
                "operator": ">",
                "value": value
            }

    class FieldMeta(type):
        def __getattr__(cls, name):
            return Filter(name)

    class Field(metaclass=FieldMeta):
        pass

    print(Field.end > datetime(2024, 1, 1))

This gives:

    {'field': 'end', 'operator': '>', 'value': datetime.datetime(2024, 1, 1, 0, 0)}

You can make python return arbitrary values for comparisons by overriding __gt__ (and lt, eq) on the first operand (which we control here since it is a Field class), it doesn't have to be a bool.

Edit:

You can even make a little adapter to use this with the current filter system if you really want to:

    from datetime import datetime

    class Filter():
        def __init__(self, name):
            self.name = name
        def __gt__(self, value):
            return {
                "field": self.name,
                "operator": "gt",
                "value": value
            }
        
        def __lt__(self, value):
            return {
                "field": self.name,
                "operator": "lt",
                "value": value
            }
        
        def __eq__(self, value):
            return {
                "field": self.name,
                "operator": "eq",
                "value": value
            }

    class FieldMeta(type):
        def __getattr__(cls, name):
            return Filter(name)

    class Field(metaclass=FieldMeta):
        pass

    def _(*args):
        kwargs = {}
        for arg in args:
            k = arg["field"] + "__" + arg["operator"]
            kwargs[k] = arg["value"]
        return kwargs

    def filter(**kwargs):
        for k, v in kwargs.items():
            print(f"{k} = {v}")

    filter(**_(Field.end > datetime(2024, 1, 1)))

This prints

end__gt = 2024-01-01 00:00:00

[Izkata]

Do this:

  def __gt__(self, value):
      return Q(**{ f"{self.name}__gt": value })

and your original code should work as-is without the need for _()

  self.filter(Field.end > self._midnight(today))

https://docs.djangoproject.com/en/6.0/topics/db/queries/#com...

[pbalau]

If you change an operation that is meant to return a Boolean to return anything else, you are insta fired.

[vtbassmatt]

I would have agreed with this, and then they did the `pathlib.Path` bit of cuteness with the `/` operator: https://github.com/python/cpython/blob/5afbb60e0283caaf34990...

And despite my misgivings, it’s really ergonomic.

[pbalau]

If you divide a Path by another Path, you get a Path. If you compare two Paths, you get a Boolean. It is not really the same.

[BrenBarn]

That's a well established pattern in Python, for instance with Numpy. That's the point. Operations in Python aren't "mean to return" anything in particular. Each class can define the operations as it wants. That's a powerful feature that allows creation of specialized expression languages, as used by other ORMs besides Django.

[zzzeek]

Hi, don't mind me, but

You don't need to return a boolean. You need to return an object that implements __bool__.

Get it ?

And even then, when you're dealing with objects that are special to expressions, you don't actually even need __bool__ that much

[echoangle]

You mean like the numpy authors that let the comparison operators return arrays?

Also, apparently SQLAlchemy does exactly what I proposed so apparently they are erring in their ways too.

I honestly don’t find it that bad.

[BrenBarn]

No it won't, because there's no requirement that the result of `>` be True or False. It can return an object that then can participate in further expressions that "keep track" of what operations are done to the fields.

[7bit]

How would you model string comparisons with LIKE?

[BrenBarn]

For those you would have a method on the field object (e.g., `Table.field.like("whatever")`).

[7bit]

Yeah, but that's different from the operator. It's a different way of thinking about it. Maybe that's the reason they used dou le underscores for everything, to keep it consistent.

[BrenBarn]

Not really. Both method calls and operators are consistent with how Python normally works. Outside of Django you never do `obj__op(value)` or `obj__meth(value)` to do the equivalent of `obj op value` or `obj.meth(value)`. It is Django that is inconsistent with how operations are done in Python.