Add public BaseModel.iter() method

Feature Request

Add BaseModel.iter() method which will behave exactly like a dict, but without converting nested values to dict by default and will be a generator instead

Why

Imagine you want to iterate through values in model, but also need to exclude some, or use aliases instead of attributes. So what you need to do is write something like this:

for attr, value in model.dict(exclude={...}, by_alias=True).items():
    do_something(attr, value)

Obviously, in this case python will firet evaluate model.dict(exclude={...}, by_alias=True). part, going through all values and only then you will be able to iterate, going through the same values again.

Also, by using dict nested values are force-converted to dict, and this might be very slow when we just want to iterate through top-level values.

We also cannot use existing _iter(), because some work is actually done in dict()

How we can solve it

By having iter() method we can bypass this double-iteration problem and converting-to-dict problems:

for attr, value in model.iter(exclude={...}, by_alias=True):
    do_something(attr, value)

[samuelcolvin]

This already exists!

It’s called ._iter().

[dmontagu]

For what it’s worth, I would be in favor of moving logic from dict into _iter, assuming it didn’t result in meaningful performance penalties in any public methods – I think it would simplify/result in a smaller diff for the implementation of dump_as_type from #812, and the related serialization performance improvements (even when not dumping as a specified type).