Change types into dataclasses

[Fokko]

Proposal to change the types into dataclasses.

This has several improvments:

• We can use the dataclasss field(repr=True) to include the fields in the representation, instead of building our own strings
• We can assign the types in the post_init when they are dynamic (List, Maps, Structs etc) , or just override them when they are static (Primitives)
• We don't have to implement any eq methods because they come for free
• The types are frozen, which is kind of nice since we re-use them
• The code is much more consise
• We can assign the min/max of the int/long/float as Final as of 3.8: https://peps.python.org/pep-0591/

My inspiration was the comment by Kyle:
#4742 (comment)

This would entail implementing eq, but why not use the generated one since we're comparing all the attributes :)

Would love to get you input

[rdblue]

Doesn't this need to check that *fields is empty?

[Fokko]

There seems to be a subtle behavioral change that requires this. Without this logic, the following test is failing:

assert str(type_var) == str(eval(repr(type_var)))

When we dig into this:

>> repr(type_var)
StructType(
    fields=(
        NestedField(field_id=1, name='optional_field', field_type=IntegerType(), is_optional=True), 
        NestedField(field_id=2, name='required_field', field_type=FixedType(length=5), is_optional=False), 
        ...
    )
)

It looks like it tries to initialize it again using a keyword argument:

>> eval(repr(type_var))
{TypeError}__new__() got an unexpected keyword argument 'fields'

Instead of variable unpacking.

Using this logic we give priority to the keyword argument:

StructField(
    NestedField(field_id=1, name='optional_field', field_type=IntegerType(), is_optional=True), 
    NestedField(field_id=2, name='required_field', field_type=FixedType(length=5), is_optional=False), 
)
or
StructField(
    fields=(
        NestedField(field_id=1, name='optional_field', field_type=IntegerType(), is_optional=True), 
        NestedField(field_id=2, name='required_field', field_type=FixedType(length=5), is_optional=False), 
    )
)

I've added an additional check to see if fields is empty.

[rdblue]

I really like how much this removes! Nice work.

[samredai]

This looks great, thanks @Fokko ! Left a few comments but overall LGTM 👍

[samredai]

Should we just rename this property to "field_type"? It might require some small changes in other parts of the library but it's probably better to not clash with a keyword and field_type is the argument name anyway. We can also do this in a follow-up PR.

[Fokko]

Yes, I had exactly the same thoughts. I would love to change this to field_type to avoid clashing with the internal type.

[Fokko]

Relative small change: f466165 :)

[samredai]

This should be nested in the if statement right above it right? If there's no fields kwarg we can't set it.

[Fokko]

This logic allows you to both pass in the fields through args and kwargs:

StructField(
    NestedField(field_id=1, name='optional_field', field_type=IntegerType(), is_optional=True), 
    NestedField(field_id=2, name='required_field', field_type=FixedType(length=5), is_optional=False), 
)
or
StructField(
    fields=(
        NestedField(field_id=1, name='optional_field', field_type=IntegerType(), is_optional=True), 
        NestedField(field_id=2, name='required_field', field_type=FixedType(length=5), is_optional=False), 
    )
)

This was a change required as explained in #4767 (comment)

[samredai]

Ah I see, makes sense!