[Python-ideas] Python 3.7 dataclasses attribute order

[Philip Martin]

Hi, I just started to use the new dataclasses module. My initial use case
boils down to somewhere between a namedtuple and a class where I want a
record with a few methods.

Mainly, I am trying to build a specific record from various sources, and
then have the class handle validation and serialization. One roadblock I
have found is that I can't use the field order I define for the class to
say write out a csv file if any of the fields have default value. I know
this bucks Python's args, kwargs ordering, but I think having the ability
define optional arguments and required arguments in any order helps improve
the classes intended usability. For instance, imagine
"account_description_2" is None for most sources, but must appear before
"other_required_field" in a CSV exported file. I think it would be useful
to be able to do the following:

import csv
from datetime import date
from dataclasses import dataclass, fields
from typing import List

OBJECT_SERIALIZERS = {date: date.isoformat}

@dataclass
class Account:
    account_id: str
    account_description: str
    account_description_2: str = None

    # INVALID
    other_required_field: str

    def serialize(self):
        for field in fields(self):
            value = getattr(self, field.name)
            serializer = OBJECT_SERIALIZERS.get(field.type, None)

            if serializer:
                value = serializer(value)
            yield value

    @property
    def field_names(self):
        return [field.name for field in fields(self)]

    @classmethod
    def from_source_a(cls, record):
        return cls(account_id=record['account_code'],
                   account_description=record['account_name'],
                   other_required_field=record['other_field'])

@dataclass
class AccountList:
    accounts: List[Account]

    @property
    def field_names(self):
        return [
            field.name for field in fields(fields(self)[0].type.__args__[0])
        ]

    @property
    def record_field(self):
        return fields(self)[0].name

    def to_csv(self, path):
        with open(path, 'w') as file:
            self.write_file(file)

    def write_file(self, file):
        records_field = self.record_field

        writer = csv.writer(file)
        writer.writerow(self.field_names)
        writer.writerows(
            record.serialize() for record in getattr(self, records_field)
        )
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