python - dict attribute 'type' to select Subclass of dataclass

Question

I have the following class

@dataclass_json
@dataclass
class Source:
    type: str =None
    label: str =None
    path: str = None

and the two subclasses:

@dataclass_json
@dataclass
class Csv(Source):
    csv_path: str=None
    delimiter: str=';'

and

@dataclass_json
@dataclass
class Parquet(Source):
    parquet_path: str=None

Given now the dictionary:

parquet={type: 'Parquet', label: 'events', path: '/.../test.parquet', parquet_path: '../../result.parquet'}

csv={type: 'Csv', label: 'events', path: '/.../test.csv', csv_path: '../../result.csv', delimiter:','}

Now I would like to do something like

Source().from_dict(csv) 

and that the output will be the class Csv or Parquet. I understand that if you initiate the class source you just "upload" the parameters with the method "from dict", but is there any posibility in doing this by some type of inheritence without using a "Constructor" which makes a if-else if-else over all possible 'types'?

Pureconfig, a Scala Library, creates different case classes when the attribute 'type' has the name of the desired subclass. In Python this is possible?

Answer 1

You can build a helper that picks and instantiates the appropriate subclass.

def from_data(data: dict, tp: type):
    """Create the subtype of ``tp`` for the given ``data``"""
    subtype = [
        stp for stp in tp.__subclasses__()  # look through all subclasses...
        if stp.__name__ == data['type']     # ...and select by type name
    ][0]
    return subtype(**data)  # instantiate the subtype

This can be called with your data and the base class from which to select:

>>> from_data(
...     {'type': 'Csv', 'label': 'events', 'path': '/.../test.csv', 'csv_path': '../../result.csv', 'delimiter':','},
...     Source,
... )
Csv(type='Csv', label='events', path='/.../test.csv', csv_path='../../result.csv', delimiter=',')

---

If you need to run this often, it is worth building a dict to optimise the subtype lookup. A simple means is to add a method to your base class, and store the lookup there:

@dataclass_json
@dataclass
class Source:
    type: str =None
    label: str =None
    path: str = None

    @classmethod
    def from_data(cls, data: dict):
        if not hasattr(cls, '_lookup'):
            cls._lookup = {stp.__name__: stp for stp in cls.__subclasses__()}
        return cls._lookup[data["type"]](**data)

This can be called directly on the base class:

>>> Source.from_data({'type': 'Csv', 'label': 'events', 'path': '/.../test.csv', 'csv_path': '../../result.csv', 'delimiter':','})
Csv(type='Csv', label='events', path='/.../test.csv', csv_path='../../result.csv', delimiter=',')