@Marvin Can you explain prefect blocks that are made of a union? This is prefect 3 with Pydantic 2. Here is a code sample:

from typing import Optional, Union

from prefect.blocks.core import Block
from prefect import flow, get_run_logger, task
from pydantic import Field


class Car(Block):
    make: Optional[str] = Field(default=None, description="Make")
    model: Optional[str] = Field(default=None, description="Model")
    year: Optional[int] = Field(default=None, description="Year")


class Bike(Block):
    brand: Optional[str] = Field(default=None, description="Brand")
    tire_size: Optional[int] = Field(default=None, description="Tire Size")


class Person(Block):
    name: Optional[str] = Field(default=None, description="Name")
    age: Optional[int] = Field(default=None, description="Age")
    transportion: Optional[Union[Car, Bike]] = Field(
        default=None, description="Mode of transportation used by this person"
    )


@task(name="Create Blocks")
def create_blocks():
    bike = Bike(brand="Giant", tire_size=26)
    person = Person(name="John", age=30, transportion=bike)
    bike.save("bike1", overwrite=True)
    person.save("person1", overwrite=True)


@task(name="Check Nested Blocks")
def check_nested_blocks():
    person = Person.load("person1")
    
    logger = get_run_logger()
    logger.info(f"Person: {person}")
    logger.info(f"{person.__dict__=}")
    logger.info(f"{person.transportion}")
    logger.info(f"{person.transportion.__dict__=}")


@flow(name="TestFlow")
def test_blocks_flow():
    create_blocks()
    check_nested_blocks()


if __name__ == "__main__":
    test_blocks_flow()

The output shows their transportation as a

Car

when it should be a

Bike

 python block_test.py                       
20:27:40.822 | INFO    | Flow run 'melodic-zebra' - Beginning flow run 'melodic-zebra' for flow 'TestFlow'
20:27:40.824 | INFO    | Flow run 'melodic-zebra' - View at <http://127.0.0.1:4203/runs/flow-run/d99d392a-319c-40ad-9c9f-4e6c4c63b0ca>
20:27:41.111 | INFO    | Task run 'Create Blocks-4c4' - Finished in state Completed()
20:27:41.152 | INFO    | Task run 'Check Nested Blocks-8cb' - Person: Person(name='John', age=30, transportion=Car(make=None, model=None, year=None))
20:27:41.153 | INFO    | Task run 'Check Nested Blocks-8cb' - person.__dict__={'name': 'John', 'age': 30, 'transportion': Car(make=None, model=None, year=None)}
20:27:41.154 | INFO    | Task run 'Check Nested Blocks-8cb' - Car(make=None, model=None, year=None)
20:27:41.154 | INFO    | Task run 'Check Nested Blocks-8cb' - person.transportion.__dict__={'make': None, 'model': None, 'year': None}
20:27:41.156 | INFO    | Task run 'Check Nested Blocks-8cb' - Finished in state Completed()
20:27:41.184 | INFO    | Flow run 'melodic-zebra' - Finished in state Completed()

When I switch the order of the

transportion: Optional[Union[Car, Bike]]

to

transportion: Optional[Union[Bike, Car]]

then the code shows

Bike

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@Marvin How do you define a block that can take one of two different types of custom fields?

To define a block in Prefect 3.x that can take one of two different types of custom fields, you can utilize Pydantic's

BaseModel

features, as Prefect blocks are built on top of it. Here's a concise guide on how you might approach this: 1. Subclass the Block: Start by creating a class that subclasses

Block

. This class will represent your custom block type. 2. Define Custom Fields: Use Pydantic's field definitions to specify the types of fields your block will accept. You can use Pydantic's

Union

to allow a field to accept multiple types. 3. Register the Block: Use Prefect's registration methods to make your custom block available for use. Here's a simple example:

python
from pydantic import BaseModel, Field
from typing import Union
from prefect.blocks.core import Block

class MyCustomBlock(Block):
    my_field: Union[int, str] = Field(..., description="This field can be an int or a str")

# Register the block
MyCustomBlock.register_type_and_schema()

This example defines a block with a field

my_field

that can be either an integer or a string. You can read more about creating custom blocks in the Prefect documentation. If you have specific needs or additional complexities, feel free to ask!