How to create custom blocks

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Create a new block type

To create a custom block type, define a class that subclasses Block. The Block base class builds on Pydantic’s BaseModel, so you can declare custom fields just like a Pydantic model.

We’ve already seen an example of a Cube block that represents a cube and holds information about the length of each edge in inches:

from prefect.blocks.core import Block

class Cube(Block):
    edge_length_inches: float


Cube.register_type_and_schema()

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Register custom blocks

In addition to the register_type_and_schema method shown above, you can register blocks from a Python module with a CLI command:

prefect block register --module prefect_aws.credentials

This command is useful for registering all blocks found within a module in a Prefect Integration library.

Alternatively, if a custom block was created in a .py file, you can register the block with the CLI command:

prefect block register --file my_block.py

Block documents can now be created with the registered block schema.

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Secret fields

All block values are encrypted before being stored. If you have values that you would not like visible in the UI or in logs, use the SecretStr field type provided by Pydantic to automatically obfuscate those values. You can use this capability for fields that store credentials such as passwords and API tokens.

Here’s an example of an AwsCredentials block that uses SecretStr:

from typing import Optional

from prefect.blocks.core import Block
from pydantic import SecretStr

class AwsCredentials(Block):
    aws_access_key_id: Optional[str] = None
    aws_secret_access_key: Optional[SecretStr] = None
    aws_session_token: Optional[str] = None
    profile_name: Optional[str] = None
    region_name: Optional[str] = None

Since aws_secret_access_key has the SecretStr type hint assigned to it, the value of that field is not exposed if the object is logged:

aws_credentials_block = AwsCredentials(
    aws_access_key_id="AKIAJKLJKLJKLJKLJKLJK",
    aws_secret_access_key="secret_access_key"
)

print(aws_credentials_block)
# aws_access_key_id='AKIAJKLJKLJKLJKLJKLJK' aws_secret_access_key=SecretStr('**********') aws_session_token=None profile_name=None region_name=None

Prefect’s SecretDict field type allows you to add a dictionary field to your block that automatically obfuscates values at all levels in the UI or in logs. This capability is useful for blocks where typing or structure of secret fields is not known until configuration time.

Here’s an example of a block that uses SecretDict:

from prefect.blocks.core import Block
from prefect.blocks.fields import SecretDict


class SystemConfiguration(Block):
    system_secrets: SecretDict
    system_variables: dict


system_configuration_block = SystemConfiguration(
    system_secrets={
        "password": "p@ssw0rd",
        "api_token": "token_123456789",
        "private_key": "<private key here>",
    },
    system_variables={
        "self_destruct_countdown_seconds": 60,
        "self_destruct_countdown_stop_time": 7,
    },
)

system_secrets is obfuscated when system_configuration_block is displayed, but system_variables show up in plain-text:

print(system_configuration_block)
# SystemConfiguration(
#   system_secrets=SecretDict('{'password': '**********', 'api_token': '**********', 'private_key': '**********'}'), 
#   system_variables={'self_destruct_countdown_seconds': 60, 'self_destruct_countdown_stop_time': 7}
# )

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Customize a block’s display

You can set metadata fields on a block type’s subclass to control how a block displays.

Available metadata fields include:

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Update custom Block types

Here’s an example of how to add a bucket_folder field to your custom S3Bucket block; it represents the default path to read and write objects from (this field exists on our implementation).

Add the new field to the class definition:

from typing import Optional

from prefect.blocks.core import Block


class S3Bucket(Block):
    bucket_name: str
    credentials: AwsCredentials
    bucket_folder: Optional[str] = None
    ...

Then register the updated block type with either your Prefect Cloud account or your self-hosted Prefect server instance.

If you have any existing blocks of this type that were created before the update and you’d prefer to not re-create them, migrate them to the new version of your block type by adding the missing values:

# Bypass Pydantic validation to allow your local Block class to load the old block version
my_s3_bucket_block = S3Bucket.load("my-s3-bucket", validate=False)

# Set the new field to an appropriate value
my_s3_bucket_block.bucket_path = "my-default-bucket-path"

# Overwrite the old block values and update the expected fields on the block
my_s3_bucket_block.save("my-s3-bucket", overwrite=True)