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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:
PropertyDescription
_block_type_nameDisplay name of the block in the UI. Defaults to the class name.
_block_type_slugUnique slug used to reference the block type in the API. Defaults to a lowercase, dash-delimited version of the block type name.
_logo_urlURL pointing to an image that should be displayed for the block type in the UI. Default to None.
_descriptionShort description of block type. Defaults to docstring, if provided.
_code_exampleShort code snippet shown in UI for how to load/use block type. Defaults to first example provided in the docstring of the class, if provided.

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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)