Custom Policies

You can define your own policies to enforce project-specific rules, confirmations, or constraints.

Have an idea that might be useful to others?
You’re warmly invited to open a pull request and contribute your policy back to the community!

Custom policies are placed in .fellow/policies/ or any directory listed in custom_policies_paths in your config.

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Quick Start

Create a new policy with:

fellow init-policy my_policy_name

This generates a file like .fellow/policies/my_policy_name.py with the basic structure.

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Structure

Each custom policy consists of:

• A config class extending PolicyConfig
• A policy class extending Policy[YourConfigClass]
• A check(...) method that returns True or an error message

Example

Below is a minimal working example of a custom policy.

It checks whether a specific string field in the command input (like filename) exceeds a maximum allowed length.
This kind of check can be useful for enforcing naming conventions, preventing long file paths, or limiting user input.

You define a config class to validate the policy’s inputs, and a policy class that implements the logic in its check(...) method.

from typing import TYPE_CHECKING, Union
from pydantic import Field
from fellow.policies.Policy import Policy, PolicyConfig

if TYPE_CHECKING:
    from fellow.commands.Command import CommandContext, CommandHandler, CommandInput


class MaxLengthFieldConfig(PolicyConfig):
    field: str = Field(..., description="Name of the CommandInput field to check")
    max_length: int = Field(..., description="Maximum allowed length of the value")


class MaxLengthField(Policy[MaxLengthFieldConfig]):
    def __init__(self, config: MaxLengthFieldConfig):
        self.config = config

    def check(
            self,
            command_name: str,
            command_handler: "CommandHandler",
            args: "CommandInput",
            context: "CommandContext",
    ) -> Union[bool, str]:
        value = getattr(args, self.config.field, None)
        if not isinstance(value, str):
            return True
        if len(value) > self.config.max_length:
            return f"Field '{self.config.field}' exceeds max length {self.config.max_length}: '{value}'"
        return True

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Usage in Config

After creating your policy, use it in your commands config:

commands:
  create_file:
    policies:
      - name: max_length_field
        config:
          field: filename
          max_length: 50