google--adk-python/.agents/skills/adk-style/references/visibility.md

Visibility Style Guide

Python does not have native access modifiers (like public, private, or package-private). ADK relies on naming conventions and module structure to define visibility boundaries.

Conventions

1. Module-Private / Internal Files

• Private by Default: All new .py module files under src/google/adk/ must be private by default (prefixed with _). This is enforced by a pre-commit hook (check-new-py-prefix).
• Even if a file contains symbols intended for the public API, the file itself must have a leading underscore. The symbols are then exposed via the package's __init__.py.
• Files intended for internal use within a package or subsystem must also be prefixed with a leading underscore (e.g., _task_models.py).
• These files should never be imported directly by code outside of the ADK framework.

2. Class and Function Visibility

• Public: No leading underscore. Intended for use by consumers of the module or package.
• Internal/Private: Leading underscore (e.g., _private_method()). Intended only for use within the defining class or module.

3. Package-Private (Subsystem Visibility)

Since Python lacks true package-private access, we simulate it by:

• Not exporting the symbol in the package's __init__.py.
• Using _-prefixed modules for internal implementation details.
• Code within the same package can import from these _ modules, but code outside should not.
• Direct Imports Required: Within the ADK framework, importing from __init__.py is not allowed. You must import from the specific module directly. This helps keep __init__.py minimal and keeps packages as self-contained as possible.

4. Public API Export

• The public API of a package must be explicitly exported in __init__.py.
• Use __all__: The __init__.py file should define __all__ to explicitly list the symbols that are part of the public API.
• Only public names (symbols intended for use outside the package) should be imported into __init__.py and listed in __all__.
• Users should be able to import public symbols directly from the package level, rather than digging into internal modules.

Examples

Exposing a Public Interface

# In src/google/adk/agents/llm/task/_task_agent.py (File is private by default)
class TaskAgent: # Public symbol
    ...

# In src/google/adk/agents/llm/task/__init__.py
from ._task_agent import TaskAgent

__all__ = [
    'TaskAgent',
]

Keeping Implementation Details Private

# In src/google/adk/agents/llm/task/_task_models.py (Internal file)
class TaskRequest(BaseModel): # Public within the module, but module is private
    ...

# In src/google/adk/agents/llm/task/__init__.py
# We DO NOT export TaskRequest here if it is only for internal use within the task package.