Reusable Packages, __init__.py & __all__ API Control

1Concept

A package is a directory containing modules. `__init__.py` marks the directory as an importable package and controls public exports. Defining `__all__ = ['SymbolA', 'SymbolB']` restricts what is imported when clients write `from package import *`.

2Architecture Diagram

package/
  ├── __init__.py  (__all__ = ['PublicApi'])
  ├── api.py       (Public class PublicApi)
  └── internal.py  (Private helpers hidden from client)

3Code Example

# Simulating package __all__ boundary control
__all__ = ['PublicService', 'public_function']

class PublicService:
    pass

def public_function():
    return "Public API"

def _internal_helper():
    return "Hidden Helper"

print(f"Exported Public Symbols: {__all__}")
print(f"Has PublicService: {'PublicService' in __all__}")

4Expected Output

Exported Public Symbols: ['PublicService', 'public_function']
Has PublicService: True

5Key Takeaways

• ✓`__all__` acts as a contract for library authors to signal intended public API boundaries.
• ✓Avoid `from module import *` in production code; it pollutes namespaces and masks linter errors.
• ✓`__init__.py` can be empty in modern Python (PEP 420).