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skills/python-test-strategy/reference/public-api.md

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+# Public API Testing
+
+## API Module
+
+Create `api.py` that exports all public interfaces (one import per line):
+
+```python
+# project/api.py
+from .math.operations import add_numbers
+from .math.operations import subtract_numbers
+from .utils.helpers import format_output
+from .core.engine import create_engine
+```
+
+One import per line makes changes clear in diffs and easier to maintain.
+
+## Test Public API
+
+Test file at `tests/test_api.py` verifies all exports are accessible:
+
+```python
+from project import api
+
+def test_api():
+    """Verify all public APIs are importable."""
+    _ = api.add_numbers
+    _ = api.subtract_numbers
+    _ = api.format_output
+    _ = api.create_engine
+```
+
+## Purpose
+
+- Catch accidental removal of public exports
+- Prevent breaking changes to API
+- Document what users can import
+- Fail tests if API changes unexpectedly
+
+## Best Practices
+
+1. **Export only stable APIs** - Functions and classes that are unlikely to change
+2. **Keep minimal** - Don't export internal utilities
+3. **Document** - Add docstring to `api.py` explaining what's exported
+4. **Group logically** - Organize imports by functional area
+
+## Deprecation
+
+Mark deprecated APIs before removal:
+
+```python
+import warnings
+
+def old_function():
+    """Deprecated: Use new_function() instead."""
+    warnings.warn("Use new_function instead", DeprecationWarning, stacklevel=2)
+    return new_function()
+```