getattro

getattro refers to the tp_getattro slot in CPython’s C API, a low-level hook that controls attribute lookup for objects of a custom type. When an attribute is accessed (for example, obj.name), the interpreter will invoke the type’s getattro function to obtain the attribute value if this slot is defined. If the tp_getattro slot is NULL, Python falls back to the default attribute lookup machinery.

The getattro function has a signature similar to PyObject* getattro(PyObject *self, PyObject *name). The name argument is the attribute name, typically a Unicode string. The function must return a new reference to the attribute value or set an exception and return NULL on error. Implementers may use PyObject_GenericGetAttr to preserve standard attribute resolution behavior, including descriptor handling and lookups in __dict__.

Common reasons to implement getattro include providing custom storage for attributes, implementing computed or lazily-evaluated attributes, or enforcing access rules. By taking control of attribute retrieval at the C level, extension types can optimize performance or implement domain-specific semantics that the default Python attribute machinery cannot easily express.

Relationship to Python-level mechanics: getattro implements the C-level attribute lookup that corresponds to Python’s __getattribute__ mechanism. If a Python class defines __getattribute__ in Python code, attribute access is routed there; if a type provides tp_getattro in C, that slot is used by the interpreter as part of the same lookup process. The descriptor protocol and __getattr__ fallback behavior still apply as part of the overall attribute resolution.

In practice, getattro is a specialized tool for CPython extension authors to customize how attributes are retrieved, balancing performance with correct interaction with Python’s descriptor and attribute semantics.