Implement a Python Built-in Function in C

Built-in Functions in C

Builtins like len, abs, and sum are C functions registered on the builtins module. They parse PyObject* arguments, do native work, and return new Python objects without ever entering the bytecode interpreter loop.

PyCFunction Pattern

A builtin uses the same C API as extensions but lives in Python/bltinmodule.c or similar:

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Every builtin you study follows the same checklist:

• Parse: PyArg_ParseTuple with a format string like "i" or "O"
• Validate: PyLong_Check, PySequence_Check, or similar type guards
• Return: a new reference (PyLong_FromLong) or NULL with PyErr_SetString

Type checks, error handling, and correct refcounting are mandatory. Fast paths often bypass generic PyNumber dispatch when the argument type is known.

For example, abs(-42) calls C code that reads the PyLong digits directly instead of re-entering the interpreter loop.

Design Notes

Fast paths should validate types once, then read internal struct fields directly. Falling back to abstract PyNumber APIs preserves semantics but costs the speed that motivated writing C in the first place.

For This Exercise

This exercise asks you to implement a Python builtin in C following CPython conventions. You will write argument parsing, type validation, and return value construction for a function callable from the REPL.

You will use the same mental model here when reading production interpreter source later in the track. Sketch one concrete input on paper, predict the outcome, then confirm with code. That discipline catches logic errors early and makes debugging far faster when you extend the implementation in follow-on tasks.