Understanding `PyBool_FromLong` in Python

What is PyBool_FromLong? 🔗

PyBool_FromLong is a function in the Python C API that converts a long integer to a Boolean value. Specifically, it takes an integer (or long) and returns Py_True if the integer is non-zero and Py_False if the integer is zero. In Python terms, it’s akin to using the bool() function to convert a number to a Boolean value.

How is PyBool_FromLong Used? 🔗

The PyBool_FromLong function is used in C extensions for Python. These extensions allow you to write Python modules in C for improved performance or to interface with C libraries. Here’s a basic example to illustrate how it’s used:

#include <Python.h>

// Example function that uses PyBool_FromLong
static PyObject* is_positive(PyObject* self, PyObject* args) {
    long number;
    
    // Parse the input from Python, expecting a single long argument
    if (!PyArg_ParseTuple(args, "l", &number)) {
        return NULL; // Return NULL to indicate an error
    }
    
    // Use PyBool_FromLong to convert the number to a boolean
    PyObject* result = PyBool_FromLong(number > 0);
    
    // Return the boolean result
    return result;
}

// Method definitions for the module
static PyMethodDef MyMethods[] = {
    {"is_positive", is_positive, METH_VARARGS, "Check if a number is positive"},
    {NULL, NULL, 0, NULL} // Sentinel value indicating the end of the array
};

// Module definition
static struct PyModuleDef mymodule = {
    PyModuleDef_HEAD_INIT,
    "mymodule", // Name of the module
    NULL,       // Module documentation
    -1,         // Size of per-interpreter state of the module, or -1 if the module keeps state in global variables
    MyMethods
};

// Module initialization function
PyMODINIT_FUNC PyInit_mymodule(void) {
    return PyModule_Create(&mymodule);
}

In this example, the is_positive function checks if a given number is positive. It uses PyBool_FromLong to convert the result of the comparison (number > 0) to a Boolean value that Python can understand.

How Does PyBool_FromLong Work? 🔗

To understand how PyBool_FromLong works, let’s use a metaphor. Imagine you have a switch that can be either on or off. This switch represents a Boolean value: on is True, and off is False. Now, if you have a number, you can think of it as a signal to flip the switch: if the number is zero, the switch is off (False); if the number is non-zero, the switch is on (True).

Internally, PyBool_FromLong does something similar. It takes a number and checks its value. If the number is zero, it returns Py_False; otherwise, it returns Py_True. Here’s a simplified look at what happens inside the function:

PyObject* PyBool_FromLong(long v) {
    return v ? Py_True : Py_False;
}

This line of code checks if v (the input number) is non-zero. If it is, it returns Py_True; otherwise, it returns Py_False.