Printing boolean values True/False with the format() method in Python

Excellent question! I believe I have the answer. This requires digging around through the Python source code in C, so bear with me.

First, format(obj, format_spec) is just syntactic sugar for obj.__format__(format_spec). For specifically where this occurs, you'd have to look in abstract.c, in the function:

PyObject *PyObject_Format(PyObject* obj, PyObject *format_spec){    PyObject *empty = NULL;    PyObject *result = NULL;    ...    if (PyInstance_Check(obj)) {        /* We're an instance of a classic class */HERE -> PyObject *bound_method = PyObject_GetAttrString(obj, "__format__");        if (bound_method != NULL) {            result = PyObject_CallFunctionObjArgs(bound_method,                                                  format_spec,                                                  NULL);    ...}

To find the exact call, we have to look in intobject.c:

static PyObject *int__format__(PyObject *self, PyObject *args){    PyObject *format_spec;    ...    return _PyInt_FormatAdvanced(self,                     ^           PyBytes_AS_STRING(format_spec),                     |           PyBytes_GET_SIZE(format_spec));               LET'S FIND THIS    ...}

_PyInt_FormatAdvanced is actually defined as a macro in formatter_string.c as a function found in formatter.h:

static PyObject*format_int_or_long(PyObject* obj,               STRINGLIB_CHAR *format_spec,           Py_ssize_t format_spec_len,           IntOrLongToString tostring){    PyObject *result = NULL;    PyObject *tmp = NULL;    InternalFormatSpec format;    /* check for the special case of zero length format spec, make       it equivalent to str(obj) */    if (format_spec_len == 0) {        result = STRINGLIB_TOSTR(obj);   <- EXPLICIT CAST ALERT!        goto done;    }    ... // Otherwise, format the object as if it were an integer}

And therein lies your answer. A simple check for whether format_spec_len is 0, and if it is, convert obj into a string. As you well know, str(True) is 'True', and the mystery is over!