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

Question

I was trying to print a truth table for Boolean expressions. While doing this, I stumbled upon the following:

>>> format(True, "") # shows True in a string representation, same as str(True)
'True'
>>> format(True, "^") # centers True in the middle of the output string
'1'

As soon as I specify a format specifier, format() converts True to 1. I know that bool is a subclass of int, so that True evaluates to 1:

>>> format(True, "d") # shows True in a decimal format
'1'

But why does using the format specifier change 'True' to 1 in the first example?

I turned to the docs for clarification. The only thing it says is:

A general convention is that an empty format string ("") produces the same result as if you had called str() on the value. A non-empty format string typically modifies the result.

So the string gets modified when you use a format specifier. But why the change from True to 1 if only an alignment operator (e.g. ^) is specified?

Answer 1

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!