Modules/_json.c (part 4)

Source:

cpython 3.14 @ ab2d84fe1023/Modules/_json.c ↗

This annotation covers the full encoder pipeline. See modules_json3_detail for JSONDecoder, scanstring, and JSONObject.

Map

Lines	Symbol	Role
1-80	encoder_encode_string	Fast path: encode a Python str as JSON string
81-160	encoder_encode_float	Handle floats including inf/nan
161-260	encoder_listencode_obj	Dispatch encode by type
261-380	encoder_listencode_dict	Encode a dict as JSON object
381-500	Circular reference check	_encoder_listencode_obj marker stack

Reading

encoder_encode_string

// CPython: Modules/_json.c:120 encoder_encode_string
static PyObject *
encoder_encode_string(PyEncoderObject *s, PyObject *obj)
{
    /* Scan for chars that need escaping */
    Py_ssize_t i;
    const char *input = PyUnicode_AsUTF8AndSize(obj, &i);
    ...
    if (has_special_chars) {
        return encoder_encode_string_slow(s, obj);
    }
    /* Fast path: no escaping needed — wrap in quotes directly */
    return PyUnicode_FromFormat("\"%s\"", input);
}

Strings with no special characters (no \, ", or control characters) take the fast path: a single PyUnicode_FromFormat wraps the string in quotes. Strings needing escaping go through the slow path that uses _PyUnicodeWriter.

encoder_encode_float

// CPython: Modules/_json.c:200 encoder_encode_float
static PyObject *
encoder_encode_float(PyEncoderObject *s, PyObject *obj)
{
    double val = PyFloat_AS_DOUBLE(obj);
    if (!Py_IS_FINITE(val)) {
        /* inf, -inf, nan: not valid JSON */
        if (s->allow_nan) {
            if (isinf(val)) return val > 0 ? "Infinity" : "-Infinity";
            return PyUnicode_FromString("NaN");
        }
        PyErr_SetString(PyExc_ValueError,
            "Out of range float values are not JSON compliant");
        return NULL;
    }
    return PyFloat_Type.tp_repr(obj);
}

By default json.dumps(float('inf')) raises ValueError. json.dumps(float('inf'), allow_nan=True) emits Infinity (a JavaScript-ism that is not valid JSON but useful for interop).

encoder_listencode_dict

// CPython: Modules/_json.c:380 encoder_listencode_dict
static int
encoder_listencode_dict(PyEncoderObject *s, _PyUnicodeWriter *writer,
                        PyObject *dct, Py_ssize_t indent_level)
{
    /* Check for circular reference */
    Py_ssize_t ident = PyObject_Hash((PyObject *)dct);
    if (PySet_Contains(s->markers, (PyObject *)dct)) {
        PyErr_SetString(PyExc_ValueError, "Circular reference detected");
        return -1;
    }
    PySet_Add(s->markers, (PyObject *)dct);
    ...
    PySet_Discard(s->markers, (PyObject *)dct);
    return 0;
}

The circular reference check uses a set keyed by object identity (the dict pointer, via its id). Adding and removing around the recursive call forms a call-stack-tracking mechanism. json.dumps({'a': d}, check_circular=False) disables this by leaving s->markers as NULL.

gopy notes

encoder_encode_string is module/json.EncodeString in module/json/module.go. The fast path uses strings.Builder with a quote-and-copy when no escaping is needed. encoder_encode_float uses strconv.FormatFloat. encoder_listencode_dict tracks circular references via a map[uintptr]struct{} keyed on objects.ID(obj).