Modules/_decimal/_decimal.c (part 5)

Source:

cpython 3.14 @ ab2d84fe1023/Modules/_decimal/_decimal.c ↗

This annotation covers the C implementation of decimal.Context operations. See lib_decimal2_detail for Decimal.fma, ln, exp, and context traps in pure Python.

Map

Lines	Symbol	Role
1-80	Context.create_decimal	Create a Decimal from various inputs under the context
81-180	Decimal.quantize	Round to a given exponent
181-260	Decimal.normalize	Remove trailing zeros
261-360	Thread-local context	getcontext() / setcontext()
361-500	decimal.localcontext	Temporary context with with statement

Reading

Context.create_decimal

// CPython: Modules/_decimal/_decimal.c:3280 ctx_create_decimal
static PyObject *
ctx_create_decimal(PyObject *v, PyObject *args)
{
    /* Create a new Decimal, applying the current context's precision
       and rounding mode. Signals are raised as configured. */
    mpd_t *mpd = mpd_qnew();
    if (PyUnicode_Check(input)) {
        mpd_qset_string(mpd, str, ctx, &status);
    } else if (PyLong_Check(input)) {
        mpd_qset_i64(mpd, val, ctx, &status);
    }
    _PyDec_CheckStatus(ctx, status);
    return PyDecType_FromMPD(&PyDec_Type, mpd, ctx);
}

Context.create_decimal applies the context's precision and rounding to the input. Decimal(x) uses the current context; ctx.create_decimal(x) uses ctx explicitly.

Decimal.quantize

// CPython: Modules/_decimal/_decimal.c:2980 dec_quantize
static PyObject *
dec_quantize(PyObject *v, PyObject *args, PyObject *kwds)
{
    /* Round self to the same exponent as exp.
       Decimal('3.14159').quantize(Decimal('0.01')) == Decimal('3.14') */
    mpd_qquantize(mpd_result, mpd_self, mpd_exp, ctx, &status);
    _PyDec_CheckStatus(ctx, status);
    return PyDecType_FromMPD(&PyDec_Type, mpd_result, ctx);
}

quantize is the key operation for financial rounding: amount.quantize(Decimal('0.01'), rounding=ROUND_HALF_UP) rounds to two decimal places.

Thread-local context

// CPython: Modules/_decimal/_decimal.c:1280 PyDec_GetCurrentContext
static mpd_context_t *
PyDec_GetCurrentContext(void)
{
    /* Retrieve the thread-local decimal context */
    PyObject *ctx = PyObject_CallNoArgs(module_state->getcontext);
    return ((PyDecContext *)ctx)->ctx;
}

Each thread has its own decimal context (precision, rounding mode, traps). decimal.getcontext() retrieves it; decimal.setcontext() replaces it. decimal.localcontext(prec=50) uses a context manager to temporarily override.

gopy notes

Context.create_decimal is module/decimal.ContextCreateDecimal in module/decimal/module.go. It delegates to shopspring/decimal or a libmpdec wrapper. Decimal.quantize calls decimal.NewFromFloat(...).StringFixed(scale). Thread-local contexts are stored in goroutine-local storage via sync.Map.