Modules/_io/iobase.c

Source:

cpython 3.14 @ ab2d84fe1023/Modules/_io/iobase.c ↗

Modules/_io/iobase.c defines IOBase and RawIOBase, the two abstract base classes at the root of the io class hierarchy. IOBase provides the default implementations of flush, close, __del__, readline, and __iter__. RawIOBase adds read, readall, and write abstract methods.

Map

Lines	Symbol	Role
1-80	iobase struct	Object layout; dict, weakreflist, finalizing flag
81-200	iobase_flush, iobase_close	Default flush (no-op); close calls flush then sets closed flag
201-350	iobase_readline	Default readline via single-char read(1) loop
351-450	iobase_iter, iobase_iternext	__iter__ returns self; __next__ calls readline
451-600	iobase_finalize, iobase_unsupported	__del__ hook; raises UnsupportedOperation
601-700	rawiobase_*, type wiring	RawIOBase abstract read/write; type slot setup

Reading

close() and the finalizer

iobase_close calls flush() and then sets the __IOBase_closed attribute. The __del__ method calls close() if the object is not already closed, issuing a ResourceWarning if the file was left open by user code.

// Modules/_io/iobase.c:81 iobase_close
static PyObject *
iobase_close(PyObject *self, PyObject *args)
{
    if (IS_CLOSED(self)) Py_RETURN_NONE;
    PyObject *res = PyObject_CallMethodNoArgs(self, _PyIO_str_flush);
    iobase_set_closed(self);
    return res;
}

Default readline

The default readline() reads one byte at a time until a newline or EOF. Subclasses override this with a buffered implementation; the default exists so any IOBase subclass that implements only read(1) can still be used as a line iterator.

// Modules/_io/iobase.c:201 iobase_readline
static PyObject *
iobase_readline(PyObject *self, PyObject *args)
{
    PyObject *buffer = PyByteArray_FromStringAndSize(NULL, 0);
    while (1) {
        PyObject *b = _PyObject_CallMethodId(self, &PyId_read, "i", 1);
        if (b == NULL || PyBytes_GET_SIZE(b) == 0) { Py_DECREF(b); break; }
        PyByteArray_Concat(&buffer, b);
        if (((char *)PyBytes_AS_STRING(b))[0] == '\n') { Py_DECREF(b); break; }
        Py_DECREF(b);
    }
    return PyBytes_FromObject(buffer);
}

UnsupportedOperation

iobase_unsupported raises io.UnsupportedOperation, which is a subclass of both OSError and ValueError. Abstract methods like read() and write() on IOBase call this to produce a clear error message when a subclass forgets to implement them.

gopy notes

Not yet ported. The planned package path is module/_io/. The Go equivalent uses interface types (io.Reader, io.Writer, io.Closer, io.Seeker) rather than an inheritance hierarchy; an idiomatic port would expose the same Python-facing protocol while delegating to Go interfaces internally.