Objects/memoryobject.c (part 2)

cpython 3.14 @ ab2d84fe1023/Objects/memoryobject.c ↗

This annotation covers the slicing, casting, and tolist facilities in Objects/memoryobject.c. For the buffer protocol basics and __getitem__ see the objects_memoryobject_c annotation.

Map

Lines	Symbol	Role
1-400	Buffer flags, PyBUF_* constants	Flag semantics for __buffer__ callers
401-800	memory_subscript, memory_ass_subscript	Slicing and item assignment
801-1200	memory_cast	Re-interpret format without copy
1201-1600	memory_tolist	Recursive unpack to nested Python lists
1601-2000	memory_tobytes, memory_hex	Copy-out and hex dump
2001-3200	Contiguity checks, copy_buffer	C- and Fortran-order contiguity validation

Reading

PyBUF_SIMPLE vs PyBUF_WRITABLE

PyBUF_SIMPLE requests a contiguous, read-only view. PyBUF_WRITABLE additionally requires that the underlying buffer permits writes. Requesting WRITABLE on a bytes object raises BufferError.

Multi-dimensional strides

A memoryview can have ndim > 1. The strides array specifies the byte offset between consecutive elements along each axis. C-contiguous arrays have strides [shape[1]*itemsize, itemsize] for 2D; Fortran-contiguous has them reversed.

// CPython: Objects/memoryobject.c:1050 mem_get_item_pointer
static char *
mem_get_item_pointer(Py_buffer *view, Py_ssize_t *indices)
{
    char *ptr = (char *)view->buf;
    for (int i = 0; i < view->ndim; i++)
        ptr += indices[i] * view->strides[i];
    return ptr;
}

cast()

memory.cast(format) re-interprets the buffer's bytes as a different item type without copying. The cast is only valid between formats of equal total byte size, and only for 1D or fully-contiguous buffers.

# CPython: Objects/memoryobject.c:870 memory_cast
# The new view shares the same buf pointer; only format/itemsize/shape change.

tolist()

tolist() unpacks the memoryview into nested Python lists by recursively applying PyObject_CallMethodObjArgs on each element, converting from the native format struct code to the appropriate Python type.

gopy notes

The first objects_memoryobject_c annotation covers construction and the buffer protocol. This annotation covers the higher-level slice/cast/tolist API. Both belong in the objects/ layer. The cast operation is a thin wrapper over buffer pointer arithmetic with no data movement.