Python objects implemented in C can export a “buffer interface.” These functions can be used by an object to expose its data in a raw, byte-oriented format. Clients of the object can use the buffer interface to access the object data directly, without needing to copy it first.
Two examples of objects that support the buffer interface are bytes and arrays. The bytes object exposes the character contents in the buffer interface’s byte-oriented form. An array can also expose its contents, but it should be noted that array elements may be multi-byte values.
An example user of the buffer interface is the file object’s write() method. Any object that can export a series of bytes through the buffer interface can be written to a file. There are a number of format codes to PyArg_ParseTuple that operate against an object’s buffer interface, returning data from the target object.
More information on the buffer interface is provided in the section Buffer Object Structures, under the description for PyBufferProcs.
Buffer objects are useful as a way to expose the data from another object’s buffer interface to the Python programmer. They can also be used as a zero-copy slicing mechanism. Using their ability to reference a block of memory, it is possible to expose any data to the Python programmer quite easily. The memory could be a large, constant array in a C extension, it could be a raw block of memory for manipulation before passing to an operating system library, or it could be used to pass around structured data in its native, in-memory format.
An array of Py_ssize_ts the length of ndim. If these suboffset numbers are greater than or equal to 0, then the value stored along the indicated dimension is a pointer and the suboffset value dictates how many bytes to add to the pointer after de-referencing. A suboffset value that it negative indicates that no de-referencing should occur (striding in a contiguous memory block).
Here is a function that returns a pointer to the element in an N-D array pointed to by an N-dimesional index when there are both non-NULL strides and suboffsets:
void *get_item_pointer(int ndim, void *buf, Py_ssize_t *strides,
Py_ssize_t *suboffsets, Py_ssize_t *indices) {
char *pointer = (char*)buf;
int i;
for (i = 0; i < ndim; i++) {
pointer += strides[i] * indices[i];
if (suboffsets[i] >=0 ) {
pointer = *((char**)pointer) + suboffsets[i];
}
}
return (void*)pointer;
}
Export obj into a Py_buffer, view. These arguments must never be NULL. The flags argument is a bit field indicating what kind of buffer the caller is prepared to deal with and therefore what kind of buffer the exporter is allowed to return. The buffer interface allows for complicated memory sharing possibilities, but some caller may not be able to handle all the complexibity but may want to see if the exporter will let them take a simpler view to its memory.
Some exporters may not be able to share memory in every possible way and may need to raise errors to signal to some consumers that something is just not possible. These errors should be a BufferError unless there is another error that is actually causing the problem. The exporter can use flags information to simplify how much of the Py_buffer structure is filled in with non-default values and/or raise an error if the object can’t support a simpler view of its memory.
0 is returned on success and -1 on error.
The following table gives possible values to the flags arguments.
Flag | Description |
---|---|
PyBUF_SIMPLE | This is the default flag state. The returned buffer may or may not have writable memory. The format will be assumed to be unsigned bytes . This is a “stand-alone” flag constant. It never needs to be |‘d to the others. The exporter will raise an error if it cannot provide such a contiguous buffer of bytes. |
PyBUF_WRITABLE | The returned buffer must be writable. If it is not writable, then raise an error. |
PyBUF_STRIDES | This implies PyBUF_ND. The returned buffer must provide strides information (i.e. the strides cannot be NULL). This would be used when the consumer can handle strided, discontiguous arrays. Handling strides automatically assumes you can handle shape. The exporter may raise an error if cannot provide a strided-only representation of the data (i.e. without the suboffsets). |
PyBUF_ND | The returned buffer must provide shape information. The memory will be assumed C-style contiguous (last dimension varies the fastest). The exporter may raise an error if it cannot provide this kind of contiguous buffer. If this is not given then shape will be NULL. |
PyBUF_C_CONTIGUOUS PyBUF_F_CONTIGUOUS PyBUF_ANY_CONTIGUOUS | These flags indicate that the contiguoity returned buffer must be respectively, C-contiguous (last dimension varies the fastest), Fortran contiguous (first dimension varies the fastest) or either one. All of these flags imply PyBUF_STRIDES and guarantee that the strides buffer info structure will be filled in correctly. |
PyBUF_INDIRECT | This implies PyBUF_STRIDES. The returned buffer must have suboffsets information (which can be NULL if no suboffsets are needed). This would be used when the consumer can handle indirect array referencing implied by these suboffsets. |
PyBUF_FORMAT | The returned buffer must have true format information if this flag is provided. This would be used when the consumer is going to be checking for what ‘kind’ of data is actually stored. An exporter should always be able to provide this information if requested. If format is not explicitly requested then the format must be returned as NULL (which means 'B', or unsigned bytes) |
PyBUF_STRIDED | This is equivalent to (PyBUF_STRIDES | PyBUF_WRITABLE). |
PyBUF_STRIDED_RO | This is equivalent to (PyBUF_STRIDES). |
PyBUF_RECORDS | This is equivalent to (PyBUF_STRIDES | PyBUF_FORMAT | PyBUF_WRITABLE). |
PyBUF_RECORDS_RO | This is equivalent to (PyBUF_STRIDES | PyBUF_FORMAT). |
PyBUF_FULL | This is equivalent to (PyBUF_INDIRECT | PyBUF_FORMAT | PyBUF_WRITABLE). |
PyBUF_FULL_RO` | This is equivalent to (PyBUF_INDIRECT | PyBUF_FORMAT). |
PyBUF_CONTIG | This is equivalent to (PyBUF_ND | PyBUF_WRITABLE). |
PyBUF_CONTIG_RO | This is equivalent to (PyBUF_ND). |
A memoryview object is an extended buffer object that could replace the buffer object (but doesn’t have to as that could be kept as a simple 1-d memoryview object). It, unlike Py_buffer, is a Python object (exposed as memoryview in builtins), so it can be used with Python code.