Certain objects available in Python wrap access to an underlying memory array or buffer . Such objects include the built-in bytes and bytearray , and some extension types like array.array . Third-party libraries may define their own types for special purposes, such as image processing or numeric analysis.
bytes
bytearray
array.array
While each of these types have their own semantics, they share the common characteristic of being backed by a possibly large memory buffer. It is then desirable, in some situations, to access that buffer directly and without intermediate copying.
Python provides such a facility at the C level in the form of the 缓冲协议 . This protocol has two sides:
on the producer side, a type can export a “buffer interface” which allows objects of that type to expose information about their underlying buffer. This interface is described in the section 缓冲对象结构 ;
on the consumer side, several means are available to obtain a pointer to the raw underlying data of an object (for example a method parameter).
Simple objects such as bytes and bytearray expose their underlying buffer in byte-oriented form. Other forms are possible; for example, the elements exposed by an array.array can be multi-byte values.
An example consumer of the buffer interface is the write() method of file objects: any object that can export a series of bytes through the buffer interface can be written to a file. While write() only needs read-only access to the internal contents of the object passed to it, other methods such as readinto() need write access to the contents of their argument. The buffer interface allows objects to selectively allow or reject exporting of read-write and read-only buffers.
write()
readinto()
There are two ways for a consumer of the buffer interface to acquire a buffer over a target object:
call PyObject_GetBuffer() with the right parameters;
PyObject_GetBuffer()
call PyArg_ParseTuple() (or one of its siblings) with one of the y* , w* or s* format codes .
PyArg_ParseTuple()
y*
w*
s*
In both cases, PyBuffer_Release() must be called when the buffer isn’t needed anymore. Failure to do so could lead to various issues such as resource leaks.
PyBuffer_Release()
Buffer structures (or simply “buffers”) are useful as a way to expose the binary data from another object 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.
Contrary to most data types exposed by the Python interpreter, buffers are not PyObject pointers but rather simple C structures. This allows them to be created and copied very simply. When a generic wrapper around a buffer is needed, a memoryview object can be created.
PyObject
For short instructions how to write an exporting object, see 缓冲对象结构 . For obtaining a buffer, see PyObject_GetBuffer() .
A pointer to the start of the logical structure described by the buffer fields. This can be any location within the underlying physical memory block of the exporter. For example, with negative strides the value may point to the end of the memory block.
strides
For contiguous arrays, the value points to the beginning of the memory block.
A new reference to the exporting object. The reference is owned by the consumer and automatically released (i.e. reference count decremented) and set to NULL by PyBuffer_Release() . The field is the equivalent of the return value of any standard C-API function.
NULL
As a special case, for temporary buffers that are wrapped by PyMemoryView_FromBuffer() or PyBuffer_FillInfo() this field is NULL . In general, exporting objects MUST NOT use this scheme.
PyMemoryView_FromBuffer()
PyBuffer_FillInfo()
product(shape) * itemsize . For contiguous arrays, this is the length of the underlying memory block. For non-contiguous arrays, it is the length that the logical structure would have if it were copied to a contiguous representation.
product(shape) * itemsize
访问 ((char *)buf)[0] up to ((char *)buf)[len-1] is only valid if the buffer has been obtained by a request that guarantees contiguity. In most cases such a request will be PyBUF_SIMPLE or PyBUF_WRITABLE .
((char *)buf)[0] up to ((char *)buf)[len-1]
PyBUF_SIMPLE
PyBUF_WRITABLE
An indicator of whether the buffer is read-only. This field is controlled by the PyBUF_WRITABLE 标志。
Item size in bytes of a single element. Same as the value of struct.calcsize() called on non- NULL format 值。
struct.calcsize()
format
Important exception: If a consumer requests a buffer without the PyBUF_FORMAT flag, format 会被设为 NULL ，但 itemsize still has the value for the original format.
PyBUF_FORMAT
itemsize
若 shape is present, the equality product(shape) * itemsize == len still holds and the consumer can use itemsize to navigate the buffer.
shape
product(shape) * itemsize == len
若 shape is NULL as a result of a PyBUF_SIMPLE 或 PyBUF_WRITABLE request, the consumer must disregard itemsize and assume itemsize == 1 .
itemsize == 1
A NULL terminated string in struct module style syntax describing the contents of a single item. If this is NULL , "B" (unsigned bytes) is assumed.
struct
"B"
This field is controlled by the PyBUF_FORMAT 标志。
The number of dimensions the memory represents as an n-dimensional array. If it is 0 , buf points to a single item representing a scalar. In this case, shape , strides and suboffsets MUST be NULL . The maximum number of dimensions is given by PyBUF_MAX_NDIM .
0
buf
suboffsets
PyBUF_MAX_NDIM
An array of Py_ssize_t 的长度 ndim indicating the shape of the memory as an n-dimensional array. Note that shape[0] * ... * shape[ndim-1] * itemsize MUST be equal to len .
Py_ssize_t
ndim
shape[0] * ... * shape[ndim-1] * itemsize
len
Shape values are restricted to shape[n] >= 0 . The case shape[n] == 0 requires special attention. See complex arrays 了解进一步信息。
shape[n] >= 0
shape[n] == 0
The shape array is read-only for the consumer.
An array of Py_ssize_t 的长度 ndim giving the number of bytes to skip to get to a new element in each dimension.
Stride values can be any integer. For regular arrays, strides are usually positive, but a consumer MUST be able to handle the case strides[n] <= 0 。见 complex arrays 了解进一步信息。
strides[n] <= 0
The strides array is read-only for the consumer.
An array of Py_ssize_t 的长度 ndim 。若 suboffsets[n] >= 0 , the values stored along the nth dimension are pointers and the suboffset value dictates how many bytes to add to each pointer after de-referencing. A suboffset value that is negative indicates that no de-referencing should occur (striding in a contiguous memory block).
suboffsets[n] >= 0
If all suboffsets are negative (i.e. no de-referencing is needed), then this field must be NULL (the default value).
This type of array representation is used by the Python Imaging Library (PIL). See complex arrays for further information how to access elements of such an array.
The suboffsets array is read-only for the consumer.
This is for use internally by the exporting object. For example, this might be re-cast as an integer by the exporter and used to store flags about whether or not the shape, strides, and suboffsets arrays must be freed when the buffer is released. The consumer MUST NOT alter this value.
Constants:
The maximum number of dimensions the memory represents. Exporters MUST respect this limit, consumers of multi-dimensional buffers SHOULD be able to handle up to PyBUF_MAX_NDIM dimensions. Currently set to 64.
Buffers are usually obtained by sending a buffer request to an exporting object via PyObject_GetBuffer() . Since the complexity of the logical structure of the memory can vary drastically, the consumer uses the flags argument to specify the exact buffer type it can handle.
所有 Py_buffer fields are unambiguously defined by the request type.
Py_buffer
The following fields are not influenced by flags and must always be filled in with the correct values: obj , buf , len , itemsize , ndim .
obj
PyBUF_WRITABLE ¶ Controls the readonly field. If set, the exporter MUST provide a writable buffer or else report failure. Otherwise, the exporter MAY provide either a read-only or writable buffer, but the choice MUST be consistent for all consumers. PyBUF_FORMAT ¶ Controls the format field. If set, this field MUST be filled in correctly. Otherwise, this field MUST be NULL .
Controls the readonly field. If set, the exporter MUST provide a writable buffer or else report failure. Otherwise, the exporter MAY provide either a read-only or writable buffer, but the choice MUST be consistent for all consumers.
readonly
Controls the format field. If set, this field MUST be filled in correctly. Otherwise, this field MUST be NULL .
PyBUF_WRITABLE can be |’d to any of the flags in the next section. Since PyBUF_SIMPLE is defined as 0, PyBUF_WRITABLE can be used as a stand-alone flag to request a simple writable buffer.
PyBUF_FORMAT can be |’d to any of the flags except PyBUF_SIMPLE . The latter already implies format B (unsigned bytes).
B
The flags that control the logical structure of the memory are listed in decreasing order of complexity. Note that each flag contains all bits of the flags below it.
Request
C or Fortran contiguity can be explicitly requested, with and without stride information. Without stride information, the buffer must be C-contiguous.
contig
PyBUF_ND
All possible requests are fully defined by some combination of the flags in the previous section. For convenience, the buffer protocol provides frequently used combinations as single flags.
In the following table U stands for undefined contiguity. The consumer would have to call PyBuffer_IsContiguous() to determine contiguity.
PyBuffer_IsContiguous()
The logical structure of NumPy-style arrays is defined by itemsize , ndim , shape and strides .
若 ndim == 0 , the memory location pointed to by buf is interpreted as a scalar of size itemsize . In that case, both shape and strides are NULL .
ndim == 0
若 strides is NULL , the array is interpreted as a standard n-dimensional C-array. Otherwise, the consumer must access an n-dimensional array as follows:
ptr = (char *)buf + indices[0] * strides[0] + ... + indices[n-1] * strides[n-1]; item = *((typeof(item) *)ptr);
As noted above, buf can point to any location within the actual memory block. An exporter can check the validity of a buffer with this function:
def verify_structure(memlen, itemsize, ndim, shape, strides, offset): """Verify that the parameters represent a valid array within the bounds of the allocated memory: char *mem: start of the physical memory block memlen: length of the physical memory block offset: (char *)buf - mem """ if offset % itemsize: return False if offset < 0 or offset+itemsize > memlen: return False if any(v % itemsize for v in strides): return False if ndim <= 0: return ndim == 0 and not shape and not strides if 0 in shape: return True imin = sum(strides[j]*(shape[j]-1) for j in range(ndim) if strides[j] <= 0) imax = sum(strides[j]*(shape[j]-1) for j in range(ndim) if strides[j] > 0) return 0 <= offset+imin and offset+imax+itemsize <= memlen
In addition to the regular items, PIL-style arrays can contain pointers that must be followed in order to get to the next element in a dimension. For example, the regular three-dimensional C-array char v[2][2][3] can also be viewed as an array of 2 pointers to 2 two-dimensional arrays: char (*v[2])[2][3] . In suboffsets representation, those two pointers can be embedded at the start of buf , pointing to two char x[2][3] arrays that can be located anywhere in memory.
char v[2][2][3]
char (*v[2])[2][3]
char x[2][3]
Here is a function that returns a pointer to the element in an N-D array pointed to by an N-dimensional 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; }
返回 1 if obj supports the buffer interface otherwise 0 。当 1 is returned, it doesn’t guarantee that PyObject_GetBuffer() will succeed. This function always succeeds.
1
Send a request to exporter to fill in view as specified by flags . If the exporter cannot provide a buffer of the exact type, it MUST raise BufferError , set view->obj to NULL 并返回 -1 .
BufferError
view->obj
-1
On success, fill in view , set view->obj to a new reference to exporter and return 0. In the case of chained buffer providers that redirect requests to a single object, view->obj MAY refer to this object instead of exporter (见 缓冲对象结构 ).
Successful calls to PyObject_GetBuffer() must be paired with calls to PyBuffer_Release() ，类似 malloc() and free() . Thus, after the consumer is done with the buffer, PyBuffer_Release() must be called exactly once.
malloc()
free()
Release the buffer view and release the 强引用 (i.e. decrement the reference count) to the view’s supporting object, view->obj . This function MUST be called when the buffer is no longer being used, otherwise reference leaks may occur.
It is an error to call this function on a buffer that was not obtained via PyObject_GetBuffer() .
Return the implied itemsize from format . On error, raise an exception and return -1.
Added in version 3.9.
返回 1 if the memory defined by the view is C-style ( order is 'C' ) or Fortran-style ( order is 'F' ) contiguous or either one ( order is 'A' ). Return 0 otherwise. This function always succeeds.
'C'
'F'
'A'
Get the memory area pointed to by the indices inside the given view . indices must point to an array of view->ndim indices.
view->ndim
Copy contiguous len 字节来自 buf to view . fort 可以是 'C' or 'F' (for C-style or Fortran-style ordering). 0 is returned on success, -1 当出错时。
拷贝 len 字节来自 src to its contiguous representation in buf . order 可以是 'C' or 'F' or 'A' (for C-style or Fortran-style ordering or either one). 0 is returned on success, -1 当出错时。
This function fails if len != src->len .
Copy data from src to dest buffer. Can convert between C-style and or Fortran-style buffers.
0 is returned on success, -1 当出错时。
Fill the strides array with byte-strides of a contiguous (C-style if order is 'C' or Fortran-style if order is 'F' ) array of the given shape with the given number of bytes per element.
Handle buffer requests for an exporter that wants to expose buf 的大小 len with writability set according to readonly . buf is interpreted as a sequence of unsigned bytes.
The flags argument indicates the request type. This function always fills in view as specified by flags, unless buf has been designated as read-only and PyBUF_WRITABLE 有设置在 flags .
On success, set view->obj to a new reference to exporter and return 0. Otherwise, raise BufferError , set view->obj to NULL 并返回 -1 ;
If this function is used as part of a getbufferproc , exporter MUST be set to the exporting object and flags must be passed unmodified. Otherwise, exporter MUST be NULL .
迭代器协议
旧缓冲协议
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