Buffer Protocol

Certain objects available in Python wrap access to an underlying memory array orbuffer.Such objects include the built-inbytesand bytearray,and some extension types likearray.array. Third-party libraries may define their own types for special purposes, such as image processing or numeric analysis.

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 thebuffer protocol.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 sectionBuffer Object Structures;

• 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 asbytesandbytearrayexpose their underlying buffer in byte-oriented form. Other forms are possible; for example, the elements exposed by anarray.arraycan be multi-byte values.

An example consumer of the buffer interface is thewrite() method of file objects: any object that can export a series of bytes through the buffer interface can be written to a file. Whilewrite()only needs read-only access to the internal contents of the object passed to it, other methods such asreadinto()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.

There are two ways for a consumer of the buffer interface to acquire a buffer over a target object:

• callPyObject_GetBuffer()with the right parameters;

• callPyArg_ParseTuple()(or one of its siblings) with one of the y*,w*ors*format codes.

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.

Buffer structure

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 notPyObjectpointers but rather simple C structures. This allows them to be created and copied very simply. When a generic wrapper around a buffer is needed, amemoryviewobject can be created.

For short instructions how to write an exporting object, see Buffer Object Structures.For obtaining a buffer, seePyObject_GetBuffer().

typePy_buffer

Part of theStable ABI(including all members) since version 3.11.

void*buf

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 negativestrides the value may point to the end of the memory block.

Forcontiguousarrays, the value points to the beginning of the memory block.

PyObject*obj

A new reference to the exporting object. The reference is owned by the consumer and automatically released (i.e. reference count decremented) and set toNULLby PyBuffer_Release().The field is the equivalent of the return value of any standard C-API function.

As a special case, fortemporarybuffers that are wrapped by PyMemoryView_FromBuffer()orPyBuffer_FillInfo() this field isNULL.In general, exporting objects MUST NOT use this scheme.

Py_ssize_tlen

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.

Accessing((char*)buf)[0]upto((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 bePyBUF_SIMPLEorPyBUF_WRITABLE.

intreadonly

An indicator of whether the buffer is read-only. This field is controlled by thePyBUF_WRITABLEflag.

Py_ssize_titemsize

Item size in bytes of a single element. Same as the value ofstruct.calcsize() called on non-NULLformatvalues.

Important exception: If a consumer requests a buffer without the PyBUF_FORMATflag,formatwill be set toNULL,butitemsizestill has the value for the original format.

Ifshapeis present, the equality product(shape)*itemsize==lenstill holds and the consumer can useitemsizeto navigate the buffer.

IfshapeisNULLas a result of aPyBUF_SIMPLE or aPyBUF_WRITABLErequest, the consumer must disregard itemsizeand assumeitemsize==1.

char*format

ANULLterminated string instructmodule style syntax describing the contents of a single item. If this isNULL,"B"(unsigned bytes) is assumed.

This field is controlled by thePyBUF_FORMATflag.

intndim

The number of dimensions the memory represents as an n-dimensional array. If it is0,bufpoints to a single item representing a scalar. In this case,shape,strides andsuboffsetsMUST beNULL. The maximum number of dimensions is given byPyBUF_MAX_NDIM.

Py_ssize_t*shape

An array ofPy_ssize_tof lengthndim indicating the shape of the memory as an n-dimensional array. Note that shape[0]*...*shape[ndim-1]*itemsizeMUST be equal to len.

Shape values are restricted toshape[n]>=0.The case shape[n]==0requires special attention. Seecomplex arrays for further information.

The shape array is read-only for the consumer.

Py_ssize_t*strides

An array ofPy_ssize_tof lengthndim 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.Seecomplex arraysfor further information.

The strides array is read-only for the consumer.

Py_ssize_t*suboffsets

An array ofPy_ssize_tof lengthndim. Ifsuboffsets[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).

If all suboffsets are negative (i.e. no de-referencing is needed), then this field must beNULL(the default value).

This type of array representation is used by the Python Imaging Library (PIL). Seecomplex arraysfor further information how to access elements of such an array.

The suboffsets array is read-only for the consumer.

void*internal

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:

PyBUF_MAX_NDIM

The maximum number of dimensions the memory represents. Exporters MUST respect this limit, consumers of multi-dimensional buffers SHOULD be able to handle up toPyBUF_MAX_NDIMdimensions. Currently set to 64.

Buffer request types

Buffers are usually obtained by sending a buffer request to an exporting object viaPyObject_GetBuffer().Since the complexity of the logical structure of the memory can vary drastically, the consumer uses theflags argument to specify the exact buffer type it can handle.

AllPy_bufferfields are unambiguously defined by the request type.

request-independent fields

The following fields are not influenced byflagsand must always be filled in with the correct values:obj,buf, len,itemsize,ndim.

readonly, format

PyBUF_WRITABLE

Controls thereadonlyfield. 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 theformatfield. If set, this field MUST be filled in correctly. Otherwise, this field MUST beNULL.

PyBUF_WRITABLEcan be |’d to any of the flags in the next section. SincePyBUF_SIMPLEis defined as 0,PyBUF_WRITABLE can be used as a stand-alone flag to request a simple writable buffer.

PyBUF_FORMATcan be |’d to any of the flags exceptPyBUF_SIMPLE. The latter already implies formatB(unsigned bytes).

shape, strides, suboffsets

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	shape	strides	suboffsets
PyBUF_INDIRECT	yes	yes	if needed
PyBUF_STRIDES	yes	yes	NULL
PyBUF_ND	yes	NULL	NULL
PyBUF_SIMPLE	NULL	NULL	NULL

contiguity requests

C or Fortrancontiguitycan be explicitly requested, with and without stride information. Without stride information, the buffer must be C-contiguous.

Request	shape	strides	suboffsets	contig
PyBUF_C_CONTIGUOUS	yes	yes	NULL	C
PyBUF_F_CONTIGUOUS	yes	yes	NULL	F
PyBUF_ANY_CONTIGUOUS	yes	yes	NULL	C or F
PyBUF_ND	yes	NULL	NULL	C

compound requests

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 tableUstands for undefined contiguity. The consumer would have to callPyBuffer_IsContiguous()to determine contiguity.

Request	shape	strides	suboffsets	contig	readonly	format
PyBUF_FULL	yes	yes	if needed	U	0	yes
PyBUF_FULL_RO	yes	yes	if needed	U	1 or 0	yes
PyBUF_RECORDS	yes	yes	NULL	U	0	yes
PyBUF_RECORDS_RO	yes	yes	NULL	U	1 or 0	yes
PyBUF_STRIDED	yes	yes	NULL	U	0	NULL
PyBUF_STRIDED_RO	yes	yes	NULL	U	1 or 0	NULL
PyBUF_CONTIG	yes	NULL	NULL	C	0	NULL
PyBUF_CONTIG_RO	yes	NULL	NULL	C	1 or 0	NULL

Complex arrays

NumPy-style: shape and strides

The logical structure of NumPy-style arrays is defined byitemsize, ndim,shapeandstrides.

Ifndim==0,the memory location pointed to bybufis interpreted as a scalar of sizeitemsize.In that case, bothshapeandstridesareNULL.

IfstridesisNULL,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,bufcan point to any location within the actual memory block. An exporter can check the validity of a buffer with this function:

defverify_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
"""
ifoffset%itemsize:
returnFalse
ifoffset<0oroffset+itemsize>memlen:
returnFalse
ifany(v%itemsizeforvinstrides):
returnFalse

ifndim<=0:
returnndim==0andnotshapeandnotstrides
if0inshape:
returnTrue

imin=sum(strides[j]*(shape[j]-1)forjinrange(ndim)
ifstrides[j]<=0)
imax=sum(strides[j]*(shape[j]-1)forjinrange(ndim)
ifstrides[j]>0)

return0<=offset+iminandoffset+imax+itemsize<=memlen

PIL-style: shape, strides and suboffsets

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-arraycharv[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 ofbuf,pointing to twocharx[2][3]arrays that can be located anywhere in memory.

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-NULLstrides and suboffsets:

void*get_item_pointer(intndim,void*buf,Py_ssize_t*strides,
Py_ssize_t*suboffsets,Py_ssize_t*indices){
char*pointer=(char*)buf;
inti;
for(i=0;i<ndim;i++){
pointer+=strides[i]*indices[i];
if(suboffsets[i]>=0){
pointer=*((char**)pointer)+suboffsets[i];
}
}
return(void*)pointer;
}

Buffer-related functions

intPyObject_CheckBuffer(PyObject*obj)

Part of theStable ABIsince version 3.11.

Return1ifobjsupports the buffer interface otherwise0.When1is returned, it doesn’t guarantee thatPyObject_GetBuffer()will succeed. This function always succeeds.

intPyObject_GetBuffer(PyObject*exporter,Py_buffer*view,intflags)

Part of theStable ABIsince version 3.11.

Send a request toexporterto fill inviewas specified byflags. If the exporter cannot provide a buffer of the exact type, it MUST raise BufferError,setview->objtoNULLand return-1.

On success, fill inview,setview->objto a new reference toexporterand return 0. In the case of chained buffer providers that redirect requests to a single object,view->objMAY refer to this object instead ofexporter(SeeBuffer Object Structures).

Successful calls toPyObject_GetBuffer()must be paired with calls toPyBuffer_Release(),similar tomalloc()andfree(). Thus, after the consumer is done with the buffer,PyBuffer_Release() must be called exactly once.

voidPyBuffer_Release(Py_buffer*view)

Part of theStable ABIsince version 3.11.

Release the bufferviewand release thestrong reference (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().

Py_ssize_tPyBuffer_SizeFromFormat(constchar*format)

Part of theStable ABIsince version 3.11.

Return the implieditemsizefromformat. On error, raise an exception and return -1.

Added in version 3.9.

intPyBuffer_IsContiguous(constPy_buffer*view,charorder)

Part of theStable ABIsince version 3.11.

Return1if the memory defined by theviewis C-style (orderis 'C') or Fortran-style (orderis'F')contiguousor either one (orderis'A'). Return0otherwise. This function always succeeds.

void*PyBuffer_GetPointer(constPy_buffer*view,constPy_ssize_t*indices)

Part of theStable ABIsince version 3.11.

Get the memory area pointed to by theindicesinside the givenview. indicesmust point to an array ofview->ndimindices.

intPyBuffer_FromContiguous(constPy_buffer*view,constvoid*buf,Py_ssize_tlen,charfort)

Part of theStable ABIsince version 3.11.

Copy contiguouslenbytes frombuftoview. fortcan be'C'or'F'(for C-style or Fortran-style ordering). 0is returned on success,-1on error.

intPyBuffer_ToContiguous(void*buf,constPy_buffer*src,Py_ssize_tlen,charorder)

Part of theStable ABIsince version 3.11.

Copylenbytes fromsrcto its contiguous representation inbuf. ordercan be'C'or'F'or'A'(for C-style or Fortran-style ordering or either one).0is returned on success,-1on error.

This function fails iflen!=src->len.

intPyObject_CopyData(PyObject*dest,PyObject*src)

Part of theStable ABIsince version 3.11.

Copy data fromsrctodestbuffer. Can convert between C-style and or Fortran-style buffers.

0is returned on success,-1on error.

voidPyBuffer_FillContiguousStrides(intndims,Py_ssize_t*shape,Py_ssize_t*strides,intitemsize,charorder)

Part of theStable ABIsince version 3.11.

Fill thestridesarray with byte-strides of acontiguous(C-style if orderis'C'or Fortran-style iforderis'F') array of the given shape with the given number of bytes per element.

intPyBuffer_FillInfo(Py_buffer*view,PyObject*exporter,void*buf,Py_ssize_tlen,intreadonly,intflags)

Part of theStable ABIsince version 3.11.

Handle buffer requests for an exporter that wants to exposebufof sizelen with writability set according toreadonly.bufis interpreted as a sequence of unsigned bytes.

Theflagsargument indicates the request type. This function always fills in viewas specified by flags, unlessbufhas been designated as read-only andPyBUF_WRITABLEis set inflags.

On success, setview->objto a new reference toexporterand return 0. Otherwise, raiseBufferError,set view->objtoNULLand return-1;

If this function is used as part of agetbufferproc, exporterMUST be set to the exporting object andflagsmust be passed unmodified. Otherwise,exporterMUST beNULL.