PEP: 688 Title: Making the buffer protocol accessible in Python Author: Jelle Zijlstra Discussions-To: https://discuss.python.org/t/15265 Status: Draft Type: Standards Track Topic: Typing Content-Type: text/x-rst Created: 23-Apr-2022 Python-Version: 3.12 Post-History: `23-Apr-2022 `__, `25-Apr-2022 `__ Abstract ======== This PEP proposes a Python-level API for the buffer protocol, which is currently accessible only to C code. This allows type checkers to evaluate whether objects implement the protocol. Motivation ========== The CPython C API provides a versatile mechanism for accessing the underlying memory of an object—the `buffer protocol `__ introduced in :pep:`3118`. Functions that accept binary data are usually written to handle any object implementing the buffer protocol. For example, at the time of writing, there are around 130 functions in CPython using the Argument Clinic ``Py_buffer`` type, which accepts the buffer protocol. Currently, there is no way for Python code to inspect whether an object supports the buffer protocol. Moreover, the static type system does not provide a type annotation to represent the protocol. This is a `common problem `__ when writing type annotations for code that accepts generic buffers. Similarly, it is impossible for a class written in Python to support the buffer protocol. A buffer class in Python would give users the ability to easily wrap a C buffer object, or to test the behavior of an API that consumes the buffer protocol. Granted, this is not a particularly common need. However, there has been a `CPython feature request `__ for supporting buffer classes written in Python that has been open since 2012. Rationale ========= Current options --------------- There are two known workarounds for annotating buffer types in the type system, but neither is adequate. First, the `current workaround `__ for buffer types in typeshed is a type alias that lists well-known buffer types in the standard library, such as ``bytes``, ``bytearray``, ``memoryview``, and ``array.array``. This approach works for the standard library, but it does not extend to third-party buffer types. Second, the `documentation `__ for ``typing.ByteString`` currently states: This type represents the types ``bytes``, ``bytearray``, and ``memoryview`` of byte sequences. As a shorthand for this type, ``bytes`` can be used to annotate arguments of any of the types mentioned above. Although this sentence has been in the documentation `since 2015 `__, the use of ``bytes`` to include these other types is not specified in any of the typing PEPs. Furthermore, this mechanism has a number of problems. It does not include all possible buffer types, and it makes the ``bytes`` type ambiguous in type annotations. After all, there are many operations that are valid on ``bytes`` objects, but not on ``memoryview`` objects, and it is perfectly possible for a function to accept ``bytes`` but not ``memoryview`` objects. A mypy user `reports `__ that this shortcut has caused significant problems for the ``psycopg`` project. Kinds of buffers ---------------- The C buffer protocol supports `many options `__, affecting strides, contiguity, and support for writing to the buffer. Some of these options would be useful in the type system. For example, typeshed currently provides separate type aliases for writable and read-only buffers. However, in the C buffer protocol, most of these options cannot be queried directly on the type object. The only way to figure out whether an object supports a particular flag is to actually ask for the buffer. For some types, such as ``memoryview``, the supported flags depend on the instance. As a result, it would be difficult to represent support for these flags in the type system. There is one exception: if a buffer is writable, it must define the ``bf_releasebuffer`` slot, so that the buffer can be released when a consumer is done writing to it. For read-only buffers, such as ``bytes``, there is generally no need for this slot. Therefore, the presence of this slot can be used to determine whether a buffer is writable. Specification ============= Python-level buffer protocol ---------------------------- We propose to add two Python-level special methods, ``__buffer__`` and ``__release_buffer__``. Python classes that implement these methods are usable as buffers from C code. Conversely, classes implemented in C that support the buffer protocol acquire synthesized methods accessible from Python code. The ``__buffer__`` method corresponds to the ``bf_getbuffer`` C slot, which is called to create a buffer from a Python object. The Python signature for this method is ``def __buffer__(self, flags: int, /) -> memoryview: ...``. The method must return a ``memoryview`` object. If the method is called from C code, the interpreter extracts the underlying ``Py_buffer`` from the ``memoryview`` and returns it to the C caller. Similarly, if the ``__buffer__`` method is called on an instance of a C class that implements ``bf_getbuffer``, the returned buffer is wrapped in a ``memoryview`` for consumption by Python code. The ``__release_buffer__`` method corresponds to the ``bf_releasebuffer`` C slot, which is called when a caller no longer needs the buffer returned by ``bf_getbuffer``. This is an optional part of the buffer protocol, and read-only buffers can generally omit it. The Python signature for this method is ``def __release_buffer__(self, buffer: memoryview, /) -> None: ...``. The buffer to be released is wrapped in a ``memoryview``. It is also possible to call ``__release_buffer__`` on a C class that implements ``bf_releasebuffer``. inspect.BufferFlags ------------------- To help implementations of ``__buffer__``, we add ``inspect.BufferFlags``, a subclass of ``enum.IntFlag``. This enum contains all flags defined in the C buffer protocol. For example, ``inspect.BufferFlags.SIMPLE`` has the same value as the ``PyBUF_SIMPLE`` constant. collections.abc.Buffer and MutableBuffer ---------------------------------------- We add two new abstract base classes, ``collections.abc.Buffer`` and ``collections.abc.MutableBuffer``. The former requires the ``__buffer__`` method; the latter requires both ``__buffer__`` and ``__release_buffer__``. These classes are intended primarily for use in type annotations: .. code-block:: python def need_buffer(b: Buffer) -> memoryview: return memoryview(b) need_buffer(b"xy") # ok need_buffer("xy") # rejected by static type checkers def need_mutable_buffer(b: MutableBuffer) -> None: view = memoryview(b) view[0] = 42 need_mutable_buffer(bytearray(b"xy")) # ok need_mutable_buffer(b"xy") # rejected by static type checkers They can also be used in ``isinstance`` and ``issubclass`` checks: .. code-block:: pycon >>> from collections.abc import Buffer >>> isinstance(b"xy", Buffer) True >>> issubclass(bytes, Buffer) True >>> issubclass(memoryview, Buffer) True >>> isinstance("xy", Buffer) False >>> issubclass(str, Buffer) False In the typeshed stub files, these classes should be defined as ``Protocol``\ s, following the precedent of other simple ABCs in ``collections.abc`` such as ``collections.abc.Iterable`` or ``collections.abc.Sized``. Example ------- The following is an example of a Python class that implements the buffer protocol: .. code-block:: python class MyBuffer: def __init__(self, data: bytes): self.data = bytearray(data) self.view = memoryview(self.data) self.held = False def __buffer__(self, flags: int) -> memoryview: if flags != inspect.BufferFlags.FULL_RO: raise TypeError("Only BufferFlags.FULL_RO supported") if self.held: raise RuntimeError("Buffer already held") self.held = True return self.view def __release_buffer__(self, view: memoryview) -> None: assert self.view is view self.held = False buffer = MyBuffer(b"capybara") with memoryview(buffer) as view: view[0] = ord("C") with memoryview(buffer) as view: assert view.tobytes() == b"Capybara" Equivalent for older Python versions ------------------------------------ New typing features are usually backported to older Python versions in the `typing_extensions `_ package. Because the buffer protocol is currently accessible only in C, this PEP cannot be fully implemented in a pure-Python package like ``typing_extensions``. As a temporary workaround, two abstract base classes, ``typing_extensions.Buffer`` and ``typing_extensions.MutableBuffer``, will be provided for Python versions that do not have ``collections.abc.Buffer`` available. After this PEP is implemented, inheriting from ``collections.abc.Buffer`` will not be necessary to indicate that an object supports the buffer protocol. However, in older Python versions, it will be necessary to explicitly inherit from ``typing_extensions.Buffer`` to indicate to type checkers that a class supports the buffer protocol, since objects supporting the buffer protocol will not have a ``__buffer__`` method. It is expected that this will happen primarily in stub files, because buffer classes are necessarily implemented in C code, which cannot have types defined inline. For runtime uses, the ``ABC.register`` API can be used to register buffer classes with ``typing_extensions.Buffer``. No special meaning for ``bytes`` -------------------------------- The special case stating that ``bytes`` may be used as a shorthand for other ``ByteString`` types will be removed from the ``typing`` documentation. With ``types.Buffer`` available as an alternative, there will be no good reason to allow ``bytes`` as a shorthand. We suggest that type checkers currently implementing this behavior should deprecate and eventually remove it. Backwards Compatibility ======================= As the runtime changes in this PEP only add new functionality, there are no backwards compatibility concerns. However, the recommendation to remove the special behavior for ``bytes`` in type checkers does have a backwards compatibility impact on their users. An `experiment `__ with mypy shows that several major open source projects that use it for type checking will see new errors if the ``bytes`` promotion is removed. Many of these errors can be fixed by improving the stubs in typeshed, as has already been done for the `builtins `__, `binascii `__, `pickle `__, and `re `__ modules. Overall, the change improves type safety and makes the type system more consistent, so we believe the migration cost is worth it. How to Teach This ================= We will add notes pointing to ``collections.abc.Buffer`` in appropriate places in the documentation, such as `typing.readthedocs.io `__ and the `mypy cheat sheet `__. Type checkers may provide additional pointers in their error messages. For example, when they encounter a buffer object being passed to a function that is annotated to only accept ``bytes``, the error message could include a note suggesting the use of ``collections.abc.Buffer`` instead. Reference Implementation ======================== An implementation of this PEP is `available `__ in the author's fork. Rejected Ideas ============== types.Buffer ------------ An earlier version of this PEP proposed adding a new ``types.Buffer`` type with an ``__instancecheck__`` implemented in C so that ``isinstance()`` checks can be used to check whether a type implements the buffer protocol. This avoids the complexity of exposing the full buffer protocol to Python code, while still allowing the type system to check for the buffer protocol. However, that approach does not compose well with the rest of the type system, because ``types.Buffer`` would be a nominal type, not a structural one. For example, there would be no way to represent "an object that supports both the buffer protocol and ``__len__``". With the current proposal, ``__buffer__`` is like any other special method, so a ``Protocol`` can be defined combining it with another method. More generally, no other part of Python works like the proposed ``types.Buffer``. The current proposal is more consistent with the rest of the language, where C-level slots usually have corresponding Python-level special methods. Keep ``bytearray`` compatible with ``bytes`` -------------------------------------------- It has been suggested to remove the special case where ``memoryview`` is always compatible with ``bytes``, but keep it for ``bytearray``, because the two types have very similar interfaces. However, several standard library functions (e.g., ``re.compile`` and ``socket.getaddrinfo``) accept ``bytes`` but not ``bytearray``. In most codebases, ``bytearray`` is also not a very common type. We prefer to have users spell out accepted types explicitly (or use ``Protocol`` from :pep:`544` if only a specific set of methods is required). Open Issues =========== Where should the ``Buffer`` and ``MutableBuffer`` ABCs live? This PEP proposes putting them in ``collections.abc`` like most other ABCs, but buffers are not "collections" in the original sense of the word. Alternatively, these classes could be put in ``typing``, like ``SupportsInt`` and several other simple protocols. Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.