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 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 mechanism for Python code to inspect whether a type supports the C-level buffer protocol. 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. Rationale ========= Current options --------------- There are two current 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, these options cannot be queried directly on the type object. The only way to figure out whether an object supports a writable buffer is to actually ask for the buffer. For some types, such as ``memoryview``, whether the buffer is writable depends on the instance: some instances are read-only and others are not. As such, we propose to expose only whether a type implements the buffer protocol at all, not whether it supports more specific options such as writable buffers. Specification ============= types.Buffer ------------ A new class, ``types.Buffer``, will be added. It cannot be instantiated or subclassed at runtime, but supports the ``__instancecheck__`` and ``__subclasscheck__`` hooks. In CPython, these will check for the presence of the ``bf_getbuffer`` slot in the type object: .. code-block:: pycon >>> from types import Buffer >>> isinstance(b"xy", Buffer) True >>> issubclass(bytes, Buffer) True >>> issubclass(memoryview, Buffer) True >>> isinstance("xy", Buffer) False >>> issubclass(str, Buffer) False The new class can also be used 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 Usage in stub files ------------------- For static typing purposes, types defined in C extensions usually require stub files, as :pep:`described in PEP 484 <484#stub-files>`. In stub files, ``types.Buffer`` may be used as a base class to indicate that a class implements the buffer protocol. For example, ``memoryview`` may be declared as follows in a stub: .. code-block:: python class memoryview(types.Buffer, Sized, Sequence[int]): ... The ``types.Buffer`` class does not require any special treatment by type checkers. 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 accessible only in C, ``types.Buffer`` cannot be implemented in a pure-Python package like ``typing_extensions``. As a temporary workaround, a ``typing_extensions.Buffer`` `abstract base class `__ will be provided for Python versions that do not have ``types.Buffer`` available. For the benefit of static type checkers, ``typing_extensions.Buffer`` can be used as a base class in stubs to mark types as supporting the buffer protocol. For runtime uses, the ``ABC.register`` API can be used to register buffer classes with ``typing_extensions.Buffer``. When ``types.Buffer`` is available, ``typing_extensions`` should simply re-export it. Thus, users who register their buffer class manually with ``typing_extensions.Buffer.register`` should use a guard to make sure their code continues to work once ``types.Buffer`` is in the standard library. 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 a new class, 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 ``types.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 ``types.Buffer`` instead. Reference Implementation ======================== An implementation of ``types.Buffer`` is `available `__ in the author's fork. Rejected Ideas ============== Buffer ABC ---------- An `earlier proposal `__ suggested adding a ``collections.abc.Buffer`` `abstract base class `__ to represent buffer objects. This idea stalled because an ABC with no methods does not fit well into the ``collections.abc`` module. Furthermore, it required manual registration of buffer classes, including those in the standard library. This PEP's approach of using the ``__instancecheck__`` hook is more natural and does not require explicit registration. Nevertheless, the ABC proposal has the advantage that it does not require C changes. This PEP proposes to adopt a version of it in the third-party ``typing_extensions`` package for the benefit of users of older Python versions. 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 =========== Read-only and writable buffers ------------------------------ To avoid making changes to the buffer protocol itself, this PEP currently does not provide a way to distinguish between read-only and writable buffers. That's unfortunate, because some APIs require a writable buffer, and one of the most common buffer types (``bytes``) is always read-only. Should we add a new mechanism in C to declare that a type implementing the buffer protocol is potentially writable? Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.