[Python-Dev] Py_SIZE vs PyXXX_GET_SIZE (original) (raw)

Brett Cannon brett at python.org
Mon Mar 20 13:04:52 EDT 2017

On Mon, 20 Mar 2017 at 04:28 Serhiy Storchaka <storchaka at gmail.com> wrote:

What is the preferable way of getting the size of tuple, list, bytes, bytearray: PySIZE or PyTupleGETSIZE, PyListGETSIZE, PyBytesGETSIZE, PyByteArrayGETSIZE? Are macros for concrete types more preferable or they are outdated?

On one hand concrete type macros are longer than PySIZE, and since concrete type macros are defined not for all PyVarObject types we need to use PySIZE for them in any case (for example for PyLongObject and PyTypeObject). On other hand we can add asserts for checking that concrete type macros are used with correct types. When I wrote a patch that replaces PySIZE with concrete type macros I found two cases of misusing PySIZE with dict object: one in json.c (already fixed in 3023ebb43f7607584c3e123aff56e867cb04a418) and other in dictobject.c (still not fixed). If prefer using concrete type macros this would unlikely happen.

Personally I have always used the concrete versions when available when it doesn't forcibly constrain the input to the function. In other words I wouldn't force a function to only take a list so I could use PyList_GET_SIZE, but if I'm constructing some internal list object or a function is defined to return a list already then I would just use the concrete versions. But I also wouldn't worry about changing uses of Py_SIZE unless I was already changing the surrounding code.

I guess we could clarify this in PEP 7 if it doesn't say when to care about this and once we reach consensus on what we all prefer. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20170320/313242f1/attachment.html>

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