Inadequate C-API to Python 3 I/O objects

[pv]

BPO	9838
Nosy	@pitrou, @tiran, @pv

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2013-07-08.15:40:30.543>
created_at = <Date 2010-09-12.13:23:45.418>
labels = ['extension-modules', 'type-feature']
title = 'Inadequate C-API to Python 3 I/O objects'
updated_at = <Date 2013-07-08.15:40:30.541>
user = 'https://github.com/pv'

bugs.python.org fields:

activity = <Date 2013-07-08.15:40:30.541>
actor = 'christian.heimes'
assignee = 'none'
closed = True
closed_date = <Date 2013-07-08.15:40:30.543>
closer = 'christian.heimes'
components = ['Extension Modules']
creation = <Date 2010-09-12.13:23:45.418>
creator = 'pv'
dependencies = []
files = []
hgrepos = []
issue_num = 9838
keywords = []
message_count = 3.0
messages = ['116188', '116192', '192662']
nosy_count = 3.0
nosy_names = ['pitrou', 'christian.heimes', 'pv']
pr_nums = []
priority = 'normal'
resolution = 'postponed'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue9838'
versions = ['Python 3.2']

[pv]

The C-API exposed by the io module on Python 3.1/3.2 is very limited, and makes interfacing with Python file objects in extension modules difficult.

In more detail:

1. Because the Python layer has buffering etc., the file handle returned by PyObject_AsFileDescriptor is not usable as-is. It requires flush and seek before use, every time there is a chance that the file object has been accessed on the Python side.

2. There are no C-API functions such as the minimal set of PyFile_Write(buf, length), PyFile_Read(buf, length), PyFile_Seek(pos, whence), PyFile_Tell().

Instead, every call must go through PyObject_CallMethod, and the file objects only handle PyBytes and PyByteArray which are cumbersome and inefficient to use in extension modules.

[pitrou]

Instead, every call must go through PyObject_CallMethod, and the file
objects only handle PyBytes and PyByteArray which are cumbersome
and inefficient to use in extension modules.

Because of the generic nature of the 3.x I/O stack, even shortcuts such as the proposed PyFile_Write will still have to use PyObject_CallMethod(obj, "write", ...) under the hood.

As for the types handled by file objects, you should be able to use a PyMemoryViewObject, which allows you to create a memory buffer without copying it (if that's what you're after).
You can also pass your own objects provided they support the new buffer API: http://docs.python.org/dev/c-api/buffer.html#bufferobjects
(I agree this documentation is not very well written, though)

So, bottom line, we could create a set of PyFile_* wrappers (or, rather, PyStream_*), but they wouldn't be much more efficient that what you can write by hand. Do you still think it's worth it? If so, I think you should float the idea on python-dev (the mailing-list).

[tiran]

I agree with Antoine. If you are still interested to improve the situation then please start a discussion on the python-ideas mailing list.