Make tempfiles subclass IOBase

[Dutcho]

BPO	33762
Nosy	@terryjreedy, @asvetlov, @vadmium, @serhiy-storchaka, @wumpus, @Dutcho, @sergii Tkachenko

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

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2019-12-11.20:46:17.479>
created_at = <Date 2018-06-03.20:53:05.205>
labels = ['type-feature', 'library', '3.9']
title = 'Make tempfiles subclass IOBase'
updated_at = <Date 2019-12-11.20:46:17.476>
user = 'https://github.com/Dutcho'

bugs.python.org fields:

activity = <Date 2019-12-11.20:46:17.476>
actor = 'terry.reedy'
assignee = 'none'
closed = True
closed_date = <Date 2019-12-11.20:46:17.479>
closer = 'terry.reedy'
components = ['Library (Lib)']
creation = <Date 2018-06-03.20:53:05.205>
creator = 'Dutcho'
dependencies = []
files = []
hgrepos = []
issue_num = 33762
keywords = []
message_count = 12.0
messages = ['318585', '319090', '319193', '344599', '358211', '358212', '358214', '358218', '358220', '358230', '358253', '358278']
nosy_count = 7.0
nosy_names = ['terry.reedy', 'asvetlov', 'martin.panter', 'serhiy.storchaka', 'wumpus', 'Dutcho', 'sergiitk']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue33762'
versions = ['Python 3.9']

[Dutcho]

I'd expect isinstance(tempfile.TemporaryFile(), io.IOBase) to be True as you can read() from and write() to the temp file.

However, on Python 3.6.5 64 bit on Windows 7 above isinstance() == False and and type(tempfile.TemporaryFile()) == tempfile._TemporaryFileWrapper, which has no super class (other than object).
Whereas, on Python 3.6.1 on iOS 11.4 (Pythonista 3.2) above isinstance() == True and type(tempfile.TemporaryFile()) == io.BufferedRandom, which has io.IOBase as its (indirect) super class.
The difference is likely caused by tempfile line 565 that equals TemporaryFile = NamedTemporaryFile in case of Windows.

This may be somewhat related to bpo-26175, but isn't a duplicate as 26175 is on a temp file's attributes, and this issue is on its type

[terryjreedy]

Martin, I added you because this Tempfile issue is related to bpo-26175.

[vadmium]

I think it is an implementation detail whether the result subclasses IOBase or just implements its API. Why do you want to check the base class, and why IOBase in particular, rather than BufferedIOBase, RawIOBase, or TextIOBase?

[wumpus]

This is breaking aiohttp client file multipart uploads from temporary files. I'd be willing to bet that a lot of libraries do isinstance(foo, io.IOBase) deep in their guts.

[SergiiTkachenko]

Confirming this to be a thing on Python 3.7.5 / OS X 10.15.1.

In [31]: f = tempfile.NamedTemporaryFile()
In [32]: isinstance(f, io.IOBase)
Out[32]: False

[SergiiTkachenko]

Affected as well:
Python 3.8.0 (default, Nov 3 2019, 10:55:54)
[Clang 11.0.0 (clang-1100.0.33.8)] on darwin

[serhiy-storchaka]

Even if make TemporaryFile a subclass of IOBase (I am not sure we should do this) you could only use this in Python 3.9 and newer. I think it is better to fix this issue on the aiohttp side. aiohttp already registers payload types for a bunch of file-like types. You can also register it yourself:

aiohttp.payload.PAYLOAD_REGISTRY.register(aiohttp.payload.IOBasePayload, tempfile.TemporaryFile)

Seems IOBasePayload needs only read() and close() methods.

[asvetlov]

Agree, aiohttp can be fixed easily.

If somebody wants to make a pull request -- you are welcome!
aio-libs/aiohttp#4432