io.FileIO.mode doesn't comply with the docs

[mikeparker]

BPO	40391
Nosy	@benjaminp, @serhiy-storchaka, @iritkatriel
Superseder	bpo-25341: File mode wb+ appears as rb+

^{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 2020-10-02.18:56:35.219>
created_at = <Date 2020-04-26.00:50:56.936>
labels = ['type-bug', '3.7', 'expert-IO', 'docs']
title = "io.FileIO.mode doesn't comply with the docs"
updated_at = <Date 2020-10-02.18:56:35.218>
user = 'https://bugs.python.org/mikeparker'

bugs.python.org fields:

activity = <Date 2020-10-02.18:56:35.218>
actor = 'serhiy.storchaka'
assignee = 'docs@python'
closed = True
closed_date = <Date 2020-10-02.18:56:35.219>
closer = 'serhiy.storchaka'
components = ['Documentation', 'IO']
creation = <Date 2020-04-26.00:50:56.936>
creator = 'mike.parker'
dependencies = []
files = []
hgrepos = []
issue_num = 40391
keywords = []
message_count = 3.0
messages = ['367288', '377824', '377827']
nosy_count = 6.0
nosy_names = ['benjamin.peterson', 'stutzbach', 'docs@python', 'serhiy.storchaka', 'mike.parker', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '25341'
type = 'behavior'
url = 'https://bugs.python.org/issue40391'
versions = ['Python 3.7']

[mikeparker]

io.FileIO.mode doesn't reflect "The mode as given in the constructor." as per the documentation(https://docs.python.org/3/library/io.html#io.FileIO.mode).
Was noted in bpo-4362, but seems to be still relevant.

Example:
>>> f = open("t.tmp", "w+b")
>>> f.mode
'rb+'

[iritkatriel]

The mode string is calculated here: https://github.com/python/cpython/blob/master/Modules/_io/fileio.c#L1055

and is based on these four values in the fileio struct:

    unsigned int created : 1;
    unsigned int readable : 1;
    unsigned int writable : 1;
    unsigned int appending : 1;

This is not enough information to distinguish between r+ and w+. That distinction was lost in _io_FileIO___init___impl, where it specified the flags that went into creating fd, but these flags are not saved.

The bit fields in this struct add up to 7 bits, so it seems possible to fix this if another bit is allocated for this purpose.

If it's not worth it, then the documentation needs to change a little.

[serhiy-storchaka]

It is a duplicate of bpo-25341.