export the set newline value on TextIOBase/TextIOWrapper

[calestyo]

BPO	43337
Nosy	@benjaminp

^{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 = None
created_at = <Date 2021-02-27.05:45:28.919>
labels = ['type-feature', 'expert-IO']
title = 'export the set newline value on TextIOBase/TextIOWrapper'
updated_at = <Date 2021-03-05.21:03:13.665>
user = 'https://bugs.python.org/calestyo'

bugs.python.org fields:

activity = <Date 2021-03-05.21:03:13.665>
actor = 'terry.reedy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['IO']
creation = <Date 2021-02-27.05:45:28.919>
creator = 'calestyo'
dependencies = []
files = []
hgrepos = []
issue_num = 43337
keywords = []
message_count = 1.0
messages = ['387767']
nosy_count = 2.0
nosy_names = ['benjamin.peterson', 'calestyo']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43337'
versions = []

[calestyo]

Hey.

It seems there is no way to determine the newline value (None, '', \n, \r, \r\n) of a given text stream.
.newlines rather gives the newlines that have been encountered so far.

Just like .encoding and .errors this would be quite useful for anything that gets a stream and operates on it, possibly needing to know which newlines it will see when reading from the stream.

One example would be, after line = stream.readline(4096+n) one want's to know whether actually a full line of 4096 characters has been read.
Putting aside newline="", for simplicity here, n already depends on whether newline was None, \n, \r or \r\n, as would any check like line.endswith(foo).

Maybe some attribute, that indicates the most recently read newline in newline="" mode, would be helpful, too.

Cheers,
Chris.