readinto is not a method on io.TextIOBase

[steverpalmer]

BPO	35848
Nosy	@benjaminp, @vadmium, @csabella, @miss-islington, @remilapeyre, @tirkarthi, @steverpalmer
PRs	bpo-35848: Move all documentation regarding the readinto #11893 [3.7] closes bpo-35848: Move all documentation regarding the readinto out of IOBase. (GH-11893) #12736

^{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 2019-04-09.04:35:30.169>
created_at = <Date 2019-01-29.09:18:40.868>
labels = ['type-bug', '3.8', 'docs']
title = 'readinto is not a method on io.TextIOBase'
updated_at = <Date 2019-04-09.04:57:42.174>
user = 'https://github.com/steverpalmer'

bugs.python.org fields:

activity = <Date 2019-04-09.04:57:42.174>
actor = 'miss-islington'
assignee = 'docs@python'
closed = True
closed_date = <Date 2019-04-09.04:35:30.169>
closer = 'benjamin.peterson'
components = ['Documentation']
creation = <Date 2019-01-29.09:18:40.868>
creator = 'steverpalmer'
dependencies = []
files = []
hgrepos = []
issue_num = 35848
keywords = ['patch']
message_count = 10.0
messages = ['334507', '334524', '334526', '334530', '334533', '334548', '334549', '335571', '339697', '339698']
nosy_count = 8.0
nosy_names = ['benjamin.peterson', 'stutzbach', 'martin.panter', 'cheryl.sabella', 'miss-islington', 'remi.lapeyre', 'xtreak', 'steverpalmer']
pr_nums = ['11893', '12736']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35848'
versions = ['Python 3.8']

[steverpalmer]

class io.IOBase states "Even though IOBase does not declare read(), readinto(), or write() because their signatures will vary, implementations and clients should consider those methods part of the interface. Also, implementations may raise a ValueError (or UnsupportedOperation) when operations they do not support are called." However, even though class io.TextIOBase is described as inheriting from io.IOBase, a call to readinto method returns AttributeError exception indicating no readinto attribute, inconsistent with the documentation.

[tirkarthi]

https://docs.python.org/3/library/io.html#io.TextIOBase

Base class for text streams. This class provides a character and line based interface to stream I/O. There is no readinto() method because Python’s character strings are immutable. It inherits IOBase. There is no public constructor.

io.TextIOBase docs say there is no readinto method in the documentation.

[remilapeyre]

I checked and io.TextIOBase is the only io.IOBase subclass to lack one of read, readinto or write:

>>> import io, inspect
>>> for name, obj in inspect.getmembers(io, predicate=inspect.isclass):
... 	missing = {'read', 'readinto', 'write'} - {name for name, _ in inspect.getmembers(obj)}
...	if issubclass(obj, io.IOBase) and missing:
...	     print(obj, missing, issubclass(obj, io.TextIOBase))

<class 'io.IOBase'> {'write', 'read', 'readinto'} False
<class '_io.StringIO'> {'readinto'} True
<class 'io.TextIOBase'> {'readinto'} True
<class '_io.TextIOWrapper'> {'readinto'} True

I can open a PR to fix the conflicts between the two parts of the documentation. I think it's appropriate to change TextIOBase to raise UnsupportedOperation when calling readinto and to change the documentation accordingly.