array: Add 'w' type and deprecate 'u' type.

[methane]

BPO	36299
Nosy	@terryjreedy, @ncoghlan, @methane, @skrah, @serhiy-storchaka
PRs	bpo-36299: array('u') uses Py_UCS4 instead of Py_UNICODE  #12497

^{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 2019-03-15.05:50:02.657>
labels = ['3.8', 'library']
title = "array: Deprecate 'u' type in array module"
updated_at = <Date 2020-04-23.00:47:43.731>
user = 'https://github.com/methane'

bugs.python.org fields:

activity = <Date 2020-04-23.00:47:43.731>
actor = 'methane'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-03-15.05:50:02.657>
creator = 'methane'
dependencies = []
files = []
hgrepos = []
issue_num = 36299
keywords = ['patch']
message_count = 12.0
messages = ['337967', '338031', '338595', '338598', '338607', '338608', '338609', '338610', '338611', '367000', '367044', '367065']
nosy_count = 5.0
nosy_names = ['terry.reedy', 'ncoghlan', 'methane', 'skrah', 'serhiy.storchaka']
pr_nums = ['12497']
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue36299'
versions = ['Python 3.8']

Linked PRs

• gh-80480: Emit DeprecationWarning for array's 'u' type code #95760
• gh-80480: array: Add 'w' typecode. #105242
• gh-80480: remove deprecated 'u' type code (array) #149535

[methane]

The doc says:

'u' will be removed together with the rest of the Py_UNICODE API.
Deprecated since version 3.3, will be removed in version 4.0.
https://docs.python.org/3/library/array.html

But DeprecationWarning is not raised yet. Let's raise it.

• 3.8 -- PendingDeprecationWarning
• 3.9 -- DeprecationWarning
• 4.0 or 3.10 -- Remove it.

[terryjreedy]

'4.0' is a stand-in for 'sometime after 2.7.final', scheduled for Jan 2020. A Pending... for 3.8.0, scheduled for Oct 2019, seems reasonable to me. Perhaps we should have a pydev discussion for the general issue of post 2.7 removals of already deprecated items.

[methane]

https://mail.python.org/pipermail/python-dev/2019-March/156807.html

We may able to convert 'u' to wchar_t to int32_t and un-deprecate it.

[methane]

I found converting Py_UNICODE to Py_UCS4 wad happened, and reverted.
ref: https://bugs.python.org/issue13072

[skrah]

I think the problem is still whether to use 'u' == UCS2 and 'w' == UCS4 like in PEP-3118.

For the project I'm currently working on I'd need these for buffer exports:

>>> from xnd import *
>>> x = xnd(["abc", "xyz"], dtype="fixed_string(10, 'utf16')")
>>> y = xnd(["abc", "xyz"], dtype="fixed_string(10, 'utf32')")
>>> 
>>> memoryview(x)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: type is not supported by the buffer protocol

The use case is not an array that represents a single utf16 string, but
an array *of* fixed strings with different encodings.

So x would be exported with format 'u' and y with format 'w'.

[skrah]

Just to demonstrate what the format would look like, this is working
for an array of fixed bytes:

>>> x = xnd([b"123", b"23456"], dtype="fixed_bytes(size=10)")
>>> memoryview(x).format
'10s'

So the formats in the previous message would be '10u' and '10w'.

[serhiy-storchaka]

array('u') is not tied with the legacy Unicode C API. It is possible to use the modern wchar_t based Unicode C API for it. See bpo-36346.

There are benefits from getting rid of the legacy Unicode C API, but not from array('u').

[skrah]

array() uses struct module characters except for 'u'. PEP-3118 was
supposed to be implemented in the struct module.

If array() continues to use 'u', the only sensible thing would be
to remove (or rename) 'a', 'u' and 'w' from PEP-3118.

[skrah]

The funny thing is that array() already knows this:

>>> import array
>>> a = array.array("u", "123")
>>> memoryview(a).format
'w'

[methane]

I closed #56706 (Py_UNICODE -> Py_UCS4).
I created #63852 (Py_UNICODE -> wchar_t) instead.

[terryjreedy]

Should this issue be closed, possibly as superseded by bpo-36346, the issue for the new PR-19653?

[methane]

While array('u') doesn't use deprecated API with #63852, I still don't like 'u' because:

• I don't have any reason to use platform dependant wchar_t. 1
• It is not consistent with PEP-3118.

How about this plan?

• Add 'w' for Py_UCS4.
• Deprecate 'u', and remove it in the future.