[docutils:bugs] #490 EncodingWarnings in io module

[r9772] breaks tests for non-UTF locales on both Linux and Windows (e.g. ISO 88591), when not using Python's UTF-8 mode.

See the following failures (from [GitHub Actions](https://github.com/AA-Turner/docutils/actions/runs/10200475399/job/28220043798?pr=16), scroll up to the first section `Run test suite (pytest ./test)`):

```
_____________________ FileInputTests.test_fallback_no_utf8 _____________________

self = <test.test_io.FileInputTests testMethod=test_fallback_no_utf8>

    @unittest.skipIf(preferredencoding in (None, 'ascii', 'utf-8'),
                     'locale encoding not set or UTF-8')
    def test_fallback_no_utf8(self):
        # If  no encoding is given and decoding with 'utf-8' fails,
        # use the locale's preferred encoding (if not None).
        # Provisional: the default will become 'utf-8'
        # (without auto-detection and fallback) in Docutils 0.22.
        source = du_io.FileInput(
            source_path=os.path.join(DATA_ROOT, 'latin1.txt'))
>       data = source.read()

test/test_io.py:321: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
docutils/io.py:412: in read
    data = self.source.read()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <encodings.utf_8.IncrementalDecoder object at 0x7f023f9906f0>
input = b'Gr\xfc\xdfe\n', final = True

>   ???
E   UnicodeDecodeError: 'utf-8' codec can't decode byte 0xfc in position 2: invalid start byte

<frozen codecs>:322: UnicodeDecodeError
```

I'm not sure what the right behaviour here should be.

------

There's also a problem on the same non-UTF-8 locales when not in UTF-8 mode:

```
======================================================================
ERROR: test_publish_cmdline (test_publisher.ConvenienceFunctionTests.test_publish_cmdline)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/docutils/docutils/docutils/docutils/io.py", line 525, in write
    self.destination.write(data)
  File "/home/runner/work/docutils/docutils/docutils/test/alltests.py", line 63, in write
    self.stream.write(string)
TypeError: write() argument must be str, not bytes

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/runner/work/docutils/docutils/docutils/docutils/io.py", line 529, in write
    self.destination.buffer.write(data)
    ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Tee' object has no attribute 'buffer'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/runner/work/docutils/docutils/docutils/test/test_publisher.py", line 160, in test_publish_cmdline
    core.publish_cmdline(writer_name='null',
  File "/home/runner/work/docutils/docutils/docutils/docutils/core.py", line 431, in publish_cmdline
    output = publisher.publish(
             ^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/docutils/docutils/docutils/docutils/core.py", line 261, in publish
    output = self.writer.write(self.document, self.destination)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/docutils/docutils/docutils/docutils/writers/__init__.py", line 81, in write
    return self.destination.write(self.output)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/docutils/docutils/docutils/docutils/io.py", line 533, in write
    raise ValueError(
ValueError: Encoding of <file> (iso8859-1) differs 
  from specified encoding (utf-8)

----------------------------------------------------------------------
Ran 1872 tests in 4.888s

FAILED (errors=1, skipped=2)
Elapsed time: 5.079 seconds
```

(On Windows it says ``ValueError: Encoding of <file> (cp1252) differs `` instead). 

This failure only happens with ``alltests.py``. Now that both ``pytest`` and ``unittest`` work with our test suite, we could consider removing  ``alltests.py``.

A



---

**[bugs:#490] EncodingWarnings in io module**

**Status:** open-fixed
**Created:** Fri Jun 28, 2024 03:34 PM UTC by Jason R. Coombs
**Last Updated:** Mon Jul 22, 2024 11:55 AM UTC
**Owner:** nobody


When running the [distutils](https://github.com/pypa/distutils) tests with `PYTHONWARNDEFAULTENCODING=1`, two warnings are emitted:

```
distutils/tests/test_check.py::TestCheck::test_check_restructuredtext
  /Users/jaraco/code/pypa/distutils/.tox/py/lib/python3.12/site-packages/docutils/io.py:381: EncodingWarning: 'encoding' argument not specified
    self.source = open(source_path, mode,

distutils/tests/test_check.py::TestCheck::test_check_restructuredtext
  /Users/jaraco/code/pypa/distutils/.tox/py/lib/python3.12/site-packages/docutils/io.py:151: EncodingWarning: UTF-8 Mode affects locale.getpreferredencoding(). Consider locale.getencoding() instead.
    fallback = locale.getpreferredencoding(do_setlocale=False)
```

Docutils should honor [PEP 597](https://peps.python.org/pep-0597/) and address these warnings (and possibly others). In my experience, adding `encoding='utf-8'` to any io operation is the best approach - it's straight-up compatible with the default on non-Windows systems and usually honoring the Unix convention is suitable if not preferable on Windows. Not only that, but that behavior will become the default in Python 3.15 or so.


---

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