[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

Brian Skinn  added the comment:

LOL. No special thanks necessary, that last post only turned into something 
coherent (and possibly correct, it seems...) after a LOT of diving into the 
source, fiddling with the code, and (((re-)re-)re-)writing! Believe me, it 
reads as a lot more knowledgeable and confident than I actually felt while 
writing it. :-D

Thanks to all of you for coming along with me on this dive into the CPython 
internals!

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Thomas Kluyver]

Thomas Kluyver  added the comment:

D'oh, yes. I missed that the failing example was displaying the captured string 
through displayhook. It makes sense now. Thanks for patiently explaining. :-)

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

Brian Skinn  added the comment:

Well, the warning content *itself* may not get passed through the displayhook 
at raise-time, in the process of being run through stderr and displayed by the 
REPL.

But, when you capture the warning content with redirect_stderr(sio) and then 
">>> sio.getvalue()", the contents of the capture from stderr, as produced by 
.getvalue(), *will* get passed through the displayhook, and thus be escaped.



In theory, I could have obtained a consistent 'want' by using print() as you've 
done. However, for my particular example (see OP), I wanted to elide the first 
part of the warning message, which is messy, irrelevant to my code example, and 
can change from Python version to Python version. However, as doctest is 
currently implemented, a 'want' can't start with an ellipsis because it 
collides with the regex that detects PS2 prompts 
(https://github.com/python/cpython/blob/4f5a3493b534a95fbb01d593b1ffe320db6b395e/Lib/doctest.py#L583-L586).

See #36714 (https://bugs.python.org/issue36714) for more information and a 
proposed enhancement/fix.

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Thomas Kluyver]

Thomas Kluyver  added the comment:

The 'single' option to compile() means it's run like at a REPL, calling 
displayhook if it's an expression returning a value.

But warnings shouldn't go through the displayhook, as far as I know:

>>> from contextlib import redirect_stdout, redirect_stderr
>>> from io import StringIO
>>> sio = StringIO()
>>> with redirect_stderr(sio):
...   exec(compile('import warnings; warnings.warn(""" \' " """)', 'dummyfile', 
'single'))
... 
>>> print(sio.getvalue())
__main__:1: UserWarning:  ' "

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

Brian Skinn  added the comment:

The application of repr() (or a repr()-equivalent) appears to occur as some 
part of the exec(compile(...)) call within doctest 
(https://github.com/python/cpython/blob/4f5a3493b534a95fbb01d593b1ffe320db6b395e/Lib/doctest.py#L1328-L1329).

On 3.6.6, in REPL:

```
>>> from contextlib import redirect_stdout
>>> from io import StringIO
>>> sio = StringIO()
>>> with redirect_stdout(sio):
... exec(compile('""" \' " """', 'dummyfile', 'single'))
...
>>> output = sio.getvalue()
>>> output
'\' \\\' " \'\n'
```

Also 3.6.6, at Win cmd:

```
>type exec_compile.py
from contextlib import redirect_stdout
from io import StringIO

exec(compile('""" \' " """', 'dummyfile', 'single'))

sio = StringIO()
with redirect_stdout(sio):
exec(compile('""" \' " """', 'dummyfile', 'single'))

output = sio.getvalue()

assert output == '\' \\\' " \'\n'

>python exec_compile.py
' \' " '

>
```

It *looks* like exec() executes the compile()'d source as if it were typed into 
a REPL -- IOW, any unassigned non-None return value X gets pushed to stdout as 
repr(X). This is then what the doctest self._fakeout captures for comparison to 
the 'want' of the example.

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Thomas Kluyver]

Thomas Kluyver  added the comment:

I'm still a bit confused why it gets escaped - as far as I know, the escaping 
only happens when you repr() a string, as the displayhook does automatically:

>>> a = """ a ' single and " double quote """
>>> a
' a \' single and " double quote '
>>> print(repr(a))
' a \' single and " double quote '
>>> print("%r" % a)
' a \' single and " double quote '
>>> print(a)
 a ' single and " double quote

The warnings code doesn't appear to ever repr() the message. So I guess it's 
some further bit of interaction with doctest. But unfortunately I don't have 
time to dig through doctest to try and understand it.

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

Brian Skinn  added the comment:

Thank you for taking the time to dig into it so deeply!

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

Thanks for the update and report, Brian.

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

Change by Brian Skinn :


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resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

Brian Skinn  added the comment:

It looks to me like it's a standard feature of the CPython string rendering 
routines, where if single and double quotes are present in any string, the 
preferred rendering is enclosure with single quotes with escaped internal 
single quotes.

On 3.6.6, regardless how I enter the following, it always returns enclosed in 
single quotes:

>>> """ ' " """
' \' " '

>>> ''' ' " '''
' \' " '

>>> ' \' " '
' \' " '

>>> " ' \" "
' \' " '

For my particular situation, then, the problem is that my warning message, as 
it sits in the source, consists of a double-quoted string that contains single 
quotes.  Then, when 3.8 doctest goes to print the source line, it has to print 
a string containing both single and double quotes, so the above default 
rendering rule kicks in and it gets printed with enclosing single-quotes. For 
3.7 doctest, where the regex doesn't match, the source line doesn't get 
printed, and so the resulting string contains no double quotes, and thus the 
string gets printed with enclosing double quotes.

Clearly, the solution is just for me to change the warning message! And indeed, 
changing to `warnings.warn("foo has no bar")` and updating the expected result 
to `'...UserWarning: foo has no bar\n...'` yields a passing test on both 3.7 
and 3.8 now.

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

I did some more debugging. doctest patches linecache which does some regex 
matching when filename is of the form [examplenumber]> to 
return example source. Before the commit seems absolute path was present in 
warning and hence this regex didn't match. With the commit returning the 
filename of this format that matches the regex the example line is again 
returned. This happens with warnings inside doctest because doctest patches 
linecache which is used by warnings.py during formatting the warning. In 
CPython for some reason presence of both single quote and double quote inside a 
triple quoted string causes the single quote to be escaped. Any concatenation 
with the escaped triple quoted string also escapes the resulting text. doctest 
seems to store the examples as single quoted strings that are escaped and 
escaping them during _formatwarnmsg_impl causes the other one also to be 
escaped. It also happens with a normal string that has an escaped double quote.

>>> a = """Test '' b""" # Two single quotes
>>> a
"Test '' b"
>>> a = """Test " b'""" # One single and double quote
>>> a
'Test " b\''
>>> a + "'c'"
'Test " b\'\'c\''
>>> a = """Test ' b"""  # Only single quote
>>> a
"Test ' b"
 a + "'c'"
"Test ' b'c'"
 a = "Test ' b\""  # Escaped double quote
 a
'Test \' b"'
 a + "'a'"
'Test \' b"\'a\''

Does anyone know why this happens with escaped quotes and single quote being 
escaped? Is this expected and is it part of spec about how single and double 
quote are swapped over representation?

Longer explanation :

Take the below sample doctest file

$ cat ../backups/bpo36695.rst
>>> import warnings  # line 0
>>> warnings.warn("Test 'a'")  # line 1

doctest patches linecache.getlines to a custom function 
`__patched_linecache_getlines` [0]

linecache.getlines = __patched_linecache_getlines

__LINECACHE_FILENAME_RE = re.compile(r'.+)'
 r'\[(?P\d+)\]>$')
def __patched_linecache_getlines(self, filename, module_globals=None):
 m = self.__LINECACHE_FILENAME_RE.match(filename)
 if m and m.group('name') == self.test.name:
  example = self.test.examples[int(m.group('examplenum'))]
  return example.source.splitlines(keepends=True)
 else:
  return self.save_linecache_getlines(filename, module_globals)

doctest forms a special filename as below that is passed to exec(compile()) and 
hence as per the commit warning is now raised as the filename "" in the warning. doctest also mocks sys.stdout internally to 
have the output captured to a StringIO buffer. [1]

# Use a special filename for compile(), so we can retrieve
# the source code during interactive debugging (see
# __patched_linecache_getlines).
filename = '' % (test.name, examplenum)

# Before commit

   cpython git:(3b0b90c8c3) ./python.exe -m doctest ../backups/bpo36695.rst
/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/doctest.py:1: 
UserWarning: Test 'a'
  # Module doctest.

# After commit

$cpython git:(11a896652e) ./python.exe -m doctest ../backups/bpo36695.rst
:1: UserWarning: Test 'a'
  warnings.warn("Test 'a'")

formatting warning message [2] calls linecache.getline with filename as 
"" after commit which in turn calls linecache.getlines 
that is patched above by doctest and hence it matches the regex and returns the 
example.source "warnings.warn("Test 'a'")". It seems to be a triple quoted 
string that is already escaped and hence in the below line calling s += " %s\n" 
% line causes the actual warning message and the example source line to be 
escaped.

  def _formatwarnmsg_impl(msg):
s =  ("%s:%s: %s: %s\n"
  % (msg.filename, msg.lineno, msg.category.__name__,
 msg.message))

if msg.line is None:
try:
import linecache
line = linecache.getline(msg.filename, msg.lineno)
except Exception:
# When a warning is logged during Python shutdown, linecache
# and the import machinery don't work anymore
line = None
linecache = None
else:
line = msg.line
if line:
line = line.strip()
s += "  %s\n" % line

[0] 
https://github.com/python/cpython/blob/29d018aa63b72161cfc67602dc3dbd386272da64/Lib/doctest.py#L1468
[1] 
https://github.com/python/cpython/blob/29d018aa63b72161cfc67602dc3dbd386272da64/Lib/doctest.py#L1452
[2] 
https://github.com/python/cpython/blob/29d018aa63b72161cfc67602dc3dbd386272da64/Lib/warnings.py#L35

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[STINNER Victor]

Change by STINNER Victor :


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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

Brian Skinn  added the comment:

TBH, now that I've tweaked tox and CI just not to run the doctests on 3.8, I 
don't really need this to be fixed.

This seems like such an edge case -- a doctest catching a warning with a 
message containing single quotes -- it might not really be worth the effort to 
figure out.  Unless someone is really invested in tracking this down, I would 
be content to close.

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brett Cannon]

Brett Cannon  added the comment:

If you look at that commit that Thomas made all it did was change where the 
string was grabbed from, not what type of object was used. So it doesn't make 
any sense as to why that would cause any specific change, so I think this may 
be doctest's doing.

Probably the next step is for someone to find in doctest where the string 
representation is being printed out to understand what would potentially shift 
its representation (and how it's even generating that representation).

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

Brian Skinn  added the comment:

, it seems like the problem must somehow stem from the new commit using 
frame.f_code.co_filename (or the C equivalent), instead of using __file__ as 
previously.

Consider this warn2.py, similar to the other but with no single quotes in the 
warning message:

```
import doctest

class Tester:
r"""Provide docstring for testing.

>>> import warnings
>>> from contextlib import redirect_stderr
>>> from io import StringIO
>>> sio = StringIO()
>>> with redirect_stderr(sio):
... warnings.warn("Warning with no quotes")
>>> sio.getvalue()
'...'

"""

doctest.run_docstring_examples(
Tester(),
{},
optionflags=doctest.ELLIPSIS,
)
```

This doctest PASSES for me in both 3.7 and 3.8; note that the expected doctest 
output from `sio.getvalue()` is *single-quote delimited*. This implies to me 
that REPL string output is single-quote delimited by default, and that there's 
some sort of "smart string formatter" functionality involved that is working 
correctly in 3.7 but not in 3.8, which REPL-prints the single-quote-containing 
string using enclosing double-quotes, so as to avoid escaping the internal 
single quotes.

Why 11a8966 would break this in this way is ... baffling to me.

---

Unfortunately, I don't think it will work to fix the doctest on my end simply 
by using `print(sio.getvalue())`, as the resulting message is one line long in 
3.7, but two lines long in 3.8. Further, doctest gets confused when you try to 
start a line of output with an ellipsis, as it thinks it's a continuation of 
the prior command.

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

> Karthikeyan, my apologies for the slow reply -- I posted this right before I 
> went to bed.

No problem, thanks for the simplified program. I wrote a similar one based on 
doctest that fails with commit and passes before it. I am still confused about 
the commit impact and warnings also uses C code so hope someone else has some 
idea over this scenario. As you mentioned it seems to be about doctest that 
uses exec and compile. I can see the change in output since doctest has it's 
own internal stdout wrapper like contextlib but using the similar exec and 
compile statement as a standalone one doesn't reproduce this.

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

Brian Skinn  added the comment:

Here is warn.py, a minimal no-dependency repro script:

```
import doctest

class Tester:
r"""Provide docstring for testing.

>>> import warnings
>>> from contextlib import redirect_stderr
>>> from io import StringIO
>>> sio = StringIO()
>>> with redirect_stderr(sio):
... warnings.warn("Warning 'containing' single quotes")
>>> sio.getvalue()
"...UserWarning: Warning 'containing' single quotes\n..."

"""


doctest.run_docstring_examples(
Tester(),
{},
optionflags=doctest.ELLIPSIS,
)
```

`python3.7 warn.py` (3.7.3) gives no output.

`python3.8 warn.py` (3.8.0a3) gives:

```
$ python3.8 warn.py

File "warn.py", line ?, in NoName
Failed example:
sio.getvalue()
Expected:
"...UserWarning: Warning 'containing' single quotes\n..."
Got:
':2: UserWarning: Warning \'containing\' single quotes\n 
 warnings.warn("Warning \'containing\' single quotes")\n'
```

The problem appears to be centered around *doctest*, as the following script 
DOES NOT raise AssertionError with either of 3.7 or 3.8:

```
import warnings
from contextlib import redirect_stderr
from io import StringIO

sio = StringIO()

with redirect_stderr(sio):
warnings.warn("Warning 'containing' single quotes")

assert " 'containing' " in sio.getvalue()
```

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

Brian Skinn  added the comment:

Karthikeyan, my apologies for the slow reply -- I posted this right before I 
went to bed.


To emphasize, the change to the formatting of the string contents, by adding 
the filename, I think is not problematic: I'm using ellipses to elide 
everything before and after my custom "warning" message.

Rather, I think the problem is that the string is being rendered as a '' 
string, instead of as a "" string; IOW:

'Test string with \'enclosing\' single quotes'

vs

"Test string with 'enclosing' double quotes"

---

In the interim, as you suggest, Karthikeyan, I can just conditionally skip the 
doctests on 3.8 with a suitable pytest -k flag.

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Thomas Kluyver]

Thomas Kluyver  added the comment:

It's not obvious to me why that change to finding the source file related to 
the warning should affect the format of the warning message printed. It might 
be something that could be fixed in the warning module. But I don't understand 
where it's going wrong at present.

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

I tried bisecting and got to this commit 
11a896652ee98aa44e59ed25237f9efb56635dcf (issue33375) . Seems this changes 
warning reporting output to add filename. I guess it's better to change the 
doctest to adopt this change. I have added the devs on the issue for 
confirmation.

commit 11a896652ee98aa44e59ed25237f9efb56635dcf (HEAD)
Author: Thomas Kluyver 
Date:   Fri Jun 8 21:28:37 2018 +0200

bpo-33375: Get filename for warnings from frame.f_code.co_filename (GH-6622)

More consistent with how other parts of Python find the filename (e.g. 
tracebacks and pdb).

$ cat ../backups/bpo36695_1.py
def foo():
'''
>>> import warnings, io
>>> from contextlib import redirect_stderr
>>> f = io.StringIO()
>>> with redirect_stderr(f):
... warnings.warn("'foo' has no 'bar'")
... err_cap = f.getvalue()
>>> print(err_cap)
'''
pass

➜  cpython git:(11a896652e) ./python.exe -m doctest ../backups/bpo36695_1.py
**
File "../backups/bpo36695_1.py", line 9, in bpo36695_1.foo
Failed example:
print(err_cap)
Expected nothing
Got:
:2: UserWarning: 'foo' has no 'bar'
  warnings.warn("'foo' has no 'bar'")

**
1 items had failures:
   1 of   5 in bpo36695_1.foo
***Test Failed*** 1 failures.

# Before 11a896652ee98aa44e59ed25237f9efb56635dcf

➜  cpython git:(11a896652e) git checkout 
11a896652ee98aa44e59ed25237f9efb56635dcf~1
Previous HEAD position was 11a896652e bpo-33375: Get filename for warnings from 
frame.f_code.co_filename (GH-6622)
HEAD is now at 3b0b90c8c3 bpo-33800: Fix default argument for parameter 
dict_type of ConfigParser/RawConfigParser (GH-7494)
➜  cpython git:(3b0b90c8c3) make -s -j4 > /dev/null
➜  cpython git:(3b0b90c8c3) ./python.exe -m doctest ../backups/bpo36695_1.py
**
File "../backups/bpo36695_1.py", line 9, in bpo36695_1.foo
Failed example:
print(err_cap)
Expected nothing
Got:
../backups/bpo36695_1.py:2: UserWarning: 'foo' has no 'bar'
  '''

**
1 items had failures:
   1 of   5 in bpo36695_1.foo
***Test Failed*** 1 failures.


I can replicate test failure as below with 
11a896652ee98aa44e59ed25237f9efb56635dcf and passes with the commit before it.

README.rst F
  [ 20%]
tests/test_stdiomgr_base.py 
  [100%]

= FAILURES 
==
___ [doctest] README.rst 

077
078 **Mock** ``stderr``\ **:**
079
080 .. code ::
081
082 >>> import warnings
083 >>> with stdio_mgr() as (in_, out_, err_):
084 ... warnings.warn("'foo' has no 'bar'")
085 ... err_cap = err_.getvalue()
086 >>> err_cap
Expected:
"...UserWarning: 'foo' has no 'bar'\n..."
Got:
':2: UserWarning: \'foo\' has no \'bar\'\n  
warnings.warn("\'foo\' has no \'bar\'")\n'

/home/karthi/stdio-mgr/README.rst:86: DocTestFailure

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

Can you please attach a single and standalone file without dependencies like 
attrs so that it would help in bisecting the issue?

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[issue36695] Change (regression?) in v3.8.0a3 doctest output after capturing the stderr output from a raised warning

[Brian Skinn]

New submission from Brian Skinn :

In [this project](https://github.com/bskinn/stdio-mgr) of mine, I have a tox 
matrix set up with Pythons from 3.3. to 3.8. I have pytest set up to run 
doctest on my 
[`README.rst`](https://github.com/bskinn/stdio-mgr/blob/6444cce8e5866e2d519c1c0630551d8867f30c9a/README.rst).
  For Pythons 3.4 to 3.7 (3.4.10, 3.5.7, 3.6.8, 3.7.2), the following doctest 
example passes:

```
>>> import warnings
>>> with stdio_mgr() as (in_, out_, err_):
... warnings.warn("'foo' has no 'bar'")
... err_cap = err_.getvalue()
>>> err_cap
"...UserWarning: 'foo' has no 'bar'\n..."
```

Under Python 3.8.0a3, though, it fails (actual local paths elided):

```
$ tox -re py38-attrs_latest
.package recreate: .../.tox/.package
.package installdeps: wheel, setuptools, attrs>=17.1
py38-attrs_latest recreate: .../.tox/py38-attrs_latest
py38-attrs_latest installdeps: attrs, pytest
py38-attrs_latest inst: .../.tox/.tmp/package/1/stdio-mgr-1.0.2.dev1.tar.gz
py38-attrs_latest installed: 
atomicwrites==1.3.0,attrs==19.1.0,more-itertools==7.0.0,pluggy==0.9.0,py==1.8.0,pytest==4.4.1,six==1.12.0,stdio-mgr==1.0.2.dev1
py38-attrs_latest run-test-pre: PYTHONHASHSEED='2720295779'
py38-attrs_latest run-test: commands[0] | pytest
===
 test session starts 

platform linux -- Python 3.8.0a3, pytest-4.4.1, py-1.8.0, pluggy-0.9.0
cachedir: .tox/py38-attrs_latest/.pytest_cache
rootdir: ..., inifile: tox.ini
collected 6 items   

   

README.rst F

 [ 16%]
tests/test_stdiomgr_base.py .   

 [100%]

=
 FAILURES 
=
___
 [doctest] README.rst 
___
077 
078 **Mock** ``stderr``\ **:**
079 
080 .. code ::
081 
082 >>> import warnings
083 >>> with stdio_mgr() as (in_, out_, err_):
084 ... warnings.warn("'foo' has no 'bar'")
085 ... err_cap = err_.getvalue()
086 >>> err_cap
Expected:
"...UserWarning: 'foo' has no 'bar'\n..."
Got:
':2: UserWarning: \'foo\' has no \'bar\'\n  
warnings.warn("\'foo\' has no \'bar\'")\n'

.../README.rst:86: DocTestFailure

 1 failed, 5 passed in 0.06 seconds 

ERROR: InvocationError for command .../.tox/py38-attrs_latest/bin/pytest 
(exited with code 1)
_
 summary 
__
ERROR:   py38-attrs_latest: commands failed

```

If I change the doctest in README to the following, where the expected output 
is surrounded by single-quotes instead of double-quotes, and the internal 
single quotes are escaped, it passes fine in 3.8.0a3:

```
>>> import warnings
>>> with stdio_mgr() as (in_, out_, err_):
... warnings.warn("'foo' has no 'bar'")
... err_cap = err_.getvalue()
>>> err_cap
'...UserWarning: \'foo\' has no \'bar\'\n...'
```

But, naturally, it fails in 3.7 and below.

It *looks* like this is probably a glitch somewhere in 3.8.0a3, where this 
string containing single quotes is rendered (at the REPL?) using enclosing 
single quotes and escaped internal single quotes, rather than enclosing 
double-quotes and non-escaped internal single-quotes?

--
components: Library (Lib)
messages: 340637
nosy: bskinn
priority: normal
severity: normal
status: open
title: Change (regression?) in v3.8.0a3 doctest output after capturing the 
stderr output from a raised warning
type: behavior
versions: Python 3.8

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