[issue32912] Raise non-silent warning for invalid escape sequences

[Daniel Carpenter]

Daniel Carpenter  added the comment:

I'm not sure if this is an issue or by design, but this DeprecationWarning 
behaves differently to other DeprecationWarnings.

A normal DeprecationWarning triggered by code in __main__ is printed by default:

$ python -c 'import warnings; warnings.warn("test", DeprecationWarning)'
:1: DeprecationWarning: test

But this one is silent:

$ python -c '"\,"'
[no output]

To see this DeprecationWarning at all, I need to type:

$ python -Wdefault -c '"\,"'
:1: DeprecationWarning: invalid escape sequence '\,'

But that enables this DeprecationWarning for all modules, not just __main__ .

I've tested this with Python 3.9 on debian bullseye and the 3.10 docker image.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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[issue32912] Raise non-silent warning for invalid escape sequences

[Steve Dower]

Change by Steve Dower :


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[issue32912] Raise non-silent warning for invalid escape sequences

[miss-islington]

miss-islington  added the comment:


New changeset b4be87a04a2a8ccfd2480e19dc527589fce53555 by Miss Islington (bot) 
(Gregory P. Smith) in branch 'master':
bpo-32912: Revert SyntaxWarning on invalid escape sequences. (GH-15195)
https://github.com/python/cpython/commit/b4be87a04a2a8ccfd2480e19dc527589fce53555


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[issue32912] Raise non-silent warning for invalid escape sequences

[Raymond Hettinger]

Raymond Hettinger  added the comment:

One possibility is to restrict the warning to a backslash followed by an 
alphabetic character or backslash, and that we define backslash followed by any 
other printable character as specifically allowed.  This would catch the likely 
sources of errors without breaking the likes of \, or \-.  Also, multiline 
strings have less of a need for a warning than a single line string.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Gregory P. Smith]

Gregory P. Smith  added the comment:

I haven't looked, so not that i'm aware of.  I suggest filing one for each of 
those.

The warning not pointing to the right line in a multiline literal sounds like a 
bug to me so that one, if fixed, seems reasonable for 3.8.  The release manager 
gets to decide.

Making the syntax warnings happen upon import of an already compiled pyc is 
more feature-ish, likely to be 3.9+.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Aaron Meurer]

Aaron Meurer  added the comment:

Are there issues tracking the things I mentioned, which should IMO happen 
before this becomes a hard error (making the warnings reproduce even if the 
file has already been compiled, and making warning message point to the correct 
line in multiline strings)? And is it too late to potentially get some of those 
things in 3.8?

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[issue32912] Raise non-silent warning for invalid escape sequences

[Gregory P. Smith]

Gregory P. Smith  added the comment:

I'm leaving this open, as we may still want to do it in 3.9+, just in a less 
disruptive manner.  (That, and how, hasn't been decided yet)

Follow the thread(s) on python-dev for the latest on that.

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stage: resolved -> 
status: closed -> open
versions: +Python 3.9 -Python 3.8

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[issue32912] Raise non-silent warning for invalid escape sequences

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Thank you!

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status: open -> closed

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[issue32912] Raise non-silent warning for invalid escape sequences

[Gregory P. Smith]

Change by Gregory P. Smith :


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pull_request: https://github.com/python/cpython/pull/15195

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[issue32912] Raise non-silent warning for invalid escape sequences

[Gregory P. Smith]

Gregory P. Smith  added the comment:


New changeset 4c5b6bac2408f879231c7cd38d67657dd4804e7c by Gregory P. Smith 
(Serhiy Storchaka) in branch '3.8':
[3.8] bpo-32912: Revert SyntaxWarning on invalid escape sequences (GH-15142)
https://github.com/python/cpython/commit/4c5b6bac2408f879231c7cd38d67657dd4804e7c


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[issue32912] Raise non-silent warning for invalid escape sequences

[Sanyam Khurana]

Change by Sanyam Khurana :


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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

PR 15142 reverts that change for 3.8.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue32912] Raise non-silent warning for invalid escape sequences

[Gregory P. Smith]

Gregory P. Smith  added the comment:

Can the pyc compilation step done by our normal package installers be made to 
treat this warning as an error so that it is forced into the package owners 
faces instead of overlooked because it was just something on stderr?

This syntax warning is absolutely the right thing to surface for 
_owners/maintainers_ of a given piece of code.  Their strings are wrong and 
their code quality will improve as a result.

It isn't the right thing to surface to _users_ of someone else's problematic 
code.  (though it does serve as a red flag proving that the owners of that code 
or person who pinned some dep to an old version haven't proactively tested with 
modern python)

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[issue32912] Raise non-silent warning for invalid escape sequences

[Ned Deily]

Change by Ned Deily :


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[issue32912] Raise non-silent warning for invalid escape sequences

[Nathaniel Smith]

Nathaniel Smith  added the comment:

I think we haven't *actually* done a proper DeprecationWarning period for this. 
We tried, but because of the issue with byte-compiling, the warnings were 
unconditionally suppressed for most users -- even the users who are diligent 
enough to enable warnings and look at warnings in their test suites.

I can see a good argument for making the change, but if we're going to do it 
then it's obviously the kind of change that requires a proper deprecation 
period, and that hasn't happened.

Maybe .pyc files need to be extended to store a list of syntax-related 
DeprecationWarnings and SyntaxWarnings, that are re-issued every time the .pyc 
is loaded? Then we'd at least have the technical capability to deprecate this 
properly.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

> Another issue that I've encountered is that ASCII art becomes gets flagged.  
> Switching to a raw string then kills the unicode escape sequences.

If you already use escape sequences in your ASCII art, what is the problem of 
using them for backslashes?

> I think anyone who starts using 3.8 on a daily basis for non-toy examples 
> will constantly run into this.

What is a better solution? The deprecation warning was emitted starting from 
3.6. Deprecation warnings were silent by default in 3.6, but they become more 
visible in 3.7. This helped some projects (which give attention to warnings in 
they tests) to fix real bugs. But it was not enough, so other bugs are fixed 
only now, when the warnings become even more visible in 3.8. We see a value of 
warnings, they help to fix bugs. If we rollback this change, it will cause yet 
undiscovered bugs be hidden for more time.

In Python 3.0 we make many abrupt breaking changes at once. People complained. 
Here is an opposite example of very gradual change: two releases with a 
DeprecationWarning, few more (at least two) releases with a SyntaxWarning, and 
finally perhaps a SyntaxError.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Aaron Meurer]

Aaron Meurer  added the comment:

Well paradoxically, the bugs that this prevents are the ones it doesn't warn 
about. If someone writes '\tan(x)' thinking it is a string representing a LaTeX 
formula for the tangent of x, they won't realize that they actually created a 
string with a tab plus "an(x)". So actually I would argue that the end goal 
*is* to make people aware of which escape characters exist, or at the very 
least, always make strings raw if there's even the remotest chance they will 
contain a backslash character. 

Is it the best way to go about this? I don't know. The whole thing sort of 
makes me think raw strings should have been the default, but it's obviously too 
late to change that. 

I personally don't feel strongly about the warnings being enabled by default or 
not. My big gripe is that if you actually want the warnings they are difficult 
to get in a reproducible way. I'm actually surprised they are so annoying for 
you. Once a py file is compiled into a pyc file the warnings completely 
disappear, even if you want them!

The fact that you can't use a real escape sequence in a raw string is annoying 
but not the end of the world given that it's trivial to concatenate strings.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Raymond Hettinger]

Raymond Hettinger  added the comment:

> Raymond, are you in agreement that these warnings should 
> at some point eventually become syntax errors?

I used to think so, but after experiencing the incessant warnings, I question 
the value.  In inactive sessions (either with the regular REPL or the ipython 
REPL), they are a recurring annoyance that interferes with data exploration 
live demos.  Perhaps, this should be left for a lint or code analysis tool.  
Why should we intentionally break code that is currently working fine.

Another issue that I've encountered is that ASCII art becomes gets flagged.  
Switching to a raw string then kills the unicode escape sequences.

This isn't really a "Raymond doesn't like this" concern.  I think anyone who 
starts using 3.8 on a daily basis for non-toy examples will constantly run into 
this.  Possibly, it catches a real error, but most often it will just be a 
recurring distractor, especially when teaching Python.  All instructors and 
runners of live demos will need to memorize exactly which characters require an 
escape and which don't -- it creates a new burden that didn't exist before.  
And if the source of the problem is in an external library, the result is 
unactionable by the user, merely making their experience unpleasant.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Aaron Meurer]

Aaron Meurer  added the comment:

Raymond, are you in agreement that these warnings should at some point 
eventually become syntax errors?

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[issue32912] Raise non-silent warning for invalid escape sequences

[Raymond Hettinger]

Raymond Hettinger  added the comment:

AFAICT, I'm currently one of the only people in the world using the 3.8 beta 
for my work on a daily basis.  I've encountered these warnings multiple times 
in multiple contexts.  If we think other people won't experience this recurring 
annoyance, we're in denial.

The entire point of beta testing is to discover problems like this.  However, 
the participants in this thread seem inclined to ignore the evidence and 
persist with the idea that inflicting this on users makes the language better.  
Respectfully, I disagree.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I'm starting to see this error even for plain text strings in existing code:

>>> dunder_methods = '''

d[k]  d.__getitem__(k) -> value 
   \--> d.__missing__(k) -> value
\---> raise KeyError(k)
d[k] = v  d.__setitem__(k, v) -> None
del d[k]  d.__delitem__(k) -> None

'''

SyntaxError: invalid escape sequence \-


These not easily suppressed messages are really annoying.  We don't have to 
inflict this on our users.  IMO, this is the least user friendly change to 
Python 3.8 and as far as I can tell, it is entirely unnecessary (we've lived 
without it for 28+ years).

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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

The warning in docutils was fixed in 
https://sourceforge.net/p/docutils/code/8255/ 1.5 months ago. It was an 
outliner, all other occurrences of \leavevmode were either with the double 
backslash or in raw string literals.

The Bottle code was written 5 years age 
(https://github.com/bottlepy/bottle/commit/8a5bfe2f70a4a5d22b93e909b207584324817760),
 it was a part of the raw string. But a bug was introduced when it it was 
backported to the 0.12 version 
(https://github.com/bottlepy/bottle/commit/1f6fda636a4ea88adfa4292f3c843e71b1a685d8).

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[issue32912] Raise non-silent warning for invalid escape sequences

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I think it is poor form to bombard end-users with warnings about things they 
can't fix.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Here's another example from the current build of the docs using Sphinx:

(venv) ~/npython/Doc $ make html
mkdir -p build
Building NEWS from Misc/NEWS.d with blurb
PATH=./venv/bin:$PATH sphinx-build -b html -d build/doctrees -D 
latex_elements.papersize=  -W . build/html
Running Sphinx v2.1.0
/Users/raymond/npython/venv/lib/python3.8/site-packages/docutils/writers/latex2e/__init__.py:2978:
 SyntaxWarning: invalid escape sequence \l
  self.out.append('}] \leavevmode ')

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[issue32912] Raise non-silent warning for invalid escape sequences

[Raymond Hettinger]

Raymond Hettinger  added the comment:

> In other words, it seems to me that getting in the way of 
> this broken end-user strategy is a *good* thing, since it
> warns of possible mistakes.

Here's an example from the current version of Bottle (0.12.17):

/Users/raymond/Dropbox/Public/sj205/notes2/bottle.py:3433: SyntaxWarning: 
invalid escape sequence \[
  _re_tok += '|([\[\{\(])'
/Users/raymond/Dropbox/Public/sj205/notes2/bottle.py:3434: SyntaxWarning: 
invalid escape sequence \]
  _re_tok += '|([\]\}\)])

These warnings would spew out during a recent Python course where we used 
3.8b2.  It mae it difficult to teach building templates in an iterative style.  
We had to abandon 3.8 and go back to 3.7 to proceed with the templating lesson.

IMO, spewing out these warnings for things users can't do anything about is a 
usability travesty.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Terry J. Reedy]

Terry J. Reedy  added the comment:

The 'filled my terminal with spaces' bug mentioned in msg344764 was 
independently reported in #37433, with a PR.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Mark Dickinson]

Mark Dickinson  added the comment:

[Aaron]

> [...] makes it very difficult for library authors to fix this.

If you're willing to use 3rd party libraries, then my own experience is that 
`pycodestyle` or `flake8` make this easy: you just need to run "flake8 --select 
W605" at the root of your repository tree to get a list of all the code 
locations that need to be fixed.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Mark Dickinson]

Mark Dickinson  added the comment:

[Raymond]

> Also, it gets in the way of the end-user strategy of "backslash anything that 
> looks special"

That's not a good strategy in the first place, though: adding an extra 
backslash for something that doesn't need to be escaped isn't benign - it's 
usually an error, since it puts that backslash into the resulting string.

>>> "abc\`d" == "abc`d"
False

In other words, it seems to me that getting in the way of this broken end-user 
strategy is a *good* thing, since it warns of possible mistakes.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Aaron Meurer]

Aaron Meurer  added the comment:

I agree. Please someone else do that. I don't know what already has issues and 
I unfortunately don't have time right now to help out with any of this. 

I simply mentioned all these things as arguments why Python should not (yet) 
make these warnings errors, which is the point of this issue. 

Also, for the pyparsing example, you would have gotten lucky because it also 
contained \w, which is not a valid escape. If it didn't, you wouldn't be 
warned. So clearly this will help things, but it will also be good to have 
linting tools that, for example, warn about any escape sequences inside 
docstrings.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Aaron, you have reported several different issues in your message:

* compiler warnings are emitted only once, therefore it is easy to miss them 
and hard to reproduce.

* warnings for multiline expressions (and multiline string literals in 
particularly) could be better.

* strange output of spaces in some cases.

And maybe more.

I think all of them deserve opening separate issues.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

In the particular case of pyparsing there was a bug in the docstring.

 def sub(self, repl): 
 """ 
 Return Regex with an attached parse action to transform the parsed 
 result as if called using `re.sub(expr, repl, string) 
`_. 
  
 Example:: 
  
 make_html = Regex(r"(\w+):(.*?):").sub(r"<\1>\2") 
 print(make_html.transformString("h1:main title:")) 
 # prints "main title" 
"""

\1 and \2 were interpreted as control characters U+0001 and U+0002, so the user 
could see 

make_html = Regex(r"(\w+):(.*?):").sub(r"<^A>^B")

or

make_html = Regex(r"(\w+):(.*?):").sub(r"<☺>☻")

or

make_html = Regex(r"(\w+):(.*?):").sub(r"<>")

depending on the output device.

So this examples proves the usefulness of the new warning.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Aaron Meurer]

Aaron Meurer  added the comment:

I agree with Raymond that third party libraries are not ready for this. 

My biggest issue is that the way Python warns about this makes it very 
difficult for library authors to fix this. Most won't even notice. The problem 
is the warnings are only shown once, when the file is compiled. So if you run 
the code in any way that causes Python to compile it, a further run of 'python 
-Wd' will show nothing. I don't know if it's reasonable, but it would be nice 
if Python recompiled when given -Wd, or somehow saved the warning so it could 
be shown if that flag is given later.

As an anecdote, for SymPy's CI, we went through five (if I am counting 
correctly) iterations of trying to test this. Each of the first four were 
subtly incorrect, until we finally managed to find the correct one (for 
reference, 'python -We:invalid -m compileall -f -q module/').  So most library 
authors who will attempt to add tests against this will get it wrong. Simply 
adding -Wd as you would expect is wrong. If the code is already compiled (which 
it probably is, e.g., if you ran setup.py), it won't show the warnings. At the 
very least the "correct" way to test this should be documented. 

Things would probably be improved if the warnings were always shown, as at 
least then devs will see the error once (although most will probably be 
confused when the warning doesn't repeat).

Another problem is the information in the warnings. It seems the line number of 
the string is now shown, which is an improvement 
(https://bugs.python.org/issue28128). It would be nice if it showed the actual 
line and column number in the file of the invalid escape. This is especially 
annoying when an escape appears in a docstring. It just shows """ as the 
offending line. 

We have a lot of LaTeX in our docstrings in SymPy, so we had quite a few of 
these to fix. SymPy doesn't have invalid escapes anymore because I was 
proactive about it, but from what I've seen, most library authors haven't been.

By the way, this looks like a bug (python 3.8b1):

$ cat test.py

"""
a
\p
"""
$ python -We:invalid test.py
  File "test.py", line 2













  












  










  

[issue32912] Raise non-silent warning for invalid escape sequences

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I'm seeing these warnings pop up from time to time with various third party 
packages (such as pyparsing which is invoked by ensurepip).

Am wondering whether this should be deferred for another version or so until be 
have good confidence that the major third party packages have all had a chance 
to adapt.  Otherwise, this will mostly be noise for end-users.  Also, it gets 
in the way of the end-user strategy of "backslash anything that looks special" 
as preferred over "remember exactly which things are special".  This may be a 
case of "the cure is worse than the disease".

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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset 6543912c90ffa579dc4c01e811f9609cf92197d3 by Serhiy Storchaka in 
branch 'master':
bpo-32912: Replace a DeprecationWarning with a SyntaxWarning (GH-9652)
https://github.com/python/cpython/commit/6543912c90ffa579dc4c01e811f9609cf92197d3


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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Since the author of PR 5849 had not the time to work on it, I have created PR 
9652 which properly replaces a DeprecationWarning with a SyntaxWarning. It also 
updates the documentation.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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pull_requests: +9043

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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

PR 5849 changes not only the Python parser, but codecs. It shouldn't. The 
Python parser and codecs will go different ways. The warning in the Python 
parser will finally be upgraded to SyntaxError (it is already replaced with 
SyntaxError if the warning is raised as error). The warning in codecs will 
become UnicodeDecodeError or ValueError. This is why the code for emitting the 
warning is not shared between the parser and codecs at first place.

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[issue32912] Raise non-silent warning for invalid escape sequences

[Emanuel Barry]

Change by Emanuel Barry :


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keywords: +patch
pull_requests: +5625
stage: needs patch -> patch review

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[issue32912] Raise non-silent warning for invalid escape sequences

[Nick Coghlan]

Nick Coghlan  added the comment:

+1 for the DeprecationWarning->SyntaxWarning->SyntaxError approach from me 
(especially as 3.7 will make the existing deprecation warning visible in 
interactive shells and __main__ modules by default).

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[issue32912] Raise non-silent warning for invalid escape sequences

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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nosy: +ncoghlan, r.david.murray, serhiy.storchaka

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[issue32912] Raise non-silent warning for invalid escape sequences

[Emanuel Barry]

New submission from Emanuel Barry :

This is a follow-up to Issue27364.

Back in Python 3.6, a silent warning was added for all invalid escape sequences 
in str and bytes. It was suggested that it would remain a silent warning (which 
still impacts tests, while not visually annoying the average user) for two 
releases (3.6 and 3.7), then would be upgraded to a non-silent warning for two 
subsequent releases (3.8 and 3.9) before becoming a full-on syntax error. With 
the 3.7 feature freeze on and going, I think it's time to evaluate the approach 
we take for 3.8 :)

I suggest upgrading the DeprecationWarning to a SyntaxWarning, which is visible 
by default, for 3.8 and 3.9. I have cross-linked #27364 to this issue as well.

-Em

--
components: Interpreter Core
messages: 312575
nosy: ebarry
priority: normal
severity: normal
stage: needs patch
status: open
title: Raise non-silent warning for invalid escape sequences
type: behavior
versions: Python 3.8

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