[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[miss-islington]

miss-islington  added the comment:


New changeset e3ce3bba9277a7c4cfde5aaf6269b6c68f334176 by Miss Islington (bot) 
in branch '3.9':
bpo-40847: Consider a line with only a LINECONT a blank line (GH-20769)
https://github.com/python/cpython/commit/e3ce3bba9277a7c4cfde5aaf6269b6c68f334176


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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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

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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[miss-islington]

Change by miss-islington :


--
nosy: +miss-islington
nosy_count: 7.0 -> 8.0
pull_requests: +19991
pull_request: https://github.com/python/cpython/pull/20795

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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:


New changeset 896f4cf63f9ab93e30572d879a5719d5aa2499fb by Lysandros Nikolaou in 
branch 'master':
bpo-40847: Consider a line with only a LINECONT a blank line (GH-20769)
https://github.com/python/cpython/commit/896f4cf63f9ab93e30572d879a5719d5aa2499fb


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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Brett Cannon]

Change by Brett Cannon :


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nosy:  -brett.cannon

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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Lysandros Nikolaou]

Change by Lysandros Nikolaou :


--
keywords: +patch
pull_requests: +19967
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/20769

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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Guido van Rossum]

Guido van Rossum  added the comment:

To be clear, I consider it a bug.

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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Adam Williamson]

Adam Williamson  added the comment:

I'm not the best person to ask what I'd "consider" to be a bug or not, to be 
honest. I'm just a Fedora packaging guy trying to make our packages build with 
Python 3.9 :) If this is still an important question, I'd suggest asking the 
folks from the Black issue and PR I linked to, that's the "real world" case if 
any.

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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Guido van Rossum]

Guido van Rossum  added the comment:

Sure looks like a tokenizer issue to me. For example this is broken in both 
versions:

pass
\

pass

It complains about an unexpected indent, but it should really be considered a 
blank line broken in two -- a backslash is supposed to just erase itself and 
the following newline.

https://docs.python.org/3/reference/lexical_analysis.html#explicit-line-joining

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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Lysandros Nikolaou]

Lysandros Nikolaou  added the comment:

This is limited to cases where the line continuation character is on an 
otherwise empty line. For example this works correctly:

$ cat t.py
print("hello world")
print("hello world 2") \

print("hello world 3")
$ ./python.exe t.py
hello world
hello world 2
hello world 3

The actual problem is at the tokenizer level, where a line with only a 
continuation character is not considered an empty line and thus two NEWLINE 
tokens get emitted, one after the other. The old parser was somehow working 
around this, probably by having this in the grammar:

file_input: (NEWLINE | stmt)* ENDMARKER

The PEG parser OTOH does not allow this.

The question now is, is it reasonable to change the tokenizer to consider a 
lone backslash an empty line? Do you also consider this a bug? Or should we 
change the new parser?

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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


--
nosy: +gvanrossum, lys.nikolaou

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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Christian Heimes]

Change by Christian Heimes :


--
nosy: +christian.heimes

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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Christian Heimes]

Change by Christian Heimes :


--
nosy: +benjamin.peterson, brett.cannon, pablogsal, yselivanov
versions: +Python 3.10

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[issue40847] New parser considers empty line following a backslash to be a syntax error, old parser didn't

[Adam Williamson]

New submission from Adam Williamson :

While debugging issues with the black test suite in Python 3.9, I found one 
which black upstream says is a Cpython issue, so I'm filing it here.

Reproduction is very easy. Just use this four-line tester:

print("hello, world")
\

print("hello, world 2")

with that saved as `test.py`, check the results:

 sh-5.0# PYTHONOLDPARSER=1 python3 test.py
hello, world
hello, world 2
 sh-5.0# python3 test.py
  File "/builddir/build/BUILD/black-19.10b0/test.py", line 3

^
SyntaxError: invalid syntax

The reason black has this test (well, a similar test - in black's test, the 
file *starts* with the backslash then the empty line, but the result is the 
same) is covered in https://github.com/psf/black/issues/922 and 
https://github.com/psf/black/pull/948 .

--
components: Interpreter Core
messages: 370618
nosy: adamwill
priority: normal
severity: normal
status: open
title: New parser considers empty line following a backslash to be a syntax 
error, old parser didn't
type: behavior
versions: Python 3.9

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