[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Serhiy Storchaka added the comment:


New changeset 89a310264000a613b7f6abd5916946aef09ae7d2 by Serhiy Storchaka in 
branch '3.6':
[3.6] bpo-29104: Fixed parsing backslashes in f-strings. (GH-490) (#1812)
https://github.com/python/cpython/commit/89a310264000a613b7f6abd5916946aef09ae7d2


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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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pull_requests: +1898

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Serhiy Storchaka added the comment:


New changeset 0cd7a3f196cf34d9bb0a52e61327f7fe289d9750 by Serhiy Storchaka in 
branch 'master':
bpo-29104: Fixed parsing backslashes in f-strings. (#490)
https://github.com/python/cpython/commit/0cd7a3f196cf34d9bb0a52e61327f7fe289d9750


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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Mariatta Wijaya]

Changes by Mariatta Wijaya :


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stage:  -> patch review

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Eric V. Smith]

Eric V. Smith added the comment:

This is on my list of things to review at PyCon, when I should finally have 
some free time.

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Eric?

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Ping again.

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Thank you Ryan. I already created a PR, but I think your recipe can be helpful 
for me in future.

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Jim Fasarakis-Hilliard]

Jim Fasarakis-Hilliard added the comment:

A PR has been submitted, Ryan. See https://github.com/python/cpython/pull/490

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Ryan Gonzalez]

Ryan Gonzalez added the comment:

Serhiy: if you want, you can give me your email and I'll submit a PR for you. 
If you want to do it yourself, just:

Download hub: https://hub.github.com/

git clone python/cpython cpython-git
cd cpython-git
git fork
curl 
https://raw.githubusercontent.com/mozilla/moz-git-tools/master/hg-patch-to-git-patch
 > hg-patch-to-git-patch
curl https://bugs.python.org/file46127/fstring_backslash_2.patch | python2 
hg-patch-to-git-patch > fstring.patch
git checkout -b fstring-fix
git apply fstring.patch
git commit -am 'bpo-29104: Commit message here'
git push -u MY_GITHUB_USERNAME fstring-fix
git pull-request

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nosy: +refi64

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Eric V. Smith]

Eric V. Smith added the comment:

Serhiy: I plan to find time in the next week or so to look at this. Sorry for 
the delay. Plus, the new workflow isn't helping me out: I need to get up to 
speed on it.

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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pull_requests: +403

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I am not experienced with git and am waiting until new workflow be described in 
the devguide.

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Jim Fasarakis-Hilliard]

Jim Fasarakis-Hilliard added the comment:

Serhiy, since review activity has dropped on b.p.o now that the move to github 
was made, would you like to make this into a pull request to get it reviewed 
and merged faster?

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Ping.

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Updated patch adds a warning.

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Added file: http://bugs.python.org/file46127/fstring_backslash_2.patch

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[STINNER Victor]

STINNER Victor added the comment:

> There is no issue with f'\\{10}'. There is an issue with f'\{10}'.

Hum, I'm not sure that my previous comment is explicit: I suggest to raise a 
syntax error on f'\{10}' (or emit a warning, and raise an error in 3.7 if you 
insist ;-)).

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

> f'\\N{AMPERSAND}': reading a local variable looks like a typo or a security 
> vulnerability, rather than a nice feature.

This can look as a typo, but how would you write this if this is not a typo? 
f'\\N{AMPERSAND}' is legitimate syntax, and it would be weird to interpret this 
expression in any other way than as '\\' + 'N' + format(AMPERSAND).

> What is the issue with having to write "\\{10}" to get "\\10" string? It's 
> less error prone.

There is no issue with f'\\{10}'. There is an issue with f'\{10}'.

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[STINNER Victor]

STINNER Victor added the comment:

The PEP 498 should be updated to define the expected behaviour of f'\{5}':
https://www.python.org/dev/peps/pep-0498/#escape-sequences

+self.assertEqual(f'\{6*7}', '\\42')
+self.assertEqual(f'\\{6*7}', '\\42')
+self.assertEqual(fr'\{6*7}', '\\42')
+
+AMPERSAND = 123
+self.assertEqual(f'\N{AMPERSAND}', '&')
+self.assertEqual(f'\\N{AMPERSAND}', '\\N123')
+self.assertEqual(fr'\N{AMPERSAND}', '\\N123')
+self.assertEqual(f'\\\N{AMPERSAND}', '\\&')

I'm not sure that I like this behaviour.

f'\\N{AMPERSAND}': reading a local variable looks like a typo or a security 
vulnerability, rather than a nice feature.

IMHO if you want an anti-slash (\) in the output string, it *must* be written 
"\\" since we *do* interpret other escape sequences:

* f'\n' returns '\n', the newline character U+000A
* f'\N{Snowman}' returns the nice snowman character U+2603 (☃)
* etc.

What is the issue with having to write "\\{10}" to get "\\10" string? It's less 
error prone.

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nosy: +haypo

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Proposed patch fixes parsing backslashes in f-strings.

P.S. Definitely we should find other name for this term. I hate misleading 
"f-string".

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keywords: +patch
Added file: http://bugs.python.org/file46080/fstring_backslash.patch

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Eric V. Smith]

Eric V. Smith added the comment:

This problem was no doubt introduced after 3.6a1 when I changed the parsing to 
disallow backslashes inside {}.

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Eric V. Smith]

Eric V. Smith added the comment:

Yes:

>>> f'\{2*5}'
'\\{10'

I agree '\\10' would make sense.

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

The problem is not that the trailing } is dropped, but that the starting { 
starts an f-string expression.

>>> f'\{2*5}'
'\\{10'

I expected either '\\10' as in '\{}'.format(2*5), or at least '\\{2*5}'.

There is other f-string parsing error:

>>> f'\\N{2*5}'
'\\N{2*5}'

I expected '\\N10'. '\\N' doesn't start a unicode name escape. This is a 
legitimate expression.

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nosy: +serhiy.storchaka

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Jim Fasarakis-Hilliard]

Jim Fasarakis-Hilliard added the comment:

Yes, should've attached in my previous message. See 
http://stackoverflow.com/questions/41330097/why-does-the-symbol-remain-when-f-10-is-evaluated-in-python-3-6

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Eric V. Smith]

Eric V. Smith added the comment:

Do you have a link to the SO question?

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Jim Fasarakis-Hilliard]

Jim Fasarakis-Hilliard added the comment:

I see, the original "complaint" about this behavior on stack overflow was made 
due to the discrepancy between the f-string the the format "equivalent": 
'\{}'.format(10)'.

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Eric V. Smith]

Eric V. Smith added the comment:

But I admit that dropping the trailing } seems odd. I think this particular 
usage should be an error: no bare } allowed, similar to:

>>> f'10}'
  File "", line 1
SyntaxError: f-string: single '}' is not allowed

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Eric V. Smith]

Eric V. Smith added the comment:

I'm not sure this counts as an error. The backslash means to treat the next 
character literally, which this does. And since \{ is not a valid escape 
sequence, it keeps both characters, exactly like:

>>> '\c'
'\\c'

Furthermore, since unknown escape sequences are deprecated, this usage is going 
to be an error in the future. You can see this now with -Werror:

$ python.exe  -Werror
Python 3.6.0b1+ (3.6:87de1f12c41c+, Sep 16 2016, 07:05:57) [MSC v.1900 64 bit 
(AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> f'\{10}'
DeprecationWarning: invalid escape sequence '\{'
>>> '\c'
DeprecationWarning: invalid escape sequence '\c'
>>>

Since this works as I expect it to now, and since it will become an error in 
the future, I don't think any change is warranted.

--
assignee:  -> eric.smith

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[SilentGhost]

Changes by SilentGhost :


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nosy: +eric.smith
type:  -> behavior

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[issue29104] Left bracket remains in format string result when '\' preceeds it

[Jim Fasarakis-Hilliard]

New submission from Jim Fasarakis-Hilliard:

In short:

>>> f"\{10}"

yields:

"\\{10"

This is reproducible only when `\` precedes the opening bracket, that is:

>>> f"\ {10}"

results in: "\\ 10"

--
components: Interpreter Core
messages: 284249
nosy: Jim Fasarakis-Hilliard
priority: normal
severity: normal
status: open
title: Left bracket remains in format string result when '\' preceeds it
versions: Python 3.6, Python 3.7

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