[3.7.5 x86_64 Linux] f-string parsing results in EOL

[yingw787]

BPO	38568
Nosy	@ericvsmith, @zware, @yingw787

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-10-23.19:17:37.480>
created_at = <Date 2019-10-23.19:09:14.827>
labels = ['interpreter-core', 'invalid', 'type-bug', '3.7']
title = '[3.7.5 x86_64 Linux] f-string parsing results in EOL'
updated_at = <Date 2019-10-29.22:03:15.804>
user = 'https://github.com/yingw787'

bugs.python.org fields:

activity = <Date 2019-10-29.22:03:15.804>
actor = 'eric.smith'
assignee = 'none'
closed = True
closed_date = <Date 2019-10-23.19:17:37.480>
closer = 'zach.ware'
components = ['Interpreter Core']
creation = <Date 2019-10-23.19:09:14.827>
creator = 'yingw787'
dependencies = []
files = []
hgrepos = []
issue_num = 38568
keywords = []
message_count = 6.0
messages = ['355254', '355255', '355256', '355259', '355260', '355684']
nosy_count = 4.0
nosy_names = ['eric.smith', 'zach.ware', 'Hobson.Lane', 'yingw787']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38568'
versions = ['Python 3.7']

[yingw787]

Hey,

I encountered an interesting bug when trying to do string parsing using f-strings. I am currently under the impression that within the curly braces is any expression that can be successfully evaluated in a REPL. Here's the error:

yingw787@yingw787-Oryx-Pro:~/src/gpudb-dev-v6.2.0/kio/kio/tests/regression/_data/csv$ python
Python 3.7.5 (default, Oct 15 2019, 21:38:37) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> blah = '"hi"'
>>> blah
'"hi"'
>>> f'{blah.strip('"')}'
  File "<stdin>", line 1
    f'{blah.strip('"')}'
                       ^
SyntaxError: EOL while scanning string literal
>>> '{0}'.format(blah.strip('"'))
'hi'
>>> 

I can use '.format()' for now, but it might be nice to align f-string expr behavior w/ .format(). Please let me know if you need more reproduction steps, or if you need a helping hand :)

Thanks
Ying

[zware]

This is because you're trying to use the string's delimiting character (') in the string itself, which won't work for fairly apparent reasons :). Try removing the f prefix and see what happens.

You can get around this by using a triple-quoted string which will allow you to use both quote characters within the string:

>>> blah = '"hi"'
>>> f'''{blah.strip('"')}'''
'hi'

[HobsonLane]

Not a bug. If you use quotes inside an f-string (or any other kind of
string) they need to be escaped.
--Hobson

On Wed, Oct 23, 2019 at 12:09 PM Ying Wang <report@bugs.python.org> wrote:

New submission from Ying Wang <yingw787.personal@gmail.com>:

Hey,

I encountered an interesting bug when trying to do string parsing using
f-strings. I am currently under the impression that within the curly braces
is any expression that can be successfully evaluated in a REPL. Here's the
error:

yingw787@yingw787-Oryx-Pro:~/src/gpudb-dev-v6.2.0/kio/kio/tests/regression/_data/csv$
python
Python 3.7.5 (default, Oct 15 2019, 21:38:37)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> blah = '"hi"'
>>> blah
'"hi"'
>>> f'{blah.strip('"')}'
  File "<stdin>", line 1
    f'{blah.strip('"')}'
                       ^
SyntaxError: EOL while scanning string literal
>>> '{0}'.format(blah.strip('"'))
'hi'
>>>

I can use '.format()' for now, but it might be nice to align f-string expr
behavior w/ .format(). Please let me know if you need more reproduction
steps, or if you need a helping hand :)

Thanks
Ying

----------
components: Interpreter Core
messages: 355254
nosy: yingw787
priority: normal
severity: normal
status: open
title: [3.7.5 x86_64 Linux] f-string parsing results in EOL
type: behavior
versions: Python 3.7

---

Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue38568\>

---

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New-bugs-announce@python.org
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[HobsonLane]

It is a limitation of f-strings, though. We're not allowed to use
backslashes within the f-string expression in curly braces. So to do what
you want you have to create another variable containing the quote character:

>>> blah = '"hi"'
>>> blah
'"hi"'
>>> q = '"'
>>> f'{blah.strip(q)}'
'hi'

--Hobson

On Wed, Oct 23, 2019 at 12:21 PM Hobson Lane <hobson@totalgood.com> wrote:

Not a bug. If you use quotes inside an f-string (or any other kind of
string) they need to be escaped.
--Hobson

On Wed, Oct 23, 2019 at 12:09 PM Ying Wang <report@bugs.python.org> wrote:

>
> New submission from Ying Wang <yingw787.personal@gmail.com>:
>
> Hey,
>
> I encountered an interesting bug when trying to do string parsing using
> f-strings. I am currently under the impression that within the curly braces
> is any expression that can be successfully evaluated in a REPL. Here's the
> error:
>
> ```bash

yingw787@yingw787-Oryx-Pro:~/src/gpudb-dev-v6.2.0/kio/kio/tests/regression/_data/csv$
python
Python 3.7.5 (default, Oct 15 2019, 21:38:37)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.

blah = '"hi"'
blah
'"hi"'
f'{blah.strip('"')}'
File "", line 1
f'{blah.strip('"')}'
^
SyntaxError: EOL while scanning string literal
'{0}'.format(blah.strip('"'))
'hi'

>
> I can use '.format()' for now, but it might be nice to align f-string
> expr behavior w/ .format(). Please let me know if you need more
> reproduction steps, or if you need a helping hand :)
>
> Thanks
> Ying
>
> ----------
> components: Interpreter Core
> messages: 355254
> nosy: yingw787
> priority: normal
> severity: normal
> status: open
> title: [3.7.5 x86_64 Linux] f-string parsing results in EOL
> type: behavior
> versions: Python 3.7
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue38568\>
> _______________________________________
> _______________________________________________
> New-bugs-announce mailing list
> New-bugs-announce@python.org
> https://mail.python.org/mailman/listinfo/new-bugs-announce
>

[yingw787]

Oh cool, I didn't know triple quotes can be used as part of f-strings! Thanks for the explanation!

[ericvsmith]

It is a limitation of f-strings, though. We're not allowed to use
backslashes within the f-string expression in curly braces.

I've considered relaxing these restrictions, although it's very complicated. The parser would need to become aware of what's inside an f-string (the braces, the expressions, the optional format spec, the doubled braces, escaped characters, etc.). See PEP-536 for a rough outline. The = format for "debugging" with f-strings (see bpo-36817) also makes this more complicated.