Multiline expression brackets with format specifiers don't work in f-strings

Bug report

Bug description:

In accordance with PEP 701, the following code works:

>>> x = 1
>>> f"___{
...     x
... }___"
'___1___'

>>> f"___{(
...     x
... )}___"
'___1___'

But the following fails:

f"__{
    x:d
}__"

This gives:

  File "<stdin>", line 1
    x:d
SyntaxError: unterminated f-string literal (detected at line 2)

Is this intended behaviour? This is not clarified in the PEP.

Similarly,

f"""__{
    x:d
}__"""

Gives:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier 'd
' for object of type 'int'

CPython versions tested on:

3.12

Operating systems tested on:

macOS

Linked PRs

Thanks for the report! I have opened #110271 to fix it

pythongh-110259: Fix f-strings with multiline expressions and format …

I'd be careful on that last one. If x is a datetime.datetime, it's a valid format spec.

>>> x=datetime.datetime.now()
>>> f"""__{
...    x:d
... }__"""
'__d\n__'

In case that's not clear: format specs for datetime.datetime can contain any characters, and are passed on to strftime. Perhaps more usefully:

>>> f"""__{
...     x:%Y
... %m
... %d
... }__"""
'__2023\n10\n03\n__'

Hummmm, then what you are saying is that we should not include the newline in the single quoted case but we should do it in the triple quoted case?

Ah, indeed, this is the case because the triple quote case works like that in 3.11:

Python 3.11.1 (main, Jan  3 2023, 20:00:19) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime
>>> f"""__{
...    datetime.datetime.now():%Y
...    %m
...    %d
... }__"""
'__2023\n   10\n   04\n__'

Hummmm, then what you are saying is that we should not include the newline in the single quoted case but we should do it in the triple quoted case?

I don't understand why this is the case. Shouldn't these both lead to the same AST?

Multi-line f-string:

f"""__{
    x:%Y
%m
%d
}__"""

Single-quoted f-string:

f"__{
    x:%Y
%m
%d
}__"

I don't think it's necessarily an issue. I was just pointing out that the error shown in the original report

f"""__{
    x:d
}__"""

(a ValueError from the bad format spec "d\n" for int) is different from the error in

f"__{
    x:d
}__"

(a SyntaxError). In any event, the format spec should be allowed to have newlines, including trailing newlines.

@lysnikolaou

Update: i tried to check how annoying would be to support the other option (support multi-line format specifiers) and turns out that is quite easy to support thanks to how we have structured this (yay!). So only if we want we can support that.

It would look like this, for clarification:


code = """\
f"__{
1
+
1:a
b
c

}__"
"""

print(ast.dump(ast.parse(code), indent=4))

which prints:

Module(
    body=[
        Expr(
            value=JoinedStr(
                values=[
                    Constant(value='__'),
                    FormattedValue(
                        value=BinOp(
                            left=Constant(value=1),
                            op=Add(),
                            right=Constant(value=1)),
                        conversion=-1,
                        format_spec=JoinedStr(
                            values=[
                                Constant(value='a\nb\nc\n\n')])),
                    Constant(value='__')]))],
    type_ignores=[])

Sorry for the back and forth @lysnikolaou @ericvsmith. I think we are free to choose whatever we prefer.

@tusharsadhwani

Notice that this has an important difference that I think it makes is less appealing: extra trailing newlines are included in the format specifier, which I think it will make formatters unhappy (@tusharsadhwani, please confirm)

Can confirm, second option will restrict code formatting, pretty much making fstrings with format specs unformattable.

But, fstrings don't get formatted by black right now as-is and there's no confirmation on my end that that's desired.

gh-110259: Fix f-strings with multiline expressions and format specs (#…

pythongh-110259: Fix f-strings with multiline expressions and format …

@tusharsadhwani

@tusharsadhwani black can't reformat the format specs without breaking datetime.datetime (for example).

@pablogsal I don't think it matters all that much. I can't imagine that there are many single quote multi-line format specs, although they no doubt exist.

Ok, I ended implementing the single-line restruction for format specs in single-quoted f-strings. Thanks everyone for your help, you rock 🚀

[3.12] gh-110259: Fix f-strings with multiline expressions and format…

@ericvsmith

@ericvsmith wasn't single quote multiline format spec invalid syntax until now?

@tusharsadhwani

@tusharsadhwani : I thought somewhere on this thread someone said it worked in 3.11, but I guess I misread that.

Fix lexing single-quoted f-string with multi-line format spec (#7787)

pythongh-110259: Fix f-strings with multiline expressions and format …

Fix lexing single-quoted f-string with multi-line format spec (#7787)

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Multiline expression brackets with format specifiers don't work in f-strings #110259

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Multiline expression brackets with format specifiers don't work in f-strings #110259

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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pablogsal commented on Oct 3, 2023

ericvsmith commented on Oct 3, 2023

ericvsmith commented on Oct 3, 2023

pablogsal commented on Oct 3, 2023

pablogsal commented on Oct 3, 2023

lysnikolaou commented on Oct 5, 2023

ericvsmith commented on Oct 5, 2023

12 remaining items

pablogsal commented on Oct 5, 2023

pablogsal commented on Oct 5, 2023

tusharsadhwani commented on Oct 5, 2023

ericvsmith commented on Oct 5, 2023

pablogsal commented on Oct 5, 2023

tusharsadhwani commented on Oct 5, 2023

ericvsmith commented on Oct 5, 2023

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