[HL33tibCe7]

> > We may be looking too deep into this but it seems like many developers think when string concatenation occurs it’s enough to declare the first string as an f-string and the other strings are turned into f-strings by osmosis. It doesn’t. We’re not suggesting this is the case for all developers that accidentally did this error, but interesting nonetheless.

I highly doubt that people believed that f-strings worked this way. Far more likely is that, for example, the expression started as one line, then got split onto two, or some such similar scenario.

[johnny_castaway]

This may be also caused by confusing syntax highlighting in some editors, for example in VSCode [1]

The variable in the second string gets highlighted (with slightly different color, but still) because it would still work with `str.format()`. GitHub doesn't seem to do this.

[1] https://imgur.com/a/9KGWVG0

[totony]

Yeah I got bitten by the same-color syntax highlighting a few times.

[_8j50]

You'd be surprised, people who expect python to be "smart" and "figure it out" might think that way.

[groestl]

Well, it's not completely unlogical.

'a' is str

b'a' is bytes

f'something' might be a separate f-str-type too?

1 is an int

1.2 is a float

(1.2 + 1) is a float

[GeorgeTirebiter]

Indeed, if "{value} is bad" can be automatically f-stringed by an external program automatically --- then why can't Python do this automatically -- so we can get rid of the f-string type as a required explicit declaration? After all, we don't specifically add a type to a number like 42 or 3.14159 --- those are implicitly 'int' and 'float' types.

I would use such a feature, as I always use f-strings when formatting.

[masklinn]

> Indeed, if "{value} is bad" can be automatically f-stringed by an external program automatically --- then why can't Python do this automatically -- so we can get rid of the f-string type as a required explicit declaration?

Because it would break existing strings containing braces, such as those used with `str.format`, or string.Template, or literal Jinja templates, ...

[GeorgeTirebiter]

Yes, my use case was "I always use F-strings" so these other breakages could not occur, by definition.

I suppose it's not that much of a problem to run a program to preprocess source and add in the F

But.. Python has a history of introducing new features that break old ones. That seems to me a balance between backward compatibility and future goodness.

the "from future import auto-fstring" construct could do it...

And, as for having to run the f-string parser: yes, but only once on static strings, which are most of them.

[chrisshroba]

It's not clear to me that folks would want this behavior globally - I personally would not, because sometimes I want to be able to include curly braces in my strings without it being interpolated, and I prefer the current syntax of having that just work. I'm very comfortable with having to add the 'f' to the string syntax to declare that this particular string should be interpolated.

There are other issues you'd have to deal with as well. Would you interpolate non-literal strings (e.g. in the code `print(input())`, if the user inputs the string "{1+1}", what would be printed?)? How would you propose dealing with jinja and other strings that typically contain curly braces that should not be interpolated? This could also be an issue with (potentially long) strings containing lots of random characters: if a '}' showed up in a string somewhere after a '{', even if separated by tens or hundreds of characters, then you'll run into errors. This could be a problem if you're dealing with pseudorandom or base-92 encoded strings, or even just strings representing code (imagine a python library that generates C++ or Java code which has lots of hard coded strings with braces).

I think overall, having to specify that a particular string should be interpolated is a better solution than having to specify that a string should not be interpolated.

[masklinn]

> Yes, my use case was "I always use F-strings" so these other breakages could not occur, by definition.

Unless you're using any dependency at all, including the standard library.

> But.. Python has a history of introducing new features that break old ones.

It doesn't tho.

> That seems to me a balance between backward compatibility and future goodness.

That assumes "everything is an fstring" is considered "future goodness", which I'm not sure is a widely shared view.

> the "from future import auto-fstring" construct could do it...

That seems unlikely as the __future__ pseudo-package has generally been used to opt into hopefully future behaviour.

Given the Python 3 experience, somehow, I don't see "let's make all string literals into fstrings" happen any time soon, but hey feel free to create a PEP proposing that.

Alternatively, create your own import hook which does this swap before handing the module off of to the compiler.

[dragonwriter]

If all strings with what looks like format specs are implicitly f-strings, how do you use reusable template strings, which use the same basic internal syntax, for formatting?

f-strings, like r-strings, have uses, but, like r-strings, I wouldn't want to replace plain strings with them.

[ArchOversight]

Because in some cases it is not desired, if for example the string is used in something that expands it further down the line (think SQL injection potential).

[bagels]

Backwards compatibility for one. Code existed before fstrings that may use curly braces, and you can currently use curly braces in non fstrings without escaping them.

Might also be a performance penalty for always having to run the fstring parser.

[HL33tibCe7]

As a beginner, maybe. But this is code in some of the biggest open-source Python repos in existence. Probably written by someone with a reasonable level of Python expertise.

[genewitch]

... and yet?

[groestl]

I think what's happening is that people assume there's type coercion going on here.

[jldugger]

I wonder if an autoformatter like black is at play here.

[saila]

Black doesn't split strings, and I doubt they'd choose to use concatenation if they did.

[hathawsh]

I kind of wish Black would split and join literal strings. I've seen several times when Black converted code like this:

    raise SomeError(
        "Explanation... "
        "yet more explanation"
    )

To this:

    raise SomeError(
        "Explanation... " "yet more explanation"
    )

That just looks odd. Black is fantastic, but not perfect.

[d0mine]

Black does split strings if configured and it does use implicit concatenation of Python string literals.

[saila]

There's an experimental option for splitting long strings using the --preview flag, but it still seems to have some potential issues. That seems like it would be extremely hard to solve correctly.

[xigoi]

Black doesn't change the semantics of code.