[Python-Dev] PEP 498: Literal String Interpolation is ready for pronouncement (original) (raw)

Guido van Rossum guido at python.org
Sat Sep 5 18🔞45 CEST 2015

On Sat, Sep 5, 2015 at 2:10 AM, haypo s <victor.stinner at gmail.com> wrote:

2015-09-05 5:01 GMT+02:00 Guido van Rossum <guido at python.org>: > And I'm ready to accept it. I'll wait until Tuesday night (Monday's a > holiday in the US) in case anything unforeseen comes up, but this is really > the Last Call for this PEP.

Would it be possible to specify a subset of the Python language allowed in f-string? For example, import or lambda should not be used in a f-string. I'm not convinced that a loop or list/dict/set-comprehension is a good idea neither.

We already went over this. You might as well argue that import or lambda should not be used as arguments to print(). It's an expression, and it must allow exactly everything that is allowed in other places where expressions are allowed.

Of course you don't have to write those things in f-string interpolations. But that's just a style guide; the language should not try to second-guess the user.

I would prefer to keep as much code as possible outside f-string because: - text editor knows well how to color it - static analyzers know how to analyze it - it encourage developers to indent and comment their code correctly, whereas f-string has more restrictons on indentation

Really, we already went over most of this. You can put whitespace (even newlines) exactly where they are allowed in other expressions, as long as they don't terminate the string. You probably shouldn't do any of those things regularly, but there are lots of other things you that you can do in Python that you shouldn't.

(is it possible to indent and comment code inside a f-string?)

Now that's an interesting question. I think the answer must be No, because we don't want to deal with ambiguities like whether a closing curly bracket or string quote should be ignored inside such comments. The processing of f-strings described by the PEP uses several phases:

The presence of comments would impede several of these stages.

For example, for me it's a common practice to write a complex list-comprehension on two lines for readability:

newlist = [verycomplexexpression(item) for item in oldlist] # sorry, it's hard to indent correctly in a mail client, especially Gmail

The list comprehension across two lines without the comment would be fine in a triple-quoted f-string (or perhaps even in a single-quoted f-string if you put a \ before the line break).

Well, I'm not convinced that we need a larger subset than what is allowed currently in str.format(), simple expressions like: obj.attr, obj[index], etc.

The PEP explains why actually.

I recall horrible examples in the previous mail threads showing how much complex code you can put inside f-string.

Yes, that was a very poorly thought out argument. All of Python should be condemned if you took it seriously.

Even the following example from the PEP seems too complex to me: print(f"Usage: {sys.argv[0]} [{'|'.join('--'+opt for opt in validopts)}]", file=sys.stderr)

Oh, first I read [...] as a list-comprehension :-p But it's part of the output string, not of the Python code... I prefer to build the second parameter outside the f-string: opts = '|'.join('--'+opt for opt in validopts) print(f"Usage: {sys.argv[0]} [{opts}]", file=sys.stderr)

The same criticism can be made for the original version (using .format()), which was lifted from real code.

f-string with complex code remembers me PHP where it was possible to mix PHP and HTML. I have bad memories such... code? template? (I don't know how to cal them). Text editors and me were unable to identify quickly the beginning and end of the code. We have a similar issue with Python unit test embedding Python code to run subprocesses. My text editor vim is unable to identify if the current code is the outter code or the code embedded in a long triple-quoted string (especially when you open the file at the embedded code).

Yeah, tools will need to be updated. There are several other languages (e.g. Ruby) that allow arbitrary expressions in strings and at least some editors have no problem with them, even vim.

Maybe we should start with a simple f-string, play with it during one cycle (Python 3.6), and then discuss again if it's necessary to extend the allowed Python expresions inside f-string?

If you really want to allow any Python inside a f-string, can you please at least explain in the PEP why you consider that it's a good thing?

Sigh. We've gone over this on python-ideas. Your objection is not new.

IMHO the main advantage of allowing expressions inside f-string is that it adds something really new compared to the current str.format() method. Without it, f-string are less interesting.

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