[Python-Dev] PEP 498: Literal String Interpolation is ready for pronouncement

[Brett Cannon]

On Sat, 5 Sep 2015 at 09:19 Guido van Rossum <guido at python.org> wrote:

> 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.
>

Right, it's the whole "consenting adults" bit along with trying to minimize
surprise by placing restrictions that are hard to remember. Having the
restriction of "it must be an expression" is pretty minor thing to remember
vs. "it must a specific f-string subset of Python as defined in section
N.N.N of the language spec". One of those fits in my brain and one of them
does not. ;)


>
>
>> 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.
>

As Guido said, we're consenting adults and if it makes the code unreadable
then don't do it. Just like we promote refactoring code to have more
functions to be more readable even though it adds overhead, I suspect we
will end up updating PEP 8 to say that f-strings should tend towards simple
expressions and should never compromise the readability of the code, in
which case the expressions should be hoisted out of the f-string and into
the surrounding code.

I also don't think that we should tie our hands in the design of the
language because it makes some work on behalf of the tool providers. If it
made it impossible then I think we should discuss it, but simply requiring
some more work from tools isn't a sufficient condition to restrict what we
do. That's like saying Guido shouldn't have gone with whitespace-delimited
formatting because most editors didn't support it back in the day and it
took some effort to support it since it wasn't such a simple "find the
matching curly brace" algorithm that applied to most languages.

I appreciate wanting to *promote* making code be readable, I don't think it
should come at the *cost* of the simplicity of the solution when coding
practices can easily make up for any ugliness that is possible.

-Brett


>
>>
>> (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:
>
> - find the end of the string (the final quote[s]) using the same algorithm
> used for all string literals
> - expand \ escapes (e.g. \uXXXX)
> - look for single {, then scan ahead to a matching } -- this skips
> matching () [] {}
> - look for optional !a,!r,!s and !<spec> inside each {...} pair
> - take what's left, enclose it in (), parse as expression
>
> 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 = [very_complex_expression(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
>> valid_opts)}]", 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 valid_opts)
>> 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.
>>
> --
> --Guido van Rossum (python.org/~guido)
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