[New-bugs-announce] [issue33754] f-strings should be part of the Grammar

[David Halter]

New submission from David Halter <davidhalter88 at gmail.com>:

Currently f-strings are a bit of a hack. They certainly work very well for users, but they are implemented in ast.c and therefore not part of the Python grammar and the tokenizer.

I want to change this. I wrote an alternative implementation of f-strings in parso (http://parso.readthedocs.io/en/latest/). The idea I have is to modify the Python grammar slightly (https://github.com/davidhalter/parso/blob/master/parso/python/grammar37.txt#L149):

fstring: FSTRING_START fstring_content* FSTRING_END
fstring_content: FSTRING_STRING | fstring_expr
fstring_conversion: '!' NAME
fstring_expr: '{' testlist [ fstring_conversion ] [ fstring_format_spec ] '}'
fstring_format_spec: ':' fstring_content*

We would push most of the hard work to the tokenizer. This obviously means that we have to add a lot of code there. I wrote a tokenizer in Python for parso here: in https://github.com/davidhalter/parso/blob/master/parso/python/tokenize.py. It is definitely working well. The biggest difference to the current tokenizer.c is that you have to work with stacks and be way more context-sensitive.

There were attempts to change the Grammar of f-strings like https://www.python.org/dev/peps/pep-0536/. It hasn't caught on, because it tried to change the semantics of f-strings. The implementation in parso has not changed the semantics of f-strings.

In a first step I would like to get this working for CPython and not tokenize.py. Modifying tokenize.py will not be part of my initial work here.

I have discussed this with Łukasz Langa, so if you guys have no objections I will start working on it. Please let me know if you support this or not.

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components: Interpreter Core
messages: 318547
nosy: davidhalter
priority: normal
severity: normal
status: open
title: f-strings should be part of the Grammar
type: enhancement
versions: Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33754>
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