f-strings for Berry

[s-hadinger]

Now that I have some decent experience with programming in Berry, I'm am not completely happy with string formatting. string.format() is slightly verbose. I would like to add an equivalent to Python's f-strings.

Example:
string.format("id=%i cluster=0x%04X name=%s", id, cluster, self.name)

would become:
f"id={id:i} cluster={cluster:04X} name={self.name}"

I made a POC that sounds promising. Changing the parser would be a daunting task. Instead, I use a "transpiler" approach. The lexer contains code to detect f-string syntax and rewrite the code to generate the string.format syntax.

The lexer transforms f"id={id:i} cluster={cluster:04X} name={self.name}" to ("id=%i cluster=0x%04X name=%s", id, cluster, self.name) and then just feeds the normal parser.

[sfromis]

Also see discussion on Tasmota Discord.
https://discord.com/channels/479389167382691863/826169659811168276/1120818883713040476

[s-hadinger]

Done: #355

[barbudor]

Hi @s-hadinger
Is that working only as compile-time like in python ? which means that I can't use a string variable ?

I'm looking if there is a way to use format string like in python:

# this is Python, not Berry
# fulltopic = "{prefix}/{topic}/{subtopic}"
fulltopic=string.replace(string.replace(tasmota.cmd("fulltopic")['FullTopic'],"%prefix%","{prefix}"),"%topic%","{topic}")+"{subtopic}"
# publish_topic = "cmnd/devname/power"
publish_topic = fulltopic.format(prefix="cmnd", topic="devname", subtopic="power")
# or
publish_topic = fulltopic.format(**{"prefix":"cmnd", "topic":"devname", "subtopic":"power"})

As you imagine the idea is to use named variable in the format string instead of positional values

Thanks

[s-hadinger]

f-strings are purely compile time synctactic sugar to the format() function. You could however write a helper function that does something similar.

[barbudor]

Yes, indeed I simply string replace:

fulltopic=tasmota.cmd("fulltopic")['FullTopic']

def make_topic(prefix, topic, subtopic)
  return string.replace(string.replace(fulltopic,"%prefix%",prefix),"%topic%",topic) + subtopic
end

[sfromis]

If you really want, you can have non-constant f-strings expanded by going via compile, to get the syntactic sugar sweetness, but that's not very efficient, and does not work well with local vars.

var prefix='stat' var topic='device' var leaf='what'
var fformat=/fst-> compile("return f'" + fst + "'")()
print(fformat('{topic}/{prefix}/{leaf}'))

[s-hadinger]

I wouldn't recommend using compile with dynamic content. This is always dangerous ; like eval() in many languages