Make sure that we forbid all f-string formatting

[sobolevn]

https://fstrings.wtf/

We need to be sure that no formatting for f strings are allowed. Maybe except = part.

In case it is already disallowed, we need to improve tests for it.

[Tapeline]

Apart from f-strings, we should also disallow heavy formatting rules for all other types of formatting (str.format and %).

Also, we should discuss, what to exclude from banlist. E.g. f"{x:.2f}" is pretty neat and I don't think we can find a better approach to it.

Another option would be, rather than banning some elements explicitly, to introduce a complexity measurement for formats, and then treat it as any other complexity rule out there.

[dolfinus]

That's the point of forbidding the stdlib formatting features? This rule will be either added to ignore immediately, or some users will tend to write their own formatting functions, probably introducing even more bugs in the process.

Could you please describe in details "why" it should be covered with a rule, other than just providing a link to some quiz?

[sobolevn]

Because it is very hard! Most of python developers I asked about this feature - had no idea how it worked. If no one (including myself) understands how it works - it should not be used, it is overly complex.

That's the point of forbidding the stdlib formatting features?

This is the whole point of WPS. We try to only keep the good parts.

[dolfinus]

I don't see how prohibiting string formatting will improve anything. If someone needs to format string in specific way, they will do it anyway, and just ignore the rule. What's the point of adding the rule which will be ignored in any case? The str.format syntax is exactly the same, so throwing away f-strings for str.format will not change anything at all.