Closes https://github.com/astral-sh/ruff/issues/6442
Python string formatting like `"hello {place}".format(place="world")`
supports format specifications for replaced content such as `"hello
{place:>10}".format(place="world")` which will align the text to the
right in a container filled up to ten characters.
Ruff parses formatted strings into `FormatPart`s each of which is either
a `Field` (content in `{...}`) or a `Literal` (the normal content).
Fields are parsed into name and format specifier sections (we'll ignore
conversion specifiers for now).
There are a myriad of specifiers that can be used in a `FormatSpec`.
Unfortunately for linters, the specifier values can be dynamically set.
For example, `"hello {place:{align}{width}}".format(place="world",
align=">", width=10)` and `"hello {place:{fmt}}".format(place="world",
fmt=">10")` will yield the same string as before but variables can be
used to determine the formatting. In this case, when parsing the format
specifier we can't know what _kind_ of specifier is being used as their
meaning is determined by both position and value.
Ruff does not support nested replacements and our current data model
does not support the concept. Here the data model is updated to support
this concept, although linting of specifications with replacements will
be inherently limited. We could split format specifications into two
types, one without any replacements that we can perform lints with and
one with replacements that we cannot inspect. However, it seems
excessive to drop all parsing of format specifiers due to the presence
of a replacement. Instead, I've opted to parse replacements eagerly and
ignore their possible effect on other format specifiers. This will allow
us to retain a simple interface for `FormatSpec` and most syntax checks.
We may need to add some handling to relax errors if a replacement was
seen previously.
It's worth noting that the nested replacement _can_ also include a
format specification although it may fail at runtime if you produce an
invalid outer format specification. For example, `"hello
{place:{fmt:<2}}".format(place="world", fmt=">10")` is valid so we need
to represent each nested replacement as a full `FormatPart`.
## Test plan
Adding unit tests for `FormatSpec` parsing and snapshots for PLE1300
In https://github.com/astral-sh/ruff/pull/6616 we are adding support for
nested replacements in format specifiers which makes actually formatting
strings infeasible without a great deal of complexity. Since we're not
using these functions (they just exist for runtime use in RustPython),
we can just remove them.
## Summary
Relates to #970.
Add new `bad-format-character` Pylint rule.
I had to make a change in `crates/ruff_python_literal/src/format.rs` to
get a more detailed error in case the format character is not correct. I
chose to do this since most of the format spec parsing functions are
private. It would have required me reimplementing most of the parsing
logic just to know if the format char was correct.
This PR also doesn't reflect current Pylint functionality in two ways.
It supports new format strings correctly, Pylint as of now doesn't. See
pylint-dev/pylint#6085.
In case there are multiple adjacent string literals delimited by
whitespace the index of the wrong format char will relative to the
single string. Pylint will instead reported it relative to the
concatenated string.
Given this:
```
"%s" "%z" % ("hello", "world")
```
Ruff will report this:
```Unsupported format character 'z' (0x7a) at index 1```
Pylint instead:
```Unsupported format character 'z' (0x7a) at index 3```
I believe it's more sensible to report the index relative to the
individual string.
## Test Plan
Added new snapshot and a small test in
`crates/ruff_python_literal/src/format.rs`.
---------
Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
