ruff_fix can overwrite file correctly but my customized ruff_fix cannot.

[otakutyrant]

From my understanding, ruff check --fix outputs either "All checks passed!" or some errors like this:

test.py:1:1: I001 [*] Import block is un-sorted or un-formatted
  |
1 | / import os
2 | | import abc
  | |__________^ I001
3 |
4 |   os.close
  |
  = help: Organize imports

Found 1 error.
[*] 1 fixable with the `--fix` option.

So I tried to customize the ruff_fix formatter like this:

local conform = {
    "stevearc/conform.nvim",
    event = "BufWritePost",
    opts = {
        formatters_by_ft = {
            -- TODO: replace them with language servers if the later is better.
            html = { "prettier" },
            css = { "prettier" },
            javascript = { "prettier" },
            typescript = { "prettier" },
            json = { "prettier" },
            yaml = { "prettier" },
            markdown = { "prettier" },
            python = {
                "ruff_fix", -- To fix auto-fixable lint errors.
                "ruff_format", -- To run the Ruff formatter.
            }, -- Although ruff is a ls, the official suggests using these formatters
            toml = {}, -- TOMP formatter is provided by taplo ls.
            lua = { "stylua" }, -- lua_ls' primitive formatter is disabled
        },
        format_on_save = {
            -- These options will be passed to conform.format()
            timeout_ms = 500,
            lsp_format = "fallback", -- use vim.lsp.buf.format() when no formatter available.
        },
        formatters = {
            -- Drop `--fix` from ruff_fix formatter in favor of pyproject.toml
            ruff_fix = {
                args = {
                    "check",
                    "--force-exclude",
                    "--exit-zero",
                    "--no-cache",
                    "--stdin-filename",
                    "$FILENAME",
                    "-",
                },
                stdin = true,
            },
        },
    },
}

And when I save any python file, the file content is overwrote as "All checks passed!" or some errors. To my surprise, the origin ruff_fix formatter works well anyway. But how?