dt_test.py

from textual.app import App, ComposeResult
from textual.widgets import DataTable
from typing import Dict

ROWS = [
    (4, "Joseph Schooling", "Singapore", 50.39),
    (1, "Aleksandr Sadovnikov", "Russia", 51.84),
    (10, "Darren Burns", "Scotland", 51.84),
]

class TableApp(App):
    def __init__(self):
        super().__init__()
        self.rowkeys: Dict[int,RowKey] = {}

    def compose(self) -> ComposeResult:
        yield DataTable()

    def on_mount(self) -> None:
        table = self.query_one(DataTable)
        table.add_columns("lane", "swimmer", "country", "time")
        for i,r in enumerate(ROWS):
            rk = table.add_row(*r)
            self.rowkeys[i] = rk


app = TableApp()
if __name__ == "__main__":
    app.run()

output.txt

$ mypy --check-untyped-defs dt_test.py 
dt_test.py:15: error: Name "RowKey" is not defined  [name-defined]
Found 1 error in 1 file (checked 1 source file)

OK with from textual.widgets._data_table import RowKey added, I get clean output:

$ mypy --check-untyped-defs dt_test.py 
Success: no issues found in 1 source file