DISC: use of typing.cast

[simonjayhawkins]

placeholder to address #32730 (comment)

[simonjayhawkins]

@jorisvandenbossche @WillAyd @jbrockmendel to avoid having similar conversations in separate PRs, we can xref back to this issue.

[jbrockmendel]

Is this actionable/closeable? I think I've seen you use typing.cast, and I'm pretty happy deferring to you on typing-related issues.

[simonjayhawkins]

We have has a few regressions caused by using asserts. This is a double edge sword, since we at least identify where our assumptions on types are incorrect.

This was a placeholder for discussion, to avoid the discussion in separate PRs.

but OK with closing for now.