Premier League Predictor: FastAPI & Redis — Chapter 10, Exercise 2
==================================================================

TASK
Call the create-fixture route with a team playing itself, with a
nonexistent season, and request an empty but valid gameweek's state.
Report the status code and body for each, and whether anything was left
behind in Redis.

SOLUTION

Team playing itself:
  POST /api/gameweeks/2/9/fixtures  {"home_team_id": 3, "away_team_id": 3}
  -> 400 {'error': 'a team cannot play itself'}
  used_teams for gameweek 2/9: empty set (nothing claimed)

Nonexistent season:
  POST /api/gameweeks/99/1/fixtures {"home_team_id": 1, "away_team_id": 2}
  -> 404 {'error': 'no such season'}
  EXISTS gameweek:99:1:used_teams -> 0 (nothing created)

Empty but valid gameweek (season 2 exists; gameweek 30 has no fixtures):
  GET /api/gameweeks/2/30
  -> 200, fixtures: [], used_team_ids: []

The first two are refused before any write, so nothing is left in Redis.
The season check is what stops a typo'd season ID from silently creating
a used_teams set, a fixture and a gameweek entry for a season that
doesn't exist - the missing-foreign-key problem from Chapter 2 again.

The third is deliberately a success: the season is real, the gameweek is
simply empty, and the page needs a 200 with empty lists to render "No
fixtures entered yet".

WHY THIS WORKS AS AN ANSWER
----------------------------
It distinguishes the three cases by status code, confirms with real
checks that the refusals leave nothing behind, and explains why the
empty gameweek is a 200 rather than a 404.