Which Formula 1 driver switched teams in the middle of the 2017 season?

Carlos Sainz Jr. switched teams in the middle of the 2017 Formula 1 season because he was signed as a loanee by Renault. Sainz started the season driving for the Toro Rosso team, but due to Renault's dissatisfaction with their current driver Jolyon Palmer's performance, they negotiated with Toro Rosso to release Sainz from his contract early. This allowed Sainz to join Renault for the final few races of the 2017 season, where he could gain experience with his new team before starting the 2018 season with them.