That is because Stroll will turn 18 on the 29th of October, and Williams' primary sponsor is alcohol brand Martini, so they want to defer the announcement until he is of legal drinking age.
Less certain is the identity of his team mate, with a number of drivers in the frame, including current Williams driver Valtteri Bottas.
However, Ex-F1 driver Mika Salo is unsure of whether his compatriot will remain at the team, amid interest from Renault.
"It's difficult to talk about Valtteri's situation," said former Ferrari driver Salo.
"Lance Stroll will be there, and Williams would probably like to keep Valtteri, but (Lance's father Lawrence) Stroll has bought half of the team so it may be him who chooses who is in the other car," he added. "He deserves a place but he may not want Valtteri as his team mate," he added.
Bahrain International Circuit - Winter testing