Mark Carney, who holds no seat, is being sworn in as Canada's 24th Prime Minister—By constitutional convention, a prime minister holds a seat in parliament… This is a dangerous precedent for the PMO.

It's not a precedent -- this has already happened several times. John Turner was PM in 1984 without having a seat. Mackenzie King won an election and became PM all while not being an MP in 1926. He was even a seat-less PM for a couple of months again in 1945.

Prior to the convention of standing for election soon after becoming PM being a hard-and-fast thing, John A. McDonald was in a similar position at Confederation in 1867, and so were two more 19th-century PMs, Abbott and Bowell.

Given the way that the Conservatives blew several conventions out of the water last time they were in power (proroguing Parliament inappropriately, and refusing to allow a coalition second crack at forming a government after an election) I agree it would be a good idea to make this a law. But ringing an alarm bell over Carney specifically is a bit too much. The Liberals are already talking about which back-bencher will resign and Carney run: somewhere in the West Island of Montreal looks like a likely candidate as they are super-safe Liberal seats.