When the bus's left blinker comes on, it has priority — not you.
On urban roads with a speed limit of 50 km/h or lower: when a bus is at a stop and turns on its left blinker, it has priority when departing. You approaching from behind must slow and yield. On roads where the limit is higher than 50 km/h, normal rules apply, the bus yields to you.
You're driving 40 km/h on a city street. A city bus is at a stop on the right, blinking left. You slow down, let the bus pull out, then follow. Pressing past on the left while the blinker is on is illegal.
Giving way to a bus signaling to leave a stop is a statutory yield rule (Traffic Ordinance). The exception applies only in acute danger, for example if traffic behind you is too close to brake safely. In all other cases, you must stop and let the bus go as soon as it is safe to do so.
You must yield to a bus signaling to pull out from a stop in a 30 or 50 km/h zone. Failing to do so can result in a fine. If an accident occurs, you're liable even if you were driving at the correct speed and in the correct lane.