How hot does it get in Paris?

If you are unlucky enough to come during a canicule (very hot weather) it can go as high as 28 C to 30C and more. Paris is not a city built for hot weather.
Usual weather is milder, around 24 C and it gets cooler at night, around 20 C, totally acceptable and no reason to buy an AC unless you are unlucky enough to live at the top of a building. It is still very rare in Paris except maybe in shops and offices.