An ability that reads “Whenever [a player, planeswalker, or battle] is attacked, . . .” triggers if one or more creatures are declared as attackers attacking that player or permanent. It won’t trigger if a creature is put onto the battlefield attacking that player or permanent.
May 3, 2019 · War of the Spark
An ability that reads “Whenever [a player or planeswalker] is attacked, . . .” triggers if one or more creatures are declared as attackers attacking that player or planeswalker. It won’t trigger if a creature is put onto the battlefield attacking that player or planeswalker.
January 25, 2019 · Ravnica Allegiance
An ability that reads "Whenever [a player or planeswalker] is attacked, . . ." triggers if one or more creatures are declared as attackers attacking that player or planeswalker. It won't trigger if a creature is put onto the battlefield attacking that player or planeswalker.
August 25, 2017 · Commander 2017
An ability that reads “Whenever [a player or planeswalker] is attacked, . . .” triggers if one or more creatures are declared as attackers attacking that player or planeswalker. It won’t trigger if a creature is put onto the battlefield attacking that player or planeswalker.
September 30, 2016 · Kaladesh
A creature that’s put onto the battlefield attacking isn’t affected by requirements or restrictions that apply to the declaration of attackers.