Don’t you defy his no-swimming edict, Mayor Bill de Blasio warns. “We’re putting fencing in and in reserve that could close off the beaches as needed.” Funny: He used to think barriers didn’t work.

For example, Blas told Fox News’ Sean Hannity that President Trump’s proposed wall on the Mexican border wouldn’t work, as illegal immigrants will enter the United States “with a wall or without a wall.” Are fences more effective?

Ironically, Hizzoner says the point of his ban is to keep people off the subways — so anyone who owns a car can drive to the beach elsewhere. It’s only those in what he used to call “the other New York” who’ll have to sweat out the Memorial Day weekend.

What a pip.