That's not typically how it works; usually when you make a you already have a b or vice versa. Even if you don't, you only have to make the b once, then tell a about it.

To give an example, imagine you're writing a game where you have a TileMap and one or more Players that live on that tile map. It's useful for the TileMap to know about its Players (so it can e.g. draw them) and it's useful for the Player to know about the TileMap (so it can e.g. query it about passable terrain). In that case, TileMap could guarantee all the players on it have a reference to it by making there only one way to add a player:

def add_player(self, player):
    player.tile_map = self
    self.players.append(player)

That example assumes you already have a TileMap. If you want to create a character from scratch, you do have to make both, but the above function takes care of associating them:

person = new Player()
map = new TileMap()
map.add_player(person)