Paul,
Functional APIs is, of course, TF specific, but, I suppose, is using basic Python capabilities, i.e, “callable” which makes a class instance callable. Here is an example.
I created a “test” object, and made it “callable” with adding “_call_() method”. (1st cell)
The 2nd cell is to instantiate the object, then, pass “10” in the 3rd cell. These two steps can be written in a single like just like the one in the 4th cell.
This is a same behavior as
A1 = tf.keras.layers.ReLU()(Z1)
