Python: function brackets. The difference between function brackets and function parentheses.

For example, function: def A (x): def B (y): Return x + y return B. The last return B function has no parentheses, but the function runs normally, when a (2) (3) is called like this, the value can be calculated normally. What is the returned B? What should I do? In addition, when a (2) is missing a parameter call, it is useless and does not cause an exception. How should I understand him?
In this way, you can see that this method cannot pass X to B to Def B (y): Return x + ydef A (x ): return B in fact, you call a --> Print the memory address of A. A (x) is the method A that is called, and return B is equivalent to printing the memory address of B directly, so a --> function a's memory address a (x) --> call method A and return function B's object equivalent to --> BA (x) (y) --> B (y) call Method B to return the values of X and Y. Here, X is the parameter value of method A, and Y is the parameter value of Method B. This can understand the parameter scope.