PYTHON Learning 0028: higher-order functions function --- ---- 2019-6-20

1, is defined: variables can point function, function parameters capable of receiving variable, a function can receive another function as a parameter, this function is called to higher order functions.
As long as the function as a parameter passed to other functions, regardless of other functions have no return value ,, called higher-order functions.
2, the function may also receive other functions as a return value.