Python high-order functions

Higher-order function definitions:
1. The parameter received by the function is a functional name;
2. Functions return a function name;
Any function that satisfies any one of the above conditions is a higher order function.

For example:
1. The function receives the parameter is the letter name

2. The function returns the result as a letter name

3. The function receives the function name, the function returns the result

Python high-order functions