Function tools
We looked at a number of higher-order functions in Chapter 5, Higher-order Functions. These functions either accepted a function as an argument or returned a function (or generator expression) as a result. All these higher-order functions had an essential algorithm which was customized by injecting another function. Functions like max(), min(), and sorted() accepted a key= function that customized their behavior. Functions like map() and filter() accept a function and an iterable and apply this function to the arguments. In the case of the map() function, the results of the function are simply kept. In the case of the filter() function, the Boolean result of the function is used to pass or reject values from the iterable.
All the functions in Chapter 5, Higher-order Functions are part of the Python __builtins__ package: they're available without the need to do an import. They are ubiquitous because they are so universally useful. The functions in this chapter must be introduced...
