Learning Python

Function Basics

Although we haven’t gotten very formal about it, we’ve already been using functions in earlier chapters. For instance, to make a file object, we call the built-in open function. Similarly, we use the len built-in function to ask for the number of items in a collection object.

In this chapter, we will learn how to write new functions in Python. Functions we write ourselves behave the same way as the built-ins we’ve already seen—they are called in expressions, are passed values, and return results. But writing functions requires a few new ideas; here’s an introduction to the main concepts:

def creates a function object and assigns it to a name

Python functions are written with a new statement, the def. Unlike functions in compiled languages such as C, def is an executable statement—when run, it generates a new function object and assigns it to the function’s name. As with all assignments, the function name becomes a reference to the function object.

return sends a result object back to the caller

When a function is called, the caller stops until the function finishes its work and returns control to the caller. Functions that compute a value send it back to the caller with a return statement.

global declares module-level variables that are to be assigned

By default, all names assigned in a function are local to that function and exist only while the function runs. To assign a name in the enclosing module, functions need to list it in a global statement.

Arguments are passed ...