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In this chapter, we will explore how to write new functions in Python. Functions wewrite behave the same way as the built-ins we’ve already seen: they are called inexpressions, are passed values, and return results. But writing new functions requiresthe application of a few additional ideas that haven’t yet been introduced. Moreover,functions behave very differently in Python than they do in compiled languages likeC. Here is a brief introduction to the main concepts behind Python functions, all ofwhich we will study in this part of the book:• def is executable code. Python functions are written with a new statement, thedef. Unlike functions in compiled languages such as C, def is an executablestatement—your function does not exist until Python reaches and runs the def.In fact, it’s legal (and even occasionally useful) to nest def statements inside ifstatements, while loops, and even other defs. In typical operation, def statementsare coded in module files, and are naturally run to generate functionswhen a module file is first imported.• def creates an object and assigns it to a name. When Python reaches and runs adef statement, it generates a new function object, and assigns it to the function’sname. As with all assignments, the function name becomes a reference to thefunction object. There’s nothing magic about the name of a function—as you’llsee, the function object can be assigned to other names, stored in a list, and soon. Functions may also be created with the lambda expression (a more advancedconcept deferred until a later chapter).• return sends a result object back to the caller. When a function is called, thecaller 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;the returned value becomes the result of the function call. Functionsknown as generators may also use the yield statement to send back a value andsuspend their state such that they may be resumed later; this is another advancedtopic covered later in this part of the book.• Arguments are passed by assignment (object reference). In Python, argumentsare passed to functions by assignment (which, as we’ve learned, means by objectreference). As you’ll see, Python’s model isn’t really equivalent to C’s passingrules or C++’s reference parameters—the caller and function share objects byreferences, but there is no name aliasing. Changing an argument name does notalso change a name in the caller, but changing passed-in mutable objects canchange objects shared by the caller.• global declares module-level variables that are to be assigned. By default, allnames assigned in a function, are local to that function and exist only while thefunction runs. To assign a name in the enclosing module, functions need to list itin a global statement. More generally, names are always looked up in scopes—places where variables are stored—and assignments bind names to scopes.Coding Functions | 301