Coding Functions

Although it wasn’t made very formal, we’ve already used some functions in earlier chapters. For instance, to make a file object, we called the built-in open function; similarly, we used the len built-in function to ask for the number of items in a collection object.

In this chapter, we will explore how to write new functions in Python. Functions we write 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 new functions requires the 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 like C. Here is a brief introduction to the main concepts behind Python functions, all of which we will study in this part of the book:

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  • def is executable code. Python functions are written with a new statement, the def. Unlike functions in compiled languages such as C, def is an executable statement—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 if statements, while loops, and even other defs. In typical operation, def statements are coded in module files and are naturally run to generate functions when a module file is first imported.
  • def creates an object and assigns it to a name. When Python reaches and runs a def statement, 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. There’s nothing magic about the name of a function—as you’ll see, the function object can be assigned to other names, stored in a list, and so on. Function objects may also have arbitrary user-defined attributes attached to them to record data.
  • lambda creates an object but returns it as a result. Functions may also be created with the lambda expression, a feature that allows us to in-line function definitions in places where a def statement won’t work syntactically (this is a more advanced concept that we’ll defer until Chapter 19).
  • 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; the returned value becomes the result of the function call.
  • yield sends a result object back to the caller, but remembers where it left off. Functions known as generators may also use the yield statement to send back a value and suspend their state such that they may be resumed later, to produce a series of results over time. This is another advanced topic covered later in this part of the book.
  • 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. More generally, names are always looked up in scopes—places where variables are stored—and assignments bind names to scopes.
  • nonlocal declares enclosing function variables that are to be assigned. Similarly, the nonlocal statement added in Python 3.0 allows a function to assign a name that exists in the scope of a syntactically enclosing def statement. This allows enclosing functions to serve as a place to retain state—information remembered when a function is called—without using shared global names.
  • Arguments are passed by assignment (object reference). In Python, arguments are passed to functions by assignment (which, as we’ve learned, means by object reference). As you’ll see, in Python’s model the caller and function share objects by references, but there is no name aliasing. Changing an argument name within a function does not also change the corresponding name in the caller, but changing passed-in mutable objects can change objects shared by the caller.
  • Arguments, return values, and variables are not declared. As with everything in Python, there are no type constraints on functions. In fact, nothing about a function needs to be declared ahead of time: you can pass in arguments of any type, return any kind of object, and so on. As one consequence, a single function can often be applied to a variety of object types—any objects that sport a compatible interface (methods and expressions) will do, regardless of their specific types.

If some of the preceding words didn’t sink in, don’t worry—we’ll explore all of these concepts with real code in this part of the book. Let’s get started by expanding on some of these ideas and looking at a few examples.