Learn Python Programming

So far, we’ve mainly been working with data structures that reside in the interpreter itself. What little interaction our programs have had with the outside world has been through input and print. In this chapter, we go one step further and let our programs catch a glimpse of a larger world: the world of files and streams. The functions and objects described in this chapter will enable you to store data between program invocations and to process data from other programs. Opening Files  You can open files with the open function, which lives in the io module but is automatically imported for you. It takes a file name as its only mandatory argument and returns a file object. Assuming that you have a text file (created with your text editor, perhaps) called somefile.txt stored in the current directory, you can open it like this: >>> f = open('somefile.txt') You can also specify the full path to the file, if it’s located somewhere else. If it doesn’t exist, however, ...

When writing computer programs, it is usually possible to discern between a normal course of events and something that’s exceptional (out of the ordinary). Such exceptional events might be errors (such as trying to divide a number by zero) or simply something you might not expect to happen very often. To handle such exceptional events, you might use conditionals everywhere the events might occur (for example, have your program check whether the denominator is zero for every division). However, this would not only be inefficient and inflexible but would also make the programs illegible. You might be tempted to ignore these exceptional events and just hope they won’t occur, but Python offers an exception-handling mechanism as a powerful alternative.     In this tutorial, you’ll learn how to create and raise your own exceptions, as well as how to handle exceptions in various ways. What Is an Exception?  To represent exceptional conditions, Python uses exception obj...

In the previous tutorial, you looked at Python’s main built-in object types (numbers, strings, lists, tuples, and dictionaries); you peeked at the wealth of built-in functions and standard libraries; and you even created your own functions. Now, only one thing seems to be missing—making your own objects. And that’s what you do in this chapter.     You may wonder how useful this is. It might be cool to make your own kinds of objects, but what would you use them for? With all the dictionaries and sequences and numbers and strings available, can’t you just use them and make the functions do the job? Certainly, but making your own objects (and especially types or classes of objects) is a central concept in Python—so central, in fact, that Python is called an objectoriented language (along with Smalltalk, C++, Java, and many others). In this chapter, you learn how to make objects. You learn about polymorphism and encapsulation, methods and attributes, superclasses, and inher...