Beginning Python - From Novice to Professional

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CHAPTER 11 ■ FILES AND STUFF 267 >>> third 'This is the third line\n' ■Note The syntax print >> file, text prints the text to the given file object. In this example, it’s important to note the following: • I’ve used print to write to the file; this automatically adds newlines after the strings I supply. • I use sequence unpacking on the opened file, putting each line in a separate variable. (This isn’t exactly common practice because you usually won’t know the number of lines in your file, but it demonstrates the “iteratorness” of the file object.) • I close the file after having written to it, to ensure that the data is flushed to disk. (As you can see, I haven’t closed it after reading from it. Sloppy, perhaps, but not critical.) A Quick Summary In this chapter, you’ve seen how to interact with the environment through files and file-like objects, one of the most important techniques for I/O (input/output) in Python. Here are some of the highlights from the chapter: File-like objects. A file-like object is (informally) an object that supports a set of methods such as read and readline (and possibly write and writelines). Opening and closing files. You open a file with the open function (in newer versions of Python, actually just an alias for file), by supplying a file name. Modes and file types. When opening a file, you can also supply a mode, such as 'r' for read mode or 'w' for write mode. By appending 'b' to your mode, you can open files as binary files. (This is necessary only on platforms where Python performs line-ending conversion, such as Windows.) Standard streams. The three standard files (stdin, stdout, and stderr, found in the sys module) are file-like objects that implement the UNIX standard I/O mechanism (also available in Windows). Reading and writing. You read from a file or file-like object using the method read. You write with the method write. Reading and writing lines. You can read lines from a file using readline, readlines, and (for efficient iteration) xreadlines. You can write files with writelines. Iterating over file contents. There are many ways of iterating over file contents. It is most common to iterate over the lines of a text file, and you can do this by simply iterating over the file itself. There are other methods too, such as readlines and xreadlines, that are compatible with older versions of Python.
268 CHAPTER 11 ■ FILES AND STUFF New Functions in This Chapter Function file(name[, mode[, buffering]]) open(name[, mode[, buffering]]) Description Opens a file and returns a file object Alias for file; use open rather than file when opening files What Now? So now you know how to interact with the environment through files; but what about interacting with the user? So far we’ve used only input, raw_input, and print, and unless the user writes something in a file that your program can read, you don’t really have any other tools for creating user interfaces. That changes in the next chapter, when I cover graphical user interfaces, with windows, buttons, and so on.
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Beginning Python From Novice to Pro
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About the Technical Reviewer ■JER
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358 CHAPTER 17 ■ EXTENDING PYTHON
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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