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CHAPTER 11 ■ FILES AND STUFF 261 ■Note An alternative to readlines that can be useful when iterating is xreadlines. For more information, see the section “Iterating Over File Contents,” later in this chapter. The method writelines is the opposite of readlines: Give it a list (or, in fact, any sequence or iterable object) of strings, and it writes all the strings to the file (or stream). Note that newlines are not added: you have to add those yourself. Also, there is no writeline method because you can just use write. ■Note On platforms that use other line separators, substitute “carriage return” (Mac) or “carriage return and newline” (Windows) for “newline.” Closing Your Files You should remember to close your files by calling their close method. Usually, a file object is closed automatically when you quit your program (and possibly before that), and not closing files you have been reading from isn’t really that important (although it can’t hurt, and might help to avoid keeping the file uselessly “locked” against modification in some operating systems and settings). But you should always close a file you have written to because Python may buffer (keep stored temporarily somewhere, for efficiency reasons) the data you have written, and if your program crashes for some reason, the data might not be written to the file at all. The safe thing is to close your files after you’re finished with them. If you want to be certain that your file is closed, you should use a try/finally statement with the call to close in the finally clause: # Open your file here try: # Write data to your file finally: file.close() ■Tip After writing something to a file, you usually want the changes to appear in that file, so other programs reading the same file can see the changes. Well, isn’t that what happens, you say. Not necessarily. As mentioned, the data may be buffered (stored temporarily somewhere in memory), and not written until you close the file. If you want to keep working with the file (and not close it) but still want to make sure the file on disk is updated to reflect your changes, call the file object’s flush method. (Note, however, that flush might not allow other programs running at the same time to access the file, due to locking considerations that depend on your operating system and settings. Whenever you can conveniently close the file, that is preferable.)
262 CHAPTER 11 ■ FILES AND STUFF Examples Assume that somefile.txt contains the text in Listing 11-3. What can you do with it? Listing 11-3. A Simple Text File Welcome to this file There is nothing here except This stupid haiku Let’s try the methods you know: read(n): >>> f = open(r'c:\text\somefile.txt') >>> f.read(7) 'Welcome' >>> f.read(4) ' to ' >>> f.close() read(): >>> f = open(r'c:\text\somefile.txt') >>> print f.read() Welcome to this file There is nothing here except This stupid haiku >>> f.close() readline(): >>> f = open(r'c:\text\somefile.txt') >>> for i in range(3): print str(i) + ': ' + f.readline(), 0: Welcome to this file 1: There is nothing here except 2: This stupid haiku >>> f.close() readlines(): >>> import pprint >>> pprint.pprint(open(r'c:\text\somefile.txt').readlines()) ['Welcome to this file\n', 'There is nothing here except\n', 'This stupid haiku'] Note that I relied on the file object being closed automatically in this example.
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Beginning Python From Novice to Pro
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For Ranveig
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■CHAPTER 23 Project 4: In the New
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viii ■CONTENTS Strings . . . . .
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x ■CONTENTS ■CHAPTER 5 Conditio
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xxii ■CONTENTS Preparations . . .
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About the Technical Reviewer ■JER
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Introduction A C program is like a
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140 CHAPTER 7 ■ MORE ABSTRACTION
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CHAPTER 9 ■ ■ ■ Magic Methods
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CHAPTER 9 ■ MAGIC METHODS, PROPER
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312 CHAPTER 14 ■ NETWORK PROGRAMM
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314 CHAPTER 15 ■ PYTHON AND THE W
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CHAPTER 16 ■ ■ ■ Testing, 1-2
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358 CHAPTER 17 ■ EXTENDING PYTHON
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CHAPTER 18 ■ ■ ■ Packaging Yo
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392 CHAPTER 20 ■ PROJECT 1: INSTA
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CHAPTER 22 ■ PROJECT 3: XML FOR A
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500 CHAPTER 27 ■ PROJECT 8: FILE
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APPENDIX A ■ ■ ■ The Short Ve
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558 APPENDIX B ■ PYTHON REFERENCE
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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■INDEX 579 cmd module 252, 501 Cm
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