Dive Into Python

After you open a file, the first thing you'll want to do is read from it, as shown in the next example.

Example 6.4. Reading a File

>>> f
<open file '/music/_singles/kairo.mp3', mode 'rb' at 010E3988>
>>> f.tell()              
0
>>> f.seek(-128, 2)       
>>> f.tell()              
7542909
>>> tagData = f.read(128) 
>>> tagData
'TAGKAIRO****THE BEST GOA         ***DJ MARY-JANE***            
Rave Mix                      2000http://mp3.com/DJMARYJANE     \037'
>>> f.tell()              
7543037

	A file object maintains state about the file it has open. The tell method of a file object tells you your current position in the open file. Since you haven't done anything with this file yet, the current position is 0, which is the beginning of the file.
	The seek method of a file object moves to another position in the open file. The second parameter specifies what the first one means; 0 means move to an absolute position (counting from the start of the file), 1 means move to a relative position (counting from the current position), and 2 means move to a position relative to the end of the file. Since the MP3 tags you're looking for are stored at the end of the file, you use 2 and tell the file object to move to a position 128 bytes from the end of the file.
	The tell method confirms that the current file position has moved.
	The read method reads a specified number of bytes from the open file and returns a string with the data that was read. The optional parameter specifies the maximum number of bytes to read. If no parameter is specified, read will read until the end of the file. (You could have simply said read() here, since you know exactly where you are in the file and you are, in fact, reading the last 128 bytes.) The read data is assigned to the tagData variable, and the current position is updated based on how many bytes were read.
	The tell method confirms that the current position has moved. If you do the math, you'll see that after reading 128 bytes, the position has been incremented by 128.

Open files consume system resources, and depending on the file mode, other programs may not be able to access them. It's important to close files as soon as you're finished with them.

Example 6.5. Closing a File

>>> f
<open file '/music/_singles/kairo.mp3', mode 'rb' at 010E3988>
>>> f.closed       
False
>>> f.close()      
>>> f
<closed file '/music/_singles/kairo.mp3', mode 'rb' at 010E3988>
>>> f.closed       
True
>>> f.seek(0)      
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
ValueError: I/O operation on closed file
>>> f.tell()
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
ValueError: I/O operation on closed file
>>> f.read()
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
ValueError: I/O operation on closed file
>>> f.close()      

	The closed attribute of a file object indicates whether the object has a file open or not. In this case, the file is still open (closed is False).
	To close a file, call the close method of the file object. This frees the lock (if any) that you were holding on the file, flushes buffered writes (if any) that the system hadn't gotten around to actually writing yet, and releases the system resources.
	The closed attribute confirms that the file is closed.
	Just because a file is closed doesn't mean that the file object ceases to exist. The variable f will continue to exist until it goes out of scope or gets manually deleted. However, none of the methods that manipulate an open file will work once the file has been closed; they all raise an exception.
	Calling close on a file object whose file is already closed does not raise an exception; it fails silently.

Now you've seen enough to understand the file handling code in the fileinfo.py sample code from teh previous chapter. This example shows how to safely open and read from a file and gracefully handle errors.

Example 6.6. File Objects in MP3FileInfo

        try:                                
            fsock = open(filename, "rb", 0) 
            try:                           
                fsock.seek(-128, 2)         
                tagdata = fsock.read(128)   
            finally:                        
                fsock.close()              
            .
            .
            .
        except IOError:                     
            pass                           

	Because opening and reading files is risky and may raise an exception, all of this code is wrapped in a try...except block. (Hey, isn't standardized indentation great? This is where you start to appreciate it.)
	The open function may raise an IOError. (Maybe the file doesn't exist.)
	The seek method may raise an IOError. (Maybe the file is smaller than 128 bytes.)
	The read method may raise an IOError. (Maybe the disk has a bad sector, or it's on a network drive and the network just went down.)
	This is new: a try...finally block. Once the file has been opened successfully by the open function, you want to make absolutely sure that you close it, even if an exception is raised by the seek or read methods. That's what a try...finally block is for: code in the finally block will always be executed, even if something in the try block raises an exception. Think of it as code that gets executed on the way out, regardless of what happened before.
	At last, you handle your IOError exception. This could be the IOError exception raised by the call to open, seek, or read. Here, you really don't care, because all you're going to do is ignore it silently and continue. (Remember, pass is a Python statement that does nothing.) That's perfectly legal; “handling” an exception can mean explicitly doing nothing. It still counts as handled, and processing will continue normally on the next line of code after the try...except block.

As you would expect, you can also write to files in much the same way that you read from them. There are two basic file modes:

Either mode will create the file automatically if it doesn't already exist, so there's never a need for any sort of fiddly "if the log file doesn't exist yet, create a new empty file just so you can open it for the first time" logic. Just open it and start writing.

Example 6.7. Writing to Files

>>> logfile = open('test.log', 'w') 
>>> logfile.write('test succeeded') 
>>> logfile.close()
>>> print file('test.log').read()   
test succeeded
>>> logfile = open('test.log', 'a') 
>>> logfile.write('line 2')
>>> logfile.close()
>>> print file('test.log').read()   
test succeededline 2

	You start boldly by creating either the new file test.log or overwrites the existing file, and opening the file for writing. (The second parameter "w" means open the file for writing.) Yes, that's all as dangerous as it sounds. I hope you didn't care about the previous contents of that file, because it's gone now.
	You can add data to the newly opened file with the write method of the file object returned by open.
	file is a synonym for open. This one-liner opens the file, reads its contents, and prints them.
	You happen to know that test.log exists (since you just finished writing to it), so you can open it and append to it. (The "a" parameter means open the file for appending.) Actually you could do this even if the file didn't exist, because opening the file for appending will create the file if necessary. But appending will never harm the existing contents of the file.
	As you can see, both the original line you wrote and the second line you appended are now in test.log. Also note that carriage returns are not included. Since you didn't write them explicitly to the file either time, the file doesn't include them. You can write a carriage return with the "\n" character. Since you didn't do this, everything you wrote to the file ended up smooshed together on the same line.