3.11. File objects

Python has a built-in function, open, for opening a file on disk. open returns a file object, which has methods and attributes for getting information about and manipulating the opened file.

Пример 3.23. Opening a file

>>> f = open("/music/_singles/kairo.mp3", "rb") 
>>> f                                           
<open file '/music/_singles/kairo.mp3', mode 'rb' at 010E3988>
>>> f.mode                                      
'rb'
>>> f.name                                      
'/music/_singles/kairo.mp3'

	The `open` method can take up to three parameters: a filename, a mode, and a buffering parameter. Only the first one, the filename, is required; the other two are optional. If not specified, the file is opened for reading in text mode. Here we are opening the file for reading in binary mode. (`print open.__doc__` displays a great explanation of all the possible modes.)
	The `open` function returns an object (by now, this should not surprise you). A file object has several useful attributes.
	The `mode` attribute of a file object tells you what mode the file was opened in.
	The `name` attribute of a file object tells you the name of the file that the file object has open.

Пример 3.24. 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 we 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 we're looking for are stored at the end of the file, we 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 which 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. (We could have simply said `read()` here, since we know exactly where we are in the file and we 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.

Пример 3.25. Closing a file


>>> f
<open file '/music/_singles/kairo.mp3', mode 'rb' at 010E3988>
>>> f.closed  
0
>>> f.close() 
>>> f
<closed file '/music/_singles/kairo.mp3', mode 'rb' at 010E3988>
>>> f.closed
1
>>> 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 `0`). 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 done with them.
	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.

Пример 3.26. 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, we want to make absolutely sure that we 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 on the way.
	At last, we handle our `IOError` exception. This could be the `IOError` exception raised by the call to `open`, `seek`, or `read`. Here, we really don't care, because all we'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.

Further reading

Python Tutorial discusses reading and writing files, including how to read a file one line at a time into a list.
eff-bot discusses efficiency and performance of various ways of reading a file.
Python Knowledge Base answers common questions about files.
Python Library Reference summarizes all the file object methods.