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An Introduction to Python
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
ISBN 0954161769
RRP £12.95 ($19.95)

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7.2.1 Methods of File Objects

The rest of the examples in this section will assume that a file object called f has already been created.

To read a file's contents, call f.read(size), which reads some quantity of data and returns it as a string. size is an optional numeric argument. When size is omitted or negative, the entire contents of the file will be read and returned; it's your problem if the file is twice as large as your machine's memory. Otherwise, at most size bytes are read and returned. If the end of the file has been reached, f.read() will return an empty string ("").

    >>> f.read()
    'This is the entire file.\n'
    >>> f.read()
    "

f.readline() reads a single line from the file; a newline character (\n) is left at the end of the string, and is only omitted on the last line of the file if the file doesn't end in a newline. This makes the return value unambiguous; if f.readline() returns an empty string, the end of the file has been reached, while a blank line is represented by '\n', a string containing only a single newline.

    >>> f.readline()
    'This is the first line of the file.\n'
    >>> f.readline()
    'Second line of the file\n'
    >>> f.readline()
    "

f.readlines() returns a list containing all the lines of data in the file. If given an optional parameter sizehint, it reads that many bytes from the file and enough more to complete a line, and returns the lines from that. This is often used to allow efficient reading of a large file by lines, but without having to load the entire file in memory. Only complete lines will be returned.

    >>> f.readlines()
    ['This is the first line of the file.\n',
     'Second line of the file\n']

An alternate approach to reading lines is to loop over the file object. This is memory efficient, fast, and leads to simpler code:

    >>> for line in f:
            print line,
            
    This is the first line of the file.
    Second line of the file

The alternative approach is simpler but does not provide as fine-grained control. Since the two approaches manage line buffering differently, they should not be mixed.

f.write(string) writes the contents of string to the file, returning None.

    >>> f.write('This is a test\n')

To write something other than a string, it needs to be converted to a string first:

    >>> value = ('the answer', 42)
    >>> s = str(value)
    >>> f.write(s)

f.tell() returns an integer giving the file object's current position in the file, measured in bytes from the beginning of the file. To change the file object's position, use ‘f.seek(offset, from_what)’. The position is computed from adding offset to a reference point; the reference point is selected by the from_what argument. A from_what value of 0 measures from the beginning of the file, 1 uses the current file position, and 2 uses the end of the file as the reference point. from_what can be omitted and defaults to 0, using the beginning of the file as the reference point.

    >>> f = open('/tmp/workfile', 'r+')
    >>> f.write('0123456789abcdef')
    >>> f.seek(5)     # Go to the 6th byte in the file
    >>> f.read(1)        
    '5'
    >>> f.seek(-3, 2) # Go to the 3rd byte before the end
    >>> f.read(1)
    'd'

When you're done with a file, call f.close() to close it and free up any system resources taken up by the open file. After calling f.close(), attempts to use the file object will automatically fail.

    >>> f.close()
    >>> f.read()
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    ValueError: I/O operation on closed file

File objects have some additional methods, such as isatty() and truncate() which are less frequently used; consult the Python Library Reference Manual for a complete guide to file objects.

ISBN 0954161769An Introduction to PythonSee the print edition