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.\012' >>> 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.\012' >>> f.readline() 'Second line of the file\012' >>> f.readline() ''
f.readlines()
uses f.readline()
repeatedly, and returns
a list containing all the lines of data in the file.
>>> f.readlines() ['This is the first line of the file.\012', 'Second line of the file\012']
f.write(string)
writes the contents of string to
the file, returning None
.
>>> f.write('This is a test\n')
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 5th 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 (innermost last): File "<stdin>", line 1, in ? ValueError: I/O operation on closed fileFile objects have some additional methods, such as
isatty()
and
truncate()
which are less frequently used; consult the Library
Reference for a complete guide to file objects.
guido@CNRI.Reston.Va.US