Core Python Programming (2nd Edition)

Regardless of your platform, these variables will be set to the correct
values when you import the os module: One less headache to worry
about.
We would also like to remind you that the comma placed at the end of the print statement is to
suppress the NEWLINE character that print normally adds at the end of output. The reason for this is
because every line from the text file already contains a NEWLINE. readline() and readlines() do not
strip off any whitespace characters in your line (see exercises.) If we omitted the comma, then your text
file display would be doublespaced one NEWLINE which is part of the input and another added by the
print statement.
File objects also have a truncate() method, which takes one optional argument, size. If it is given, then
the file will be truncated to, at most, size bytes. If you call TRuncate() without passing in a size, it will
default to the current location in the file. For example, if you just opened the file and call TRuncate(),
your file will be effectively deleted, truncated to zero bytes because upon opening a file, the "read head"
is on byte 0, which is what tell() returns.
Before moving on to the next section, we will show two more examples, the first highlighting output to
files (rather than input), and the second performing both file input and output as well as using the seek
() and tell() methods for file positioning.
filename = raw_input('Enter file name: ')
fobj = open(filename, 'w')
while True:
aLine = raw_input("Enter a line ('.' to quit): ")
if aLine != ".":
fobj.write('%s%s' % (aLine, os.linesep)
else:
break
fobj.close()
Here we ask the user for one line at a time, and send them out to the file. Our call to the write()
method must contain a NEWLINE because raw_input() does not preserve it from the user input. Because
it may not be easy to generate an end-of-file character from the keyboard, the program uses the period
( . ) as its end-of-file character, which, when entered by the user, will terminate input and close the file.
The second example opens a file for read and write, creating the file from scratch (after perhaps
truncating an already existing file). After writing data to the file, we move around within the file using
seek(). We also use the tell() method to show our movement.
>>> f = open('/tmp/x', 'w+')
>>> f.tell()
0
>>> f.write('test line 1\n') # add 12-char string [0-11]
>>> f.tell()
12
>>> f.write('test line 2\n') # add 12-char string [12-23]
>>> f.tell() # tell us current file location (end))
24