On 01/26/2012 06:20 PM, amt wrote:
Exercise 17, extra credit 6 Learn python the hard way: Find out why
you had to do output.close() in the code.
Code:
from sys import argv
from os.path import exists
script, from_file, to_file = argv
print "Copying from %s to %s" % (from_file, to_file)
input = open(from_file)
indata = input.read()
print "The input file is %d bytes long" % len(indata)
print "Does the output file exist? %r" % exists(to_file)
print "Ready, hit RETURN to continue, CTRL-C to abort."
raw_input()
output = open(to_file, 'w')
output.write(indata)
print "Alright, all done."
output.close()
input.close()
I don't get it. If you don't close input and output it works exactly
the same as if you would close them, so why do you have to do
output.close() and input.close()?
Also does it matter if you do: input.close() and then output.close()?
Is there an order to follow?
This is an extra credit question, after all. So it must be a little
subtle, and it is.
First, Python (the language) does not promise that it will close the
files for you. The operating system does, when the program exits. In
this trivial program, that's right away. But if your program does
something else for a while, or repeats this sequence of steps dozens of
times, you could run out of resources, or overwrite something.
CPython, the most common implementation, does promise that the file will
be closed when the usage count of the file object goes away. So if you
did any of the following, it would close the file:
1) del output
2) output = None
3) output = open(someother_file, "w")
However, that's an optimization within CPython, and not necessarily
guaranteed for other Python implementations, like IronPython or Jython.
Second, some operating system platforms won't let the same file be
simultaneously open for readonly and for write. So if the two filenames
happened to be the same file (through symlink or other mechanism, or
even using two different ways to describe the same file), you might get
an error trying to write without having closed the input file.
Try putting the same name for both input and output file, see what it
does. Or use an absolute path for the input filename and a relative
path to the same place for the output file.
The place to close the input file is right after you're done reading the
data, and before opening the output file.
--
DaveA
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