write a .txt file

[Cameron Simpson]

On 13Jul2010 02:46, Jia Hu <hujia06 at gmail.com> wrote:
| Hi:
| 
| Do you mean the following code?
| 
| #!/usr/bin/python
| # OS: Ubuntu
| import subprocess
| fileName = open ('final.txt', 'a')
| fileName.write ('%s %s %s \n' % (12,25,9))
| 
| fileName.flush()   # add
| fileName.close()  # add

You should not need the .close().

| desLrr = subprocess.Popen('ls -a >> final.txt', shell=True)  # change to "ls
| -a"

You're still not waiting for the Popen subprocess to finish before
continuing.

| fileName=open ('final.txt', 'a')

You shouldn't need the open() if you skip the .close()

| fileName.seek(0,2)

If you _do_ do an open(..., "a") then you don't need the seek - append
mode will write at the end for you.

| fileName.write ('%s %s %s \n' % (85,25,12))
| fileName.close()
| 
| I run that, the result showed the list of file is located after the number
| list 85 25 12
| Is that possible that I put the fileName.flush() in a wrong position ?
| I am new to Python and do not quite understand the "flush" concept.

First: please don't top post. Post below the relevant points as I do
above, like a conversation. That way it is clear exactly what each
remark is for.

Second: "flush" is a general idea used with buffers.

When you .write() to a file the data does not normally go directly to
disc.  This is because a real write to the disc is expensive in time
(the CPU is much much faster than a hard drive).

Also, a OS-level "write", which transfers the data from your program into
the OS's buffers (where it queues, for eventual writing to the disc)
is _also_ a comparitively expensive operation. So when you .write()
in python (or C using stdio, and in many other languages) the data is
normally kept in a memory buffer inside your program. Only when that
buffer is filled is an OS-level "write" done. In this way, OS calls are
few. This is a performance gain.

So, in your program, when you .write() your first line of numbers
the data has not been handed to the OS at all, and therefore the
"final.txt" file has not seen it. A .flush() call is an explicit request
to empty the buffer, making an OS-level "write" immediately. In your
program, this is necessary to get the data to "final.txt" before the
"echo" or "ls" commands are run.

If you .close() a file, that also empties the buffer. But it also closes
the file! SO you need to re-open it. For your purposes, a .flush() is
enough and leaves the file open for you to continue using it later.

Regarding the .seek(): after the "echo" or "ls" command has run
the file  has grown. Python's "file" object does not know the file has
grown because it was not involved. So a .seek(0 is needed to position
Python's .write() location to the end of the file instead of where it
though things were.

Um. Because you have opened the file in 'a' mode you should not need the
.seek() - an "append" mode file will always write to the end of the
file.

So: your first problem only requires a .flush(), to empty the buffer
before "echo" or "ls" runs. However, you still have not waited for the
"echo" or "ls" to finish - Popen kicks the command off, but it will run
at the same time as your program! Call:

  desLrr.wait()

to wait for the "echo" or "ls" to finish before continuing!

Cheers,
-- 
Cameron Simpson <cs at zip.com.au> DoD#743
http://www.cskk.ezoshosting.com/cs/

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