write a .txt file

[Jia Hu]

On Tue, Jul 13, 2010 at 3:17 AM, Cameron Simpson <cs at zip.com.au> wrote:

> 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().
>
>
Thank you.


> | 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.



> OK, it is a good suggestion. By the way, do you add the pipe "|" or

they are automatically generated ?

>
> 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()

I initially thought only desLrr.wait() is needed even if without
fileName.flush() , that is
because the first line of number will transfer to the memory buffer and then
"echo, ls" is
also kept in the memory buffer and then wait() to make them finish. But when
I actually did this,
I find this method is not very correct. The first few words generated
by "ls" are missing.
When I add  "fileName.flush()" again and got a correct output.

Should I always write to the file at one time and then run "flush" or
"close()" before the next
write to the file (e.g. "echo" or the second line of number) ?

Thank you.



>

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/
>
> Quick Guide to Resources to break crypto systems of given strength:
> Difficulty Resources required
>  2**0      Pencil and paper
>  2**8      Pencil and paper and a lot of patience
>  2**16     Warez d00d with a Commodore 64
>  2**32     Amateur with an average PC
>  2**40     Smart amateur with a good PC
>  2**56     Network of workstations, custom-built hardware
>  2**64     Thousands of PCs working together for several years
>  2**80     NSA, Microsoft, the Illuminati
>  2**128    Cubic kilometers of sci-fi nanotech
>  2**160    Dyson spheres
> Eddy L O Jansson and Matthew Skala, from:
> http://hem.passagen.se/eddy1/reveng/cp4/cp4break.html
>
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