[Tutor] Difference between popens

[Bernard Lebel]

Hey, thanks for the nice explanation Michael!


Bernard



On 6/9/06, Michael P. Reilly <arcege at gmail.com> wrote:
> Excuse the slightly pedantic discussion before I get to a real-world
> example, but the differences get into something a bit deeper than Python and
> into what is called "interprocess communication".  Each program that's
> running has an input data stream and an output data steam and on many modern
> computer systems there is a third stream for error messages.  The idea is
> that the program doesn't need to know that the input is coming from a user
> or a keyboard; it could be coming from a file or another program - the
> program shouldn't care.  The same thing with the output and the error
> messages.
>
> These three data streams have traditionally been called "stdin" (standard
> input), "stdout" (standard output) and "stderr" (standard error).  And in
> Python, they are known by the same names in the 'sys' module.
>
> Now before I said that the concept of these is that the program should not
> care where its data is going to or coming from, including another program.
> This is where interprocess communication comes into play.  Specifically,
> this is accomplished with a data construct called "pipes" (the name 'popen'
> comes from "pipe open" in the C world, akin to 'fopen' for "file open").
> Pipes are created between the programs to redirect data between inputs and
> outputs.  The most well known and viewed form of this is the vertical bar
> "|" on command-lines in UNIX, then DOS had to adopt it:  type README.txt |
> more.
>
> Back to the question at hand: why and how would you use the files returned.
> Let's say you have a large dictionary of values in your Python program, and
> the system has a very nice spelling program out there.  You would like to
> use that program so you don't have to write you own routine.  How the
> spelling program is normally called is that words are sent to its standard
> input (stdin) stream, separated by newlines, and the misspelled words are
> printed on its stdout, also separated by newlines.  We want to start the
> program (called "spawning"), write our dictionary values to its stdin and
> read its stdout.  For this, we need to gain access to the program's stdin
> and stdout.  Since we are spawning it, it is called our "child" process.
>
> def verify_dictionary(dict_o_words):
>     reverse_dict = {}  # to get back to the keys
>     (child_stdin, child_stdout) = os.popen2('spell')
>     for (key, value) in dict_o_words.items():
>         if reverse_dict.has_key(value):
>             reverse_dict[value].append(key)
>         else:
>             reverse_dict[value] = [key]
>         child_stdin.write('%s\n' % value)
>     child_stdin.close() # close the data stream to tell other program we're
> finished
>     misspellings = []
>     for line in child_stdout.readlines():
>         value = line.strip()
>         if reverse_dict.has_key(value):
>             misspellings.extend(reverse_dict[value])
>     close_stdout.close() # tells other program we're finished reading from
> it
>     return misspellings
>
> I hope this discussion answers your questions and hasn't been too beneath
> you. :)  Some of it might be arcane history now, but I always feel it is
> good to know the roots to understand where you are going.
>   -Michael
>
>
> On 6/9/06, Bernard Lebel <3dbernard at gmail.com> wrote:
> >
>  Hi,
>
> I'd like to know what are the differences at the various os.popenX
> flavors. I read the documentation and I can see they return file
> objects..... so what can you do with these file objects? I mean, why
>  would you need a set of file objects rather than another?
>
> Sorry the difference is very not clear to me.
>
>
> Thanks
> Bernard
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>
>
> --
> There's so many different worlds,
> So many different suns.
> And we have just one world,
> But we live in different ones.