[Tutor] A question about using stdin/out/err vs named files
Hi, When you run a python program, it appears that stdin, stdout, and stderr are opened automatically. I've been trying to find out how you tell if there's data in stdin (like when you pipe data to a python program) rather than in a named input file. It seems like most/all the Unix/Linux commands are able to figure this out. Do you know how Python programs do this or might do this? MANY thanks for any/all help/hints/tips/suggestions, George... ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] A question about using stdin/out/err vs named files
George R Goffe writes: > When you run a python program, it appears that stdin, stdout, and > stderr are opened automatically. That's true of any program on a POSIX-compliant operating system. > I've been trying to find out how you tell if there's data in stdin > (like when you pipe data to a python program) rather than in a named > input file. What does “there's data in [a stream]” mean here, and how is it distinct from there being “data in … a named input file”? The advantage of the standard POSIX streams is that processes can treat them as very nearly normal files. I don't doubt you have a distinction you want to detect, but can you be clearer about what that distinction is? > It seems like most/all the Unix/Linux commands are able to figure this > out. Do you know how Python programs do this or might do this? Hmm. The standard input stream is a separate object from any named input file you might otherwise open. That's a trivially obvious difference to detect, so I guess you must not mean that. Perhaps you mean “is the ‘stdin’ stream connected to an interactive terminal?”. That's quite a different question from what you seem to be asking, so I don't know. -- \ “The best is the enemy of the good.” —Voltaire, _Dictionnaire | `\Philosophique_ | _o__) | Ben Finney ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] A question about using stdin/out/err vs named files
Are you planning to pipe data to a python program? If so please specify and you will get more complete answers. Specifically I am thinking you want information pertaining to subprocess in the standard library. https://docs.python.org/3/library/subprocess.html On Sat, Oct 18, 2014 at 2:36 PM, George R Goffe wrote: > Hi, > > When you run a python program, it appears that stdin, stdout, and stderr are > opened automatically. > > I've been trying to find out how you tell if there's data in stdin (like when > you pipe data to a python program) rather > than in a named input file. It seems like most/all the Unix/Linux > commands are able to figure this out. Do you know how Python programs do this > or might do this? > > MANY thanks for any/all help/hints/tips/suggestions, > > George... > ___ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > https://mail.python.org/mailman/listinfo/tutor ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] A question about using stdin/out/err vs named files
On Sat, Oct 18, 2014 at 11:36:43AM -0700, George R Goffe wrote: > Hi, > > When you run a python program, it appears that stdin, stdout, and > stderr are opened automatically. > > I've been trying to find out how you tell if there's data in stdin > (like when you pipe data to a python program) rather than in a named > input file. It seems like most/all the Unix/Linux commands are able to > figure this out. Do you know how Python programs do this or might do > this? Hmmm, good question. I've never actually done this before, but I think the right way is to just try reading from stdin and see if anything is there. Let's try it, using Python 2.7: # stdin_demo.py import sys c = sys.stdin.read(1) if c: text = c + sys.stdin.read() # Slurp everything. print "Read text from stdin:" print text else: print "Nothing in stdin." Alas, that doesn't do what I expected. It works if I pipe something into stdin: [steve@ando ~]$ echo "Some data here" | python2.7 stdin_demo.py Read text from stdin: Some data here but if stdin is empty, the call to read() blocks, waiting for input. I have to manually type something (or nothing, as the case may be) then type Ctrl-D to force end-of-file: [steve@ando ~]$ python2.7 stdin_demo.py # enter something, then Ctrl-D hello world Read text from stdin: hello world [steve@ando ~]$ python2.7 stdin_demo.py # immediately Ctrl-D Nothing in stdin. Here's my second attempt: # stdin_demo.py import sys if sys.stdin.isatty(): print "Ignoring stdin." else: c = sys.stdin.read(1) if c: text = c + sys.stdin.read() # Slurp everything. print "Read text from stdin:" print text else: print "Nothing in stdin." This version seems to do what I think you want: [steve@ando ~]$ python2.7 stdin_demo.py Ignoring stdin. [steve@ando ~]$ echo "Some data here" | python2.7 stdin_demo.py Read text from stdin: Some data here [steve@ando ~]$ echo -n "" | python2.7 stdin_demo.py Nothing in stdin. I haven't tried this under Windows, and I'm not sure if it is the "correct" way to do it under POSIX systems like Unix, Linux or Mac, but it seems to work. Oh, in case it wasn't obvious... it's probably not a good idea to slurb everything as I do above, but I leave processing stdin line-by-line as an exercise. -- Steven ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
