Re: [Tutor] A question about using stdin/out/err vs named files
George R Goffe wrote: 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? I don't think there is a way to guess that. Instead there is an optional commandline argument; if that is missing or '-' the script assumes stdin as the default. With argparse you spell it like this: $ cat upper.py #!/usr/bin/env python3 import argparse import sys parser = argparse.ArgumentParser(description=Convert input to uppercase) parser.add_argument( input, type=argparse.FileType(r), default=sys.stdin, nargs=?, help=Input file or '-' for stdin. Default: stdin.) for line in parser.parse_args().input: print(line.upper(), end=) $ ./upper.py Hello HELLO $ ./upper.py upper.py #!/USR/BIN/ENV PYTHON3 IMPORT ARGPARSE IMPORT SYS PARSER = ARGPARSE.ARGUMENTPARSER(DESCRIPTION=CONVERT INPUT TO STDIN) PARSER.ADD_ARGUMENT( INPUT, TYPE=ARGPARSE.FILETYPE(R), DEFAULT=SYS.STDIN, NARGS=?, HELP=INPUT FILE OR '-' FOR STDIN. DEFAULT: STDIN.) FOR LINE IN PARSER.PARSE_ARGS().INPUT: PRINT(LINE.UPPER(), END=) There is also the fileinput module. ___ 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 18/10/14 19:36, George R Goffe wrote: When you run a python program, it appears that stdin, stdout, and stderr are opened automatically. correct. I've been trying to find out how you tell if there's data in stdin Same way you tell if there's data in any file/stream - you read from it. In the old days when I programmed in C there were a pair of calls often used for this: getc() and ungetc(). You read a character with getc() then pout it back with ungetc(). Unfortunately they don't exist in Python stdlib. But, there is a getch()/ungetch() in the msvcrt for Windows, so you could use that. The curses module on linux has an ungetch() but no getch() - which seems bizarre... Steven has posted a solution using read(1) but that blocks so you need to use the isatty() method which all seems a bit messy and doesn't work with tty input. On further investigation the curses.screen object has a getch() method, but its not clear how the curses.ungetch() relates to that (there's no help() documentation) and I've run out of time to experiment. But if you need a linux solution that works with any kind of stdin that might be worth exploring. Sorry, not a great answer but hopefully its of some use. -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ http://www.flickr.com/photos/alangauldphotos ___ 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
* Peter Otten __pete...@web.de [2014-10-19 10:05]: George R Goffe wrote: 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? There is also the fileinput module. I use fileinput all the time. This iterates over the lines of all files listed in sys.argv[1:], defaulting to sys.stdin if the list is empty. If a filename is '-', it is also replaced by sys.stdin. To specify an alternative list of filenames, pass it as the first argument to input(). A single file name is also allowed. It gives a fairly clean way to just do the Right Thing whether you are feeding files, or reading from stdin. -- David Rock da...@graniteweb.com ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
[Tutor] A question about using stdin/out/err vs named files
Hi, Wow. Lots of feedback. REALLY GOOD FEEDBACK! This was my first question to this list. Let me clarify my question. I want to use tst.py as follows: tst.py input-file output-file OR cat data-file | tst.py - output-file OR cat data-file | tst.py output-file tst.py input-file output-file works well tst.py - output-file works well The question boils down to tst.py output-file... which is a parsing the args question which you guys have caused me to think more clearly about. If there's just 1 arg, consider it an output-file and read from stdin and write to output-file ONLY if output-file does not exist. Thank you all for your very helpful and informative responses. Regards, 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
On 10/18/2014 02:36 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... Command line argument parsing aside, perhaps something like this would be useful: script.py --- #!/usr/bin/env python3 import os, stat mode = os.fstat(0).st_mode if stat.S_ISFIFO(mode): print(stdin is a pipe) elif stat.S_ISREG(mode): print(stdin is a redirected file) elif stat.S_ISCHR(mode): print(stdin is terminal) else: print(stdin is weird) --- $ ./script.py stdin is terminal $ cat script.py | ./script.py stdin is a pipe $ ./script.py script.py stdin is a redirected file ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
[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 grgo...@yahoo.com.dmarc.invalid 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 grgo...@yahoo.com.dmarc.invalid 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
