In Python, how do you check if sys.stdin has data or not?
I found that os.isatty(0) can not only check if stdin is connected to a TTY device, but also if there is data available.
But if someone uses code such as
sys.stdin = cStringIO.StringIO("ddd") and after that uses os.isatty(0), it still returns True. What do I need to do to check if stdin has data?
I've been using
if not sys.stdin.isatty() Here's an example:
import sys def main(): if not sys.stdin.isatty(): print "not sys.stdin.isatty" else: print "is sys.stdin.isatty" Running
$ echo "asdf" | stdin.py not sys.stdin.isatty sys.stdin.isatty() returns false if stdin is not connected to an interactive input device (e.g. a tty).
isatty(...) isatty() -> true or false. True if the file is connected to a tty device. 
grep is mentioned in another post, I'll mention that less uses isatty. I came here looking to do something like less: read input from a file if a file path is given as an input argument, but read input from stdin otherwise. An alternative is to use - as an alias for stdin and just accept the file argument.
select answers whether the "data stream" has any more data in it.On Unix systems you can do the following:
import sys import select if select.select([sys.stdin, ], [], [], 0.0)[0]: print("Have data!") else: print("No data") On Windows the select module may only be used with sockets though so you'd need to use an alternative mechanism.
select which calls the operating system's select function can work only with those files. StringIO is a virtual file known only to the Python interpreter therefore it doesn't have a file descriptor and you can't use select on it.
sys.stdin.isatty() however did work for me within cron.
<_io.TextIOWrapper name='<stdin>' mode='r' encoding='UTF-8'> and not []Depending on the goal here:
import fileinput for line in fileinput.input(): do_something(line) can also be useful.
As mentioned by others, there's no foolproof way to know if data will become available from stdin, because UNIX doesn't allow it (and more generally because it can't guess the future behavior of whatever program stdin connects to).
Always wait for stdin, even if there may be nothing (that's what grep etc. do), or ask the user for a - argument.
printf "" | cat or while read l; do true; done; echo finishing (in the second case you have to input EOF/^D manually to go through)cat on the command line, it will wait for input indefinitely. In the above example printf sends EOFI found a way to use a future from concurrent.futures library. This is a gui program that will allow you to scroll and hit the wait button when you are still waiting for STDIN. Whenever the program sees it has a line of information from STDIN then it will print it in the multi-line box. But it does not lock the script lock out the user from the gui interface why it is checking stdin for a new line. (Please note I tested this in python 3.10.5)
import concurrent.futures import PySimpleGUI as sg import sys def readSTDIN(): return sys.stdin.readline() window = sg.Window("Test Window", [[sg.Multiline('', key="MULTI", size=(40, 10))], [sg.B("QUIT", bind_return_key=True, focus=True), sg.B("Wait")]], finalize=True, keep_on_top=True) for arg in sys.argv[1:]: window["MULTI"].print(f"ARG={arg}") with concurrent.futures.ThreadPoolExecutor() as pool: futureResult = pool.submit(readSTDIN) while True: event, values = window.read(timeout=500) if event != "__TIMEOUT__": window['MULTI'].print(f"Received event:{event}") if event in ('QUIT', sg.WIN_CLOSED): print("Quit was pressed") window.close() break if futureResult.done(): # flag that the "future" task has a value ready; #therefore process the line from STDIN x = futureResult.result() window["MULTI"].print(f"STDIN:{x}") futureResult = pool.submit(readSTDIN) #add a future for next line of STDIN Saw a lot of answers above, in which almost all the answers required either sys, select, os, etc. However, I got a very simple idea about how continuously to take input from stdin if it has data. We can use try, except block to do so. as,
while(1): try: inp = input() except: break The above loop will run and we are checking for input continuously and storing the input in variable "inp", if there is no input in the stdin then try will throw an error however in except block break statement is present, so the while loop will be terminated
Using built in modules this can be achieved with following code as Gregg gave already the idea:
import fileinput isStdin = True for line in fileinput.input: # check if it is not stdin if not fileinput.isstdin(): isStdin = False break # continue to read stdin print(line) fileinput.close() 
fileinput.input is a function that not iterable, and this will block forever when no data
os.isatty(0)checks if the file associated to the file descriptor (fd) 0 is a TTY. When you change thesys.stdinvariable, you're not changing the file associated with fd0. fd0 is still pointing to the original stdin (which is a TTY in your case).cStringIO.StringIO("ddd")could block; it always has data available, except when EOF is reached of course.