Programs on all major operating systems are connected to input and output streams (and usually an error stream). When run from the commandline without a redirect operator, a program is normally connnected to the shells standard input stream (where a user can type commands into the program) and the standard output stream (which prints output back on the console).
However, we aren’t limited to such streams. It’s perfectly possible to use the contents of a file as a program’s input stream or even use the output of one program and link it’s output to another program’s input stream. Such chaining of streams allows for powerful OS scripting.
This isn’t really a Python feature, but here is an example found in Programming Python: Powerful Object-Oriented Programming that demonstraes connecting the output of one OS stream to the input stream of a Python program. I added some comments that help explain the program.
def interact(): print('Hello stream world') while True: try: # Input normally reads from the keyboard because our program # is connected to that input stream. However, if we execute this program # in a way that connects the program's input to some other stream, # the input command reads from there instead! reply = input('Enter a number => ') except EOFError: # We have reached the end of our input stream (for example user entered ctrl+c at the shell) # So we exit the looop break else: num = int(reply) print("%d squared is %d" % (num, num ** 2)) print('Bye') if __name__ == '__main__': interact()
When this program is run on it’s own, it will collect input from the keyboard until we press ctrl+c. That’s not the part that we are demonstrating here. Let’s suppose we have a text file that has the following contents.
1 2 3 4 5 6 7 8 9 10
Now when we run our program with a redirect operator, we get the following output.
Patricks-MacBook-Pro:Streams stonesoup$ python teststreams.py 1 squared is 1 Enter a number => 2 squared is 4 Enter a number => 3 squared is 9 Enter a number => 4 squared is 16 Enter a number => 5 squared is 25 Enter a number => 6 squared is 36 Enter a number => 7 squared is 49 Enter a number => 8 squared is 64 Enter a number => 9 squared is 81 Enter a number => 10 squared is 100 Enter a number => Bye
Notice the unix redirect operator. This program was run
python teststreams.py < input.txt. That < input.txt connects the contents of input.txt to the teststreams.py script. Thus, when input is called, the function simply collects the next line in input.txt rather than waiting for the keyboard.