Stdin Not Working As Intended [Python]

Question

I'm running through some exercises in Python right now, including a simple line counter that can take input from the command line or stdin:

#### line_count1.py ####

import sys

def count_lines(file):
    n = 0
    for line in file:
        n = n + 1
    return n

if len(sys.argv) == 1:
    print("Needs stdin")
    file = sys.stdin
else:
    print("File given at command line")
    file = open(sys.argv[1])

print (count_lines(file))

If I enter a file at the command line, ie python line_count1.py file_with_4_lines.txt, it works great, and I get the output:

File given at command line
4

However, if I enter it so that it DOES need stdin via python line_count1.py, I get the following output:

Needs stdin
_

But never actually does anything with my stdin entry. I can enter file_with_4_lines.txt, but then it just takes it and waits for me to input another stdin line, never breaking out until I have to kill the code in Task Manager.

What would be causing this to happen? From my understanding, as soon as I enter something for stdin that should trigger the rest of the code to go through. But it's not. What am I missing?

Are you expecting it to read the contents of file_with_4_lines.txt when you type file_with_4_lines.txt in the stdin case? Cause that's not how stdin works... — ShadowRanger
– ShadowRanger, Commented Mar 14, 2017 at 0:35
It's supposed to equate the file variable with stdin, which is then used in count_lines(file). — Chase
– Chase, Commented Mar 14, 2017 at 0:40

Community · Accepted Answer · 2017-04-13 12:36:24Z

1

This is not related to your code, but related to the stdin read behaviour on a terminal. See the following post for more information: https://unix.stackexchange.com/questions/16333/how-to-signal-the-end-of-stdin-input.

EDIT: As @Chase said, the key to terminate stdin on window is Ctrl+Z, and on linux it is Ctrl+D.

edited Apr 13, 2017 at 12:36

CommunityBot

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answered Mar 14, 2017 at 0:33

Stewart Smith

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2 Comments

Chase Over a year ago

This doesn't seem to solve it, entering file_with_4_lines.txt ^D doesn't exit stdin, and neither does file_with_4_lines.txt, then ^D on the next line.

Chase Over a year ago

TO FUTURE READERS: The problem was that I'm on Windows, not Linux. Windows ends with Ctrl+Z, not Ctrl+D.

ShadowRanger · Accepted Answer · 2017-03-14 00:46:47Z

It sounds like you want to accept a filename from stdin if not given at the command line, when what you're doing right now is attempting to count stdin itself.

If the goal is to process a given file, where the name comes from either stdin or the command line, then the code should be changed to:

if len(sys.argv) == 1:
    # Prompt for and read a single line from stdin to get the desired file name
    filename = input("Needs stdin")  # On Py2, use raw_input, not input
else:
    print("File given at command line")
    # Use argument as filename
    filename = sys.argv[1]

# Open the name provided at stdin or command line
# Use with statement so it's properly closed when you're done
with open(filename) as file:
    print(count_lines(file))

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