Taking multiple lines of input in Python from the system.in

Question

For one of my assignments, rather than reading directly from a text file, we are directly taking the input from sys.in. I was wondering what the best way of obtaining this input and storing it would be? So far, I've tried using:

sys.stdin.readlines() -- But this will not terminate unless it recieves an EOF statement, while a zero in my input signals the end of the file.

sys.stdin.readline() -- It will only read the final line of the input code.

input() -- I can only read in the first line of the input and when I try to loop it, I end up with a syntax error.

An example of the input is below:

My attempt at obtaining the input:

input_list = []

    while True:
        input_str = input("")
        if input_str == '0':
            break
        else:
            input_list.append(input_str)

    print(input_list)

Returns this error while parsing the second line through:

Any help with this would be greatly appreciated. I've probably spent longer trying to get the input to work now than the actual assignment now.

EDIT: The version of Python we are using is 3.4

FINAL EDIT: Like both the answers said, it turns out that the university labs have a older version of python running when run in cmd prompt in the labs, where the input() method started behaving differently. Once I tried the code at home, it worked as intended.

mhawke · Accepted Answer · 2015-07-30 02:24:53Z

1

Avoid using input() as this behaves differently for different versions of Python. Instead, you can read from sys.stdin, this should work:

import sys

input_list = []

for line in sys.stdin:
    if line.strip() == '0':
        break
    input_list.append(line)
else:
    print('Warning: EOF occurred before "0" terminator')

print(input_list)

The for loop will terminate when a line with a single 0 is read, or when EOF is seen. The else lets you handle the case when the input terminates before 0 is seen; you can treat that as an error if that suits your application.

For your input, the output would be:

['3\n', '2 1 3\n', '3 2 1\n', '1 3 2\n', '2 1 3\n', '3 2 1\n', '1 3 2\n', '2\n', '1 2\n', '1 2\n', '2 1\n', '2 1\n']

answered Jul 30, 2015 at 2:24

mhawke

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Anand S Kumar · Accepted Answer · 2015-07-30 02:09:27Z

1

For Python 2.x , you should use raw_input() .

In Python 2.x , using input() would actually try to evaluate the whatever was inputted, which can lead to issues (like the one you encountered) , and is dangerous (since it evaluates whatever the user inputs) .

raw_input() would not evaluate the inputted value, but would only return it as a string.

answered Jul 30, 2015 at 2:09

Anand S Kumar

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7 Comments

Jeffrey Chen Over a year ago

Thank you for your quick response. Unfortunately we are using python 3.4 at the moment, I'll edit the post now to adknowledge that .

Anand S Kumar Over a year ago

Are you 100% sure? Try creating a script - import sys; print(sys.version) , and then run the script (without specifying python executable , like you did for your actual code).

Jeffrey Chen Over a year ago

Yep, the current version is : 3.4.2 (v3.4.2:ab2c023a9432, Oct 6 2014, 22:16:31)

Anand S Kumar Over a year ago

And you surely created a new script for this and ran? Or just add that line to the start of your hoopla.py and again run the same code and try to update the question with the new screenshot for that.

Jeffrey Chen Over a year ago

It seems like you were right, when I ran it through cmd this time it came up with 2.7.3 . Im terribly sorry! Is there a way that I can specify which version of python the script should run? Thanks.

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