How do I read a file as the default user input?

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How do I read a whole file in python 3.6.x using the user input() function? Like the algorithms challenge sites do. For example:

  • I have a file with the following content:

    line 1
line 2
line 3
end

  • My program would do something like:

    x = input()
while x != 'end':
    print("I am at", x)
    x = input()

  • Then I would have as output:

    I am at line 1
I am at line 2
I am at line 3

I guess I need to wrap my python sample progam with another program (possible also python, or OS script) that call the first one passing the file as user input. But how could I do it?

user-input io python-3.x python
2 Answers

Usually such challenges will pipe the input via stdin, and expected the response via stdout.

To do that, run your command from a command line like so;

python program.py < input.txt

If you want to save the output of the script into a file, you can redirect it to one.

python program.py < input.txt > output.txt
user_15942860

The usual standard lib tool for this job is the fileinput module:

import fileinput

for line in fileinput.input():
    print(f'working with line {line}')

Then from the shell, take your pick of the following (they all work):

$ python3 my_script.py my_file.txt
$ python3 my_script.py < my_file.txt
$ cat my_file.txt | python3 my_script.py
user_6740390
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