Python: Reading input from a file?

Question

I have a text file of the format:

2
1 3
2 4

I have written a python program that asks for number of inputs and prints out the values sum .

n = int(raw_input())
for _ in range(n):
     a,b = map(int,raw_input().split())
     print a+b

But

How should I take input from file such that first line is number of test cases and next lines are inputs?
Print the output to a file?

score 5 · Answer 1 · answered Apr 11 '15 at 18:57

5

1 + 2. Shell redirection.

$ python foo.py < input.txt > output.txt

answered Apr 11 '15 at 18:57

Ignacio Vazquez-Abrams

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score 1 · Answer 2 · edited May 23 '17 at 12:06

You can use the fileinput module to make your script support both reading input from a file and reading from stdin.

import fileinput
num_inputs = None
for line in fileinput.input():
    if num_inputs is None:
        num_inputs = int(line)
    else:
        a,b = map(int, line.split())
        print a+b

This way, if you pass a filename as an argument to your script, it reads from it, else it reads from stdin.

Note that in this snippet I just read all the inputs in file after the first line, and make no real use of the num_inputs variable, but you can easily change that to enforce the expected number of inputs is actually read.

As to writing to a file, this is a basic python question, which has already been answered here.

score 1 · Accepted Answer · answered Apr 11 '15 at 19:03

Assuming your input comes from a file, inp.txt, like:

$ cat inp.txt 
2
1 3
2 4

You can use the code below, which basically reads the input file line by line, and computes the values and stores them:

with open("inp.txt", "r") as inp, open("out.txt", "w") as out:
    no_of_cases = int(inp.readline())
    for line in inp:
        a,b = map(int,line.split())
        out.write("%s\n" % (a + b))

Note that no_of_cases has not been used in the code anywhere above because it is redundant for this example.

This will produce the output file out.txt for you, with following data:

$ cat out.txt 
4
6

Python: Reading input from a file?

3 Answers3