How do you read from stdin?

Question

I\'m trying to do some of the code golf challenges, but they all require the input to be taken from stdin. How do I get that in Python?

Answer 1

This will echo standard input to standard output:

import sys
line = sys.stdin.readline()
while line:
    print line,
    line = sys.stdin.readline()

Answer 2

argparse is an easy solution

Example compatible with both Python versions 2 and 3:

#!/usr/bin/python

import argparse
import sys

parser = argparse.ArgumentParser()

parser.add_argument('infile',
                    default=sys.stdin,
                    type=argparse.FileType('r'),
                    nargs='?')

args = parser.parse_args()

data = args.infile.read()

You can run this script in many ways:

1. Using stdin

echo 'foo bar' | ./above-script.py

  or shorter by replacing echo by here string:

./above-script.py <<< 'foo bar'

2. Using a filename argument

echo 'foo bar' > my-file.data
./above-script.py my-file.data

3. Using stdin through the special filename -

echo 'foo bar' | ./above-script.py -

Answer 3

You could use the fileinput module:

import fileinput

for line in fileinput.input():
    pass

fileinput will loop through all the lines in the input specified as file names given in command-line arguments, or the standard input if no arguments are provided.

Note: line will contain a trailing newline; to remove it use line.rstrip()

Answer 4

The problem I have with solution

import sys

for line in sys.stdin:
    print(line)

is that if you don't pass any data to stdin, it will block forever. That's why I love this answer: check if there is some data on stdin first, and then read it. This is what I ended up doing:

import sys
import select

# select(files to read from, files to write to, magic, timeout)
# timeout=0.0 is essential b/c we want to know the asnwer right away
if select.select([sys.stdin], [], [], 0.0)[0]:
    help_file_fragment = sys.stdin.read()
else:
    print("No data passed to stdin", file=sys.stderr)
    sys.exit(2)

Answer 5

Here's from Learning Python:

import sys
data = sys.stdin.readlines()
print "Counted", len(data), "lines."

---

On Unix, you could test it by doing something like:

% cat countlines.py | python countlines.py 
Counted 3 lines.

On Windows or DOS, you'd do:

C:\> type countlines.py | python countlines.py 
Counted 3 lines.

Answer 6

import sys

for line in sys.stdin:
    print(line)

Note that this will include a newline character at the end. To remove the newline at the end, use line.rstrip() as @brittohalloran said.