How do I read from stdin?

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 -

I use the following method, it returns a string from stdin (I use it for json parsing). It works with pipe and prompt on Windows (not tested on Linux yet). When prompting, two line breaks indicate end of input.

def get_from_stdin():

  lb = 0
  stdin = ''

  for line in sys.stdin:
    if line == "\n":
        lb += 1
        if lb == 2:
            break
    else:
        lb = 0
        stdin += line

  return stdin

For Python 3 that would be:

# Filename e.g. cat.py
import sys

for line in sys.stdin:
    print(line, end="")

This is basically a simple form of cat(1), since it doesn't add a newline after each line. You can use this (after You marked the file executable using chmod +x cat.py such as:

echo Hello | ./cat.py

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)

Since Python 3.8 you can use assignment expression:

while (line := input()):
    print(line)

When using -c command, as a tricky way, instead of reading the stdin (and more flexible in some cases) you can pass a shell script command as well to your python command by putting the shell command in quotes within a parenthesis started by $ sign.

e.g.

python3 -c "import sys; print(len(sys.argv[1].split('\n')))" "$(cat ~/.goldendict/history)"

This will count the number of lines from goldendict's history file.

There is os.read(0, x) which reads xbytes from 0 which represents stdin. This is an unbuffered read, more low level than sys.stdin.read()