How to make a python script "pipeable" in bash?

See this simple echo.py:

import sys

if __name__ == "__main__":
    for line in sys.stdin:
        sys.stderr.write("DEBUG: got line: " + line)
        sys.stdout.write(line)

running:

ls | python echo.py 2>debug_output.txt | sort

output:

echo.py
test.py
test.sh

debug_output.txt content:

DEBUG: got line: echo.py
DEBUG: got line: test.py
DEBUG: got line: test.sh

I'll complement the other answers with a grep example that uses fileinput to implement the typical behaviour of UNIX tools: 1) if no arguments are specified, it reads data from stdin; 2) many files can be specified as arguments; 3) a single argument of - means stdin.

import fileinput
import re
import sys

def grep(lines, regexp):
    return (line for line in lines if regexp.search(line))

def main(args):
    if len(args) < 1:
        print("Usage: grep.py PATTERN [FILE...]", file=sys.stderr)
        return 2 

    regexp = re.compile(args[0])
    input_lines = fileinput.input(args[1:])

    for output_line in grep(input_lines, regexp):
        sys.stdout.write(output_line)

if __name__ == "__main__":
    sys.exit(main(sys.argv[1:]))

Example:

$ seq 1 20 | python grep.py "4"
4
14