Implementation of head using docopt

head.py

""" 
Usage: head [options] [FILE]...

  -c K, --bytes=K        print the first K bytes of each file;
                         with the leading `-', print all but the last
                         K bytes of each file
  -n K, --lines=K        print the first K lines instead of the first 10;
                         with the leading `-', print all but the last
                         K lines of each file [default: 10]
  -q, --quiet, --silent  never print headers giving file names
  -v, --verbose          always print headers giving file names
      --help             display this help and exit
      --version          output version information and exit
"""

import sys
from collections import namedtuple
import docopt


HeadParams = namedtuple("HeadParams", "version bytes lines print_header files")


def err(msg):
    sys.stdout.write("head: {0}\n".format(msg))
    sys.exit(1)


def get_params(args):
    try:
        bytes = int(args["--bytes"]) if args["--bytes"] else None
    except ValueError:
        err("invalid value for bytes")

    try:
        lines = int(args["--lines"]) if args["--lines"] else None
    except ValueError:
        err("invalid value for lines")

    silent = args["--silent"]
    verbose = args["--verbose"]
    version = args["--version"]

    files = args["FILE"]
    if not files:
        files.append("-")

    print_header = True
    if len(files) == 1:
        print_header = False

    if silent:
        print_header = False

    if verbose:
        print_header = True

    return HeadParams(version, bytes, lines, print_header, files)


def print_one_head(fp, params):
    if params.print_header:
        sys.stdout.write("==> {0} <==\n".format(fp.name))
    if params.bytes:
        sys.stdout.write(fp.read()[:params.bytes])
    else:
        sys.stdout.write("".join(fp.readlines()[:params.lines]))


def head():
    args = docopt.docopt(__doc__)
    params = get_params(args)

    if params.version:
        print("head version: 1.0.0")
        return

    for filename in params.files:
        try:
            if filename == "-":
                print_one_head(sys.stdin, params)
            else:
                with open(filename) as fp:
                    print_one_head(fp, params)
        except IOError as e:
            err("error opening {0}: {1}".format(filename, e))


if __name__ == "__main__":
    head()