Implementing ‘cat’ in Python (reading a file character by character)

Though I like messing about with stuff and enjoy programming ‘just because I can’ I don’t really have a favorite programming language and tend to just use what ever is available, which in the case of my next little project looks like it will by Python.

Trouble is that I can’t be bothered to spend too much time trying to remember the boring stuff like syntax and layout, so I generally find that it is easier to modify a bit of existing code than start from scratch. Unfortunately in this case I couldn’t as all my existing Python scripts were on another machine, so as it had been a while since I last used it and I hadn’t tried using character I/O before I decided see how a few things might out work in a relatively simple test program.

I figured that once that was working I’d have started remembered all the little details like the indenting rules and so on. It didn’t take very long to put together, once I’d worked out that you can’t have an assignment statement in an expression!

One of the little features of the Python language that I’d forgotten – again.

import sys, os
for _arg in sys.argv[1:]:
    with open(_arg, 'r') as _file:
      _char =
      while _char:
        _char =
  except IOError as _err:
    sys.stderr.write(os.path.basename(sys.argv[0]) + ": " + _arg +
    ": " + _err.strerror + "\n")

That wasn’t too hard. It might have been better to use an iterator to avoid having to read from the file at the beginning and end of the loop, but I particularly wanted to read the data one character at a time.

import sys, os
for _arg in sys.argv[1:]:
    with open(_arg, 'r') as _file:
      for _line in iter((_file.readline),""):
        print _line,
  except IOError as _err:
    sys.stderr.write(os.path.basename(sys.argv[0]) + ": " + _arg +
    ": " + _err.strerror + "\n")

At this point scope creep set in, and I began to wonder if I could get my script to behave in the same sort of way as the real ‘cat’ command. This meant adding quite a bit of extra code to parse the command line arguments, keep track of blank lines, and so on but it was all good practice. I know I should probably have used a module like ‘argparse’ to parse the command line arguments, but I as I’ve never used it before I decided not to.

#!/usr/bin/python -s
#  py-cat
#  Implements a subset of the GNU 'cat' functionality in python, with  added
#  option of inserting delay between characters...
#  This  program is free software: you can redistribute it and/or modify  it
#  under  the  terms of the GNU General Public License as published  by  the
#  Free  Software  Foundation, either version 3 of the License, or (at  your
#  option) any later version.
#  This  program  is  distributed in the hope that it will  be  useful,  but
#  WITHOUT   ANY   WARRANTY;   without  even   the   implied   warranty   of
#  Public License for more details.
#  You  should have received a copy of the GNU General Public License  along
#  with this program.  If not, see <>.
#  20 Mar 16   0.1   - Initial version - MEJT
#   3 Apr 16   0.2   - Reads input files line by line, added ability to read
#                      from standard input - MEJT
#  25 Nov 17   0.3   - Tidied up parameter parsing code and added ability to
#                      parse values for parameters - MEJT
#                    - Added an optional delay between characters - MEJT
#                    - Version number now a 'constant' - MEJT
import sys, os
import time
def _about():
    "Usage: " + sys.argv[0] + "[OPTION]... [FILE]...\n" +
    "Concatenate FILE(s)to standard output.\n" + "\n" +
    "  -b, --number-nonblank    number nonempty output lines," + 
    " overrides -n\n" + 
    "  -n, --number             number all output lines \n" +
    "  -r, --restart            line numbers start at zero, implies -n\n" +
    "  -s, --squeeze-blank      suppress repeated empty output lines\n" +
    "  -d, --delay              delay between each byte\n" +
    "  -?, --help               display this help and exit\n" +
    "      --version            output version information and exit\n\n" +
    "Example:\n" +
    "  " + _os.path.basename(sys.argv[0]) + " f g\t   output f's contents, then g's contents.\n")
  raise SystemExit
def _version():
  sys.stdout.write(os.path.basename(sys.argv[0]) + " " + str(VERSION) +"\n"
    "License GPLv3+: GNU GPL version 3 or later <>.\n"
    "This is free software: you are free to change and redistribute it.\n"
    "There is NO WARRANTY, to the extent permitted by law.\n")
  raise SystemExit
def _error(_error):
  sys.stderr.write(os.path.basename(sys.argv[0]) + ": " + _error + "\n")
  raise SystemExit
def _print(_line):
  global _number, _nonblank, _strip, _count, _blanks
  if len(_line) > 1: _blanks = 0
  if not(_blanks > 1 and _strip):
    if _number and not(len(_line) == 1 and _nonblank):
      _count += 1
      sys.stdout.write("%6d  " % _count)
    for _char in _line:
  _blanks += 1
_names = []
_restart = False
_number = False
_nonblank = False
_strip = False
_delay = 0
_count = 1
while _count < len(sys.argv):
  _arg = sys.argv [_count]
  if _arg[:1] == "-" and len(_arg) > 1:
    if _arg in ["--squeeze-blank""-s"]:
      _strip = True
    elif _arg in ["--restart""-r"]:
      _number = True
      _restart = True
    elif _arg in ["--number""-n"]:
      _number = True
    elif _arg in ["--number-nonblank""-b"]:
      _number = True
      _nonblank = True
    elif _arg in ["--delay","-d"]:
      _count += 1
      if _count < len(sys.argv):
        _delay = sys.argv [_count]
          _delay = int(_delay)
          if _delay < 0 or _delay > 5000:
            _error("delay out of range")
        except ValueError:
          _error("delay invalid")
    elif _arg in ["--help""-?"]:
    elif _arg in "--version":
      if _arg[:2] == "--":
        _error ("unrecognized option -- '" + (_arg + "'"))
        _error ("invalid option -- '" + (_arg[1:] + "'"))
    _names.append(_arg) # If it isn't a qualified 
  _count += 1
if not len(_names) : _names.append("-"# Default to stdin.
_count = 0
_blanks = 0
for _name in _names: 
  if _restart : _count = 0
    if _name== "-"# Handle input from standard input.
      _line = sys.stdin.readline() # Read ahead.
      while _line:
        _line = sys.stdin.readline()
    else# Read from file.
      with open(_name, 'r'as _file:
        for _line in _file:
  except IOError as _err:
    _error(_name + ": " + _err.strerror)
  except KeyboardInterrupt# Catch ^C

After implementing a couple of the options available in ‘cat’, and adding one of my own devising, I decided that I’d had enough – besides I needed to actually start work on the original problem! However, I now have a useful template to get me started on other projects.

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