A utility for converting strings to different cases.

case.py

#!/usr/bin/env python

"""case.py

A utility for converting strings to different cases.

Usage info is autogenerated by the argparse module and can be accessed
by running the program with the '-h' option.
"""

import sys
import argparse
import string

def get_args():
    """Obtain the arguments entered for our program on the command line."""
    # Define all of our arguments and their help messages
    description = ("case - A utility for converting strings to different"
                   " cases.")
    epilog = ("Note: Leading, trailing, and multiple whitespace is not"
              " preserved for -C/-t.")
    parser = argparse.ArgumentParser(add_help=False, description=description,
                                     epilog=epilog)

    # If we're running interactively, require a string
    if sys.stdin.isatty():
        parser.add_argument("string", help="The string to convert.")

    cases = parser.add_mutually_exclusive_group(required=True)
    cases.add_argument("-c", "--capitalize", action="store_true",
                       help="Capitalize the first letter of the string.")
    cases.add_argument("-C", "--capwords", action="store_true",
                       help="Capitalize every word.")
    cases.add_argument("-t", "--titlecase", action="store_true",
                       help="Capitalize words according to titlecase"
                            " rules.")
    cases.add_argument("-u", "--uppercase", action="store_true",
                       help="Make every letter uppercase.")
    cases.add_argument("-l", "--lowercase", action="store_true",
                       help="Make every letter lowercase.")
    cases.add_argument("-s", "--swapcase", action="store_true",
                       help="Make lowercase letters uppercase and"
                            " vice-versa.")

    parser.add_argument("-h", "--help", action="help",
                        help="Print this help message.")

    # Display a help message and exit if the program is run without arguments
    if len(sys.argv) == 1:
        parser.print_help()
        sys.exit(1)

    args = parser.parse_args()

    return args

def case_to_title(input_string):
    """Capitalize principle words."""
    # Build a *simplified* list of non-principles
    articles = ["a", "an", "the"]
    conjunctions = ["and", "but", "or"]
    prepositions = ["at", "by", "of"]

    excludes = articles + conjunctions + prepositions
    output_string = []

    for index, word in enumerate(input_string.split()):
        # Always capitalize the first word
        if index == 0:
            output_string.append(word.capitalize())
        else:
            lowercase = word.lower()
            if lowercase in excludes:
                output_string.append(lowercase)
            else:
                output_string.append(word.capitalize())

    return " ".join(output_string)

def process_string(args):
    """Call the function corresponding to the chosen case and print the
    processed string.
    """
    if sys.stdin.isatty():
        input_string = args.string
    else:
        input_string = sys.stdin.read()

    if args.capitalize:
        output_string = input_string.capitalize()
    elif args.capwords:
        output_string = string.capwords(input_string)
    elif args.titlecase:
        output_string = case_to_title(input_string)
    elif args.uppercase:
        output_string = input_string.upper()
    elif args.lowercase:
        output_string = input_string.lower()
    elif args.swapcase:
        output_string = input_string.swapcase()

    print output_string
    sys.exit(0)

def main():
    """Get program arguments and process them."""
    args = get_args()
    process_string(args)

if __name__ == "__main__":
    main()