Naïve LZW implementation for a school assignment using Python.

lzw.py

class LZW:
    def __init__(self):
        self.ctx = ""
        self.table = [ chr(i) for i in range(256) ]
        self.empty_index = 256
        self.last_index = None
        self._model = []

    def encode(self, c):
        """Receives a character to encode. Updates the table."""
        curr = self.ctx + c
        try:
            self.last_index = self.table.index(curr)
            self.ctx = curr
        except ValueError:
            self.table.append(curr)
            self._model.append(self.last_index)
            self.ctx = c
            self.last_index = self.table.index(c)

    def model(self):
        """Returns a human-readable version of the compressed file."""
        m = ""
        for c in self._model:
            if c < 256:
                m += "({})".format(repr(chr(c)))
            else:
                m += "({})".format(c)

        return m

    def compressed(self):
        """Returns a string containing a text representation of the compressed binary file."""
        c = "".join("{:09b}".format(c) for c in self._model)
        return c + "0" * (len(c) % 8)

    def print_table(self):
        """Prints the current table from 256 onwards."""
        print("pos | chars")
        print("~~~~~~~~~~~")
        for i in range(256, len(self.table)):
            print("{} | {}".format(i, repr(self.table[i])))


if __name__ == '__main__':
    import argparse
    import os

    parser = argparse.ArgumentParser(description='Encodes the text passed through stdin using the LZW algorithm. Input should end with an empty line.')
    parser.add_argument('-no-info', action="store_true", help='Hide the extra information like the model representation and the table.')
    args = parser.parse_args()
    coder = LZW()
    for line in os.sys.stdin:
        for c in line:
            coder.encode(c)

    if args.no_info:
        print(coder.compressed())
    else:
        print("compressed: {}".format(coder.compressed()))
        print()
        print("model:      {}".format(coder.model()))
        print()
        print("table:")
        coder.print_table()