Basic lzw compression help in python

Question

I'm just trying to write a really basic script that'll take some input text and compress it with lzw, using this package: http://packages.python.org/lzw/

I've never tried any encoding with python before and am thoroughly confused =( - I also can't find any documentation online about it, other than the package info.

Here's what I have:

import lzw

file = lzw.readbytes("collectemailinfo.txt", buffersize=1024)
enc = lzw.compress(file)
print enc

Any help or pointers of any kind would be much appreciated!

Thanks =)

Answer 1

Here is the package API : http://packages.python.org/lzw/lzw-module.html

You can read psuedo-code of compression and decompression here

Is there anything else you are confused about?

Here is an example:

Python

In this version the dicts contain mixed typed data:

def compress(uncompressed):
    """Compress a string to a list of output symbols."""

    # Build the dictionary.
    dict_size = 256
    dictionary = dict((chr(i), chr(i)) for i in xrange(dict_size))
    # in Python 3: dictionary = {chr(i): chr(i) for i in range(dict_size)}

    w = ""
    result = []
    for c in uncompressed:
        wc = w + c
        if wc in dictionary:
            w = wc
        else:
            result.append(dictionary[w])
            # Add wc to the dictionary.
            dictionary[wc] = dict_size
            dict_size += 1
            w = c

    # Output the code for w.
    if w:
        result.append(dictionary[w])
    return result



def decompress(compressed):
    """Decompress a list of output ks to a string."""

    # Build the dictionary.
    dict_size = 256
    dictionary = dict((chr(i), chr(i)) for i in xrange(dict_size))
    # in Python 3: dictionary = {chr(i): chr(i) for i in range(dict_size)}

    w = result = compressed.pop(0)
    for k in compressed:
        if k in dictionary:
            entry = dictionary[k]
        elif k == dict_size:
            entry = w + w[0]
        else:
            raise ValueError('Bad compressed k: %s' % k)
        result += entry

        # Add w+entry[0] to the dictionary.
        dictionary[dict_size] = w + entry[0]
        dict_size += 1

        w = entry
    return result

How to use:

compressed = compress('TOBEORNOTTOBEORTOBEORNOT')
print (compressed)
decompressed = decompress(compressed)
print (decompressed)

Output:

['T', 'O', 'B', 'E', 'O', 'R', 'N', 'O', 'T', 256, 258, 260, 265, 259, 261, 263]
TOBEORNOTTOBEORTOBEORNOT

NOTE: this example is taken from here