Issues with character conversion

[rexrf]

So I'm trying to learn python and I came up with an idea for a little program that reads a text file, converts all of the characters into pseudo-random ones, and then creates a key to return it to human readable form. The issue is that, when I decode the message sometimes one character will get decoded incorrectly. So all 'a' characters might show up as '0', or 'M', etc.

---

import random
import pickle
import copy


"""
Encodes and decodes files
"""


def load(string):
    """
    function that converts characters to pseudo-random
    characters and creates a key to decipher later.
    """

    ## APPEND PATH HERE*
    ## opens file to store converted data in
    datafile = open('/your/path/here/data.dep', 'wb')
    ## oens file to store key in
    keyfile = open('/your/path/here/key.dep', 'wb')
    ## defines some default values
    key = []
    user_string = []
    working_pool = {}
    out_key = {}
    new_string = []

    ## character map, gets copied to create character pools
    all_char = ['a', 'b', 'c', 'd', 'e',
                'f', 'g', 'h', 'i', 'j',
                'k', 'l', 'm', 'n', 'o',
                'p', 'q', 'r', 's', 't',
                'u', 'v', 'w', 'x', 'y',
                'z', 'A', 'B', 'C', 'D',
                'E', 'F', 'G', 'H', 'I',
                'J', 'K', 'L', 'M', 'N',
                'O', 'P', 'Q', 'R', 'S',
                'T', 'U', 'V', 'W', 'X',
                'Y', 'Z', '0', '1', '2',
                '3', '4', '5', '6', '7',
                '8', '9', '!', '@', '#',
                '$', '%', '^', '&', '*',
                '+', '-', '_', '=', '~',
                ' ', '.', ',', ';', '(',
                ')', '<', '>', '?', ':',
                '|', '/', '~', '[', ']',
                '{', '}', '\'', '"', '\t',
                '\n', '\\']
    ## above list copied
    char_copy = copy.deepcopy(all_char)
    pool = copy.deepcopy(all_char)

    ## for each character in the character map,
    for char in char_copy[:]:
        ## pick a random character from pool
        temp_char = random.choice(pool)
        ## appends working_pool dictionary
        ## {'random': 'actual'} EXAMPLE
        working_pool[temp_char] = char
        ## appends my_key dictionary, creates key
        ## {'actual': 'random'} EXAMPLE
        out_key[char] = temp_char
        ## remove the random character assigned
        ## from the pool so it cant be picked twice
        pool.remove(temp_char)

    ## opens user defined file supplied by function parameter
    with open(string) as infile:
        ## while true, examine each character
        while True:
            ## append 'user_string' with each character in file
            ch = infile.read(1)
            user_string.append(ch)
            ## if the character is in the conversion pool
            if ch in working_pool:
                ## convert character
                new_string.append(working_pool[ch])
            else:
                ## keep character as is
                new_string.append(ch)
            ## repeat until EOF
            if not ch:
                eof = 'true'
                break
        ## start DEBUG
        #test = ''.join(str(z) for z in new_string)
        #ustring = ''.join(str(y) for y in user_string)
        #print(test)
        #print('\n')
        #print(ustring)
        #print(working_pool)
        ## end DEBUG

    ## serializes converted string and stores it
    pickle.dump(new_string, datafile)
    pickle.dump(out_key, keyfile)
    datafile.close()
    keyfile.close()

def unload(data, key):
    """
    function that, given the correct keyfile,
    will translate converted string to human
    readable clear text
    """

    ## opens files supplies by function parameter
    input_file = open(data, 'rb')
    decoder = open(key, 'rb')

    active = True
    ## while active is true
    while active:
        try:
            ## dump data into 'contents' until EOF
            contents = pickle.load(input_file)
        except EOFError:
            ## if EOF, active is false
            active = False


    ## same as above but for decoder
    active = True
    while active:
        try:
            load_key = pickle.load(decoder)
        except EOFError:
            active = False

    ## sets empty list for converting
    converted_string = []
    ## for each value in the encrypted data,
    for ch in contents[:]:
        ## if the value is in the key,
        if ch in load_key:
            ## add the decoded character to converted_string
            converted_string.append(load_key[ch])
        #else:
            #converted_string.append(ch)

    clear_text = ''.join(str(e) for e in converted_string)
    ## start DEBUG
    #print('\n')
    #print(load_key)
    ## end DEBUG
    ## print unencrypted results
    print(clear_text)
    input_file.close()
    decoder.close()


## CALL FUNCTIONS HERE
load('/fake/data/someFile.txt')
unload('/your/path/here/data.dep', '/foo/bar/key.dep')

any help or general python advice is greatly appreciated :)