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import random
import string
def shift(current_position, distance, direction: (0, 1)):
 direction = 1 if direction else -1
 return current_position + direction * distance
def stream_cipher(message, key, do_encrypt=True):
 """
 This function will receive a message, a key, and the
 decision to encrypt or decrypt the message.
 The function uses a stream cipher encryption algorithm 
 which replaces each letter in the message with a 
 pseudo-random character from a given character set.
 Example:
 >>> stream_cipher("This is a test", 1234)
 'wPwV~#5;:D"905'
 >>> stream_cipher('wPwV~#5;:D"905', 1234, False)
 'This is a test'
 """
 random.seed(key)
 # This must be *2 so a character shifted
 # beyond the end of the character set will
 # loop back to the beginning.
 characters = 2 * (
 string.ascii_letters +
 string.digits +
 string.punctuation + ' '
 )
 # I declare this in a variable so the 
 # program can work with a variable length character set
 lenchars = len(characters)//2
 # This will replace each character in the message
 # with a pseudo-random character selected from
 # the character set.
 return ''.join([characters[shift(characters.index(message[each_char]), lenchars - int(lenchars * random.random()), do_encrypt)] for each_char in range(len(message))])
def main():
 message = input("Input a message: ")
 key = input("Input a key: ")
 while True:
 do_encrypt = input("Encrypt or decrypt the message? (1,0): ")
 if do_encrypt in ('1', '0'):
 break
 print("Please input a valid number,\n"
 "either 0 for decryption or 1 for encryption.")
 print(encrypt(message, key, do_encrypt == '1'))
 input("Press enter to exit.")
if __name__ == '__main__':
 main()

import random
import string
def shift(current_position, distance, direction: (0, 1)):
 direction = 1 if direction else -1
 return current_position + direction * distance
def stream_cipher(message, key, do_encrypt=True):
 """
 This function will receive a message, a key, and the
 decision to encrypt or decrypt the message.
 The function uses a stream cipher encryption algorithm 
 which replaces each letter in the message with a 
 pseudo-random character from a given character set.
 Example:
 >>> stream_cipher("This is a test", 1234)
 'wPwV~#5;:D"905'
 >>> stream_cipher('wPwV~#5;:D"905', 1234, False)
 'This is a test'
 """
 random.seed(key)
 # The character set must be multiplied by 2 so
 # a character shifted beyond the end of the 
 # character set will loop back to the beginning.
 characters = 2 * (
 string.ascii_letters +
 string.digits +
 string.punctuation + ' '
 )
 # I declare this in a variable so the 
 # program can work with a variable length character set
 lenchars = len(characters)//2
 # This will replace each character in the message
 # with a pseudo-random character selected from
 # the character set.
 return ''.join([characters[shift(characters.index(message[each_char]), lenchars - int(lenchars * random.random()), do_encrypt)] for each_char in range(len(message))])
def main():
 message = input("Input a message: ")
 key = input("Input a key: ")
 while True:
 do_encrypt = input("Encrypt or decrypt the message? (1,0): ")
 if do_encrypt in ('1', '0'):
 break
 print("Please input a valid number,\n"
 "either 0 for decryption or 1 for encryption.")
 print(encrypt(message, key, do_encrypt == '1'))
 input("Press enter to exit.")
if __name__ == '__main__':
 main()

import random
def Crypt(string,key,encrypt=1):
 """
 This program will input a message, a key, and the
 decision to encrypt or decrypt the message.
 It will then output the ciphertext created by running
 the message through a stream cipher encryption algorithm.
 What is a stream cipher encryption algorithm?:
 A stream cipher encryption algorithm shifts each letter
 in the message along the alphabet by a pseudorandom
 amount determined by the inputted key.
 """
 random.seed(key)
 # This must be *2 so a letter shifted beyond the end of the alphabet will loop back to the beginning
 alphabet = 2 * " AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz1234567890.;:,'?/|{}[]-=+_!@#$%^&*()<>`~"
 # I declare this in a variable so the program can work with a variable length alphabet
 lenalpha = int(len(alphabet)/2)
 if encrypt:
 # This will shift each letter up the alphabet by a pseudo-random amount based on the key
 return ''.join([alphabet[alphabet.index(string[p]) + lenalpha - int(lenalpha * random.random())] for p in range(len(string))])
 else:
 # This will shift each letter down the alphabet by a pseudo-random amount based on the key
 return ''.join([alphabet[alphabet.index(string[p]) - lenalpha + int(lenalpha * random.random())] for p in range(len(string))])
if __name__ == '__main__':
 message = input("Input your message: ")
 key = input("Input a key: ")
 finished_with_the_user = False
 while not finished_with_the_user:
 encrypt_question = input("Encrypt or decrypt a message?(1,0): ")
 if encrypt_question.isdigit():
 finished_with_the_user = True
 encrypt_question = int(encrypt_question)
 else:
 print("Please input a valid number.")
 print(Crypt(message, key, encrypt_question))
 input("Press enter to exit.")

import random
def Crypt(string,key,encrypt=1):
 """
 This program will input a message, a key, and the
 decision to encrypt or decrypt the message.
 It will then output the ciphertext created by running
 the message through a stream cipher encryption algorithm.
 What is a stream cipher encryption algorithm?:
 A stream cipher encryption algorithm shifts each letter
 in the message along the alphabet by a pseudorandom
 amount determined by the inputted key.
 """
 random.seed(key)
 # This must be *2 so a letter shifted beyond the end of the alphabet will loop back to the beginning
 alphabet = 2 * " AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz1234567890.;:,'?/|{}[]-=+_!@#$%^&*()<>`~"
 # I declare this in a variable so the program can work with a variable length alphabet
 lenalpha = int(len(alphabet)/2)
 if encrypt:
 # This will shift each letter up the alphabet by a pseudo-random amount based on the key
 return ''.join([alphabet[alphabet.index(string[p]) + lenalpha - int(lenalpha * random.random())] for p in range(len(string))])
 else:
 # This will shift each letter down the alphabet by a pseudo-random amount based on the key
 return ''.join([alphabet[alphabet.index(string[p]) - lenalpha + int(lenalpha * random.random())] for p in range(len(string))])
if __name__ == '__main__':
 message = input("Input your message: ")
 key = input("Input a key: ")
 finished_with_the_user = False
 while not finished_with_the_user:
 encrypt_question = input("Encrypt or decrypt a message?(1,0): ")
 if encrypt_question.isdigit():
 finished_with_the_user = True
 encrypt_question = int(encrypt_question)
 else:
 print("Please input a valid number.")
 print(Crypt(message, key, encrypt_question))
 input("Press enter to exit.")

Any suggested edits and edits that I think of will be applied below.

import random
import string
def shift(current_position, distance, direction: (0, 1)):
 direction = 1 if direction else -1
 return current_position + direction * distance
def stream_cipher(message, key, do_encrypt=True):
 """
 This function will receive a message, a key, and the
 decision to encrypt or decrypt the message.
 The function uses a stream cipher encryption algorithm 
 which replaces each letter in the message with a 
 pseudo-random character from a given character set.
 Example:
 >>> stream_cipher("This is a test", 1234)
 'wPwV~#5;:D"905'
 >>> stream_cipher('wPwV~#5;:D"905', 1234, False)
 'This is a test'
 """
 random.seed(key)
 # This must be *2 so a character shifted
 # beyond the end of the character set will
 # loop back to the beginning.
 characters = 2 * (
 string.ascii_letters +
 string.digits +
 string.punctuation + ' '
 )
 # I declare this in a variable so the 
 # program can work with a variable length character set
 lenchars = len(characters)//2
 # This will replace each character in the message
 # with a pseudo-random character selected from
 # the character set.
 return ''.join([characters[shift(characters.index(message[each_char]), lenchars - int(lenchars * random.random()), do_encrypt)] for each_char in range(len(message))])
def main():
 message = input("Input a message: ")
 key = input("Input a key: ")
 while True:
 do_encrypt = input("Encrypt or decrypt the message? (1,0): ")
 if do_encrypt in ('1', '0'):
 break
 print("Please input a valid number,\n"
 "either 0 for decryption or 1 for encryption.")
 print(encrypt(message, key, do_encrypt == '1'))
 input("Press enter to exit.")
if __name__ == '__main__':
 main()