python - The message packer reads a string

The message packer reads a string

You need to implement a Message Packer. The message packer reads a string

Rules:

• adds two chars ZZ to mark the beginning of the message
• add number 0 to mark the end of the message.
• The input string is reversed inside the message.
• If the input string contains char Z then it is replaced by # in the message. The lower case character z is retained
• If the input string contains numeric character 0 then it is replaced by $.
• Only lower case letter ([a-z]), upper case letter ([A-Z]) and numeric characters ([0-9]) are accepted. If the string contains any other characters an error message Invalid String
• Maximum number of characters accepted in the string is 8. If the number exceeds 8 then Exceeded Limit   is output

Sample:

• Input : You need to read a string from STDIN.
• Output :  Message Packed as per rules above or Invalid String or Exceeded Limit is  printed.
• 
  
• Input: PQRS
• Output: ZZSRQP0
• 
  
• Input: ABbcZdz
• Output: ZZzd#cbBA0
• 
  
• Input: X1340yZ
• Output: ZZ#y$431X0
• 
  
• Input: AB*D1$34
• Output: Invalid String
• 
  
• Input: ABCD12345
• Output: Exceeded Limit

Program:

def main():

    message = raw_input("Enter value :")

    count = 0

    reverse = ''.join(reversed(message))    # Reversed the string , must need join

    lis = list(reverse)          # Reverse the string

    lengh = len(reverse)

    if lengh > 8:                # Length should be less than 8

        print "Exceeded Limit"

        return

    count = 0

    for char in lis:

        if char == 'Z':

            lis[count] = '#'

        elif char == '0':

            lis[count] = '$'

        elif not (char.isupper() or char.islower() or char.isdigit()):

            print "Invalid String"

            return

        count = count + 1

    lis.append('0')

    lis = ['Z'] + lis                 # Append the Z to the begging of the list

    lis = ['Z'] + lis

    result = ''.join(lis)            # Convert list to the string

    print result

main()

Output:

acp_python$ python messagePacker.py

Enter value :ABbcZdz

ZZzd#cbBA0

acp_python$ vi messagePacker.py

acp_python$ python messagePacker.py

Enter value :X1340yZ

ZZ#y$431X0

acp_python$ python messagePacker.py

Enter value :AB*D1$34

Invalid String

acp_python$ python messagePacker.py

Enter value :ABCD12345

Exceeded Limit