caesar python dec

# program to implement ceasar cipher decryption
import string


# this method takes string and shiftvalue as input and returns encrypted string as output
def decrypt(original_msg, shift_value = 1):

    allchars = string.digits + string.ascii_letters + " " + string.punctuation

    decrypted_msg = ""

    for letter in original_msg:
        if letter in allchars:
            position = allchars.index(letter)
            position -= shift_value

            # if the position becomes negative then we start from the back of the allchars
            if position < 0:
                position += len(allchars)

            position %= len(allchars)
            e_letter = allchars[position]
            decrypted_msg += e_letter
        else:
            decrypted_msg += letter

    return decrypted_msg

# method that uses the decrypt method and decrypts files by reading the file path and storing the decrypted file on desktop
def decrypt_file(shift_value):
    file_path = input("Enter File path : ")
    try:
        file1 = open(file_path, 'r+')
        f2 = open("C:/Users/MOHIT/Desktop/decrypted_file.txt", 'w')
        print("File opened Successfully!!")
        file_data = file1.read()
        print("File read Successfully!!")
        decrypted_data = decrypt(file_data, shift_value)
        f2.write(decrypted_data)
        print("Decrypted File stored on Desktop successfully!!")

    except Exception as e:
        print("Problems with opening the file")
        print(str(e))

## main function

# encrypt("hello my name is mohit" , 3)
print("Program to implement Ceasar Cipher decryption")
shift_value = int(input("Enter the shift value : "))
decrypt_file(shift_value)
# decrypt(data)

'''

Program to implement Ceasar Cipher decryption
Enter File path : F:/python_3.4/sample.txt
File opened Successfully!!
File read Successfully!!
Decrypted File stored on Desktop successfully!!

'''