main.py

# Login and Register System using Python

import re
import os

# Class for verification of Username
class Username:

    def __init__(self, new_username=" "):
        self.new_username = new_username

    def validation(self, new_username=" "):
# Username verification conditions
        if re.search("^[0-9]", new_username):
            print("Username shouldn't start with Numerical digit.")
            return
        if re.search("^[!@#$%^&*]", new_username):
            print("Username shouldn't start with special character.")
            return
        if re.search("[A-Z]", new_username):
            print("Username shouldn't have uppercase letter.")
            return
        if not re.search("[a-z0-9]{2,20}@[a-z]{2,10}[.][a-z]{2,4}", new_username):
            return
        return True


# Class for verification of Password
class Password:

    def __init__(self, new_password=" "):
        self.new_password = new_password

    def verify(self, new_password=" "):
# Password Verification Conditions
        if not re.search("[A-Z]", new_password):
            print("Password must have 1Uppercase character")
            return
        if not re.search("[a-z]", new_password):
            print("Password must have 1 Lowercase character")
            return
        if not re.search("[0-9]", new_password):
            print("Password must have 1 Numerical character")
            return
        if not 16 >= len(new_password) > 5:
            print("Password must have 5 to 16 characters")
            return
        if not re.search(r'[@#!$%^&*|;:_]', new_password):
            print("Password must contain 1 special character")
            return
        return True


# user have to select they want to Login/Register
userchoice = input("Please choose Login / Register: ")
new_username = "notvalid"
new_password = "notvalid"
corrected_password = "notvalid"
File = open("Credentials.txt", "a")

# Login Part - if user chooses login
if userchoice == "Login":
   username = input("Username: ")
   password = input("Password: ")

# This part checks if the username and password entered are valid
   File = open("Credentials.txt", "r")
   lines = File.readlines()
   File.close()
   for itr in lines:
     temp = itr.split()
     if username and password in temp:
      print("User Validated")
      break
   else:
     print("Invalid Credentials.Type 'O' to retrieve your old password or 'N' to enter new password.")

# Here, user can Retrieve their old password or reset with new password
     A = input()

# Old Password Retrieving Part
     if A == "O":
       reenter_username = input("Username: ")
       for itr in lines:
         temp = itr.split()
         if reenter_username in temp:
           p = temp[1:]
           print("Your Password is:", *p)
           break
       else:
        print("User not registered! Go to Registration.")

# Overwriting Existing Password part
     elif A == "N":
         reenter_username = input("Username: ")
         File1 = open("Credentials.txt", "r")
         lines = File1.readlines()
         for itr in lines:
             temp = itr.split()
             Temp_File = open("Temp_File.txt", "a")
             if len(lines) > 0:
                if temp[0] == reenter_username:
                    for x in corrected_password:
                        corrected_password = input("Please, Enter New Password: ")
                        cor = Password()
                        if cor.verify(corrected_password):
                            Temp_File.write(reenter_username+" "+corrected_password+"\n")
                            print("Password updated Successfully.")
                            break
                else:
                    Temp_File.writelines(temp[0]+" "+temp[1])
                    Temp_File.write("\n")
         File1.close()
         Temp_File.close()
         os.remove("Credentials.txt")
         os.rename("Temp_File.txt", "Credentials.txt")

     else:
        print("User not found, please go to Registration.")
   File.close()

# Registration part - If user chooses Registration
elif userchoice == "Register":
    for x in new_username:
        new_username = input("E-Mail id: ")

# This part verifies the username with the help of class Username
        c = Username()
        if c.validation(new_username):
                File = open("Credentials.txt", "r")
                lines = File.readlines()
                File.close()
                for itr in lines:
                    temp = itr.split()
                    if new_username in temp:
                        print("Username already exists!")
                        break
                else:
                    break
        else:
            print("provide valid email address")

# This part verifies the password with the help of Password Class
    for i in new_password:
        new_password = input("Password: ")
        c = Password()
        if c.verify(new_password):
            print("Registration Successful!")
            break
    File = open("Credentials.txt", "a")
    File.write(new_username+" "+new_password+"\n")
    File.close()
else:
    print("Please make a valid choice")

# ....End of the Program....