Username-Password Matching Program
Validate input
You should be certain that your program wouldn't fail for any kind of bad input, something the user shouldn't have entered that might cause the program to behave incorrectly
Is this possible for your program? Yes, take this line
username = input(&quot;Please enter your username: &quot;)

If the user enters Ctrl+Z here, it causes an error and the program stops.

EOFError

We can catch these exceptions using try and except in Python
try:
    username = input(&quot;Please enter your username: &quot;)
except (Exception, KeyboardInterrupt): 
    print(&quot;Please enter a valid username!&quot;)

The same applies to the input for password
One step further, I would also like to check the length of the details, i.e if the username or password is too short. Show a custom message if that is true

On an exception
Due to the changes we made previously, we can easily print an error message if we catch an exception rather than the termination of the program. But what happens after?
You have two options, either to stop the program there because you cannot process bad input or, you can ask for input again. 
If you choose the second option, that will take a few more lines of code but is surely better because the user gets another chance rather than running the whole program again
For that, we need to wrap it in a while loop that will iterate till it gets valid input
def username_input():
    while True:
        try:
            username = input(&quot;Please enter your username: &quot;)
        except Exception:
            print(&quot;Please a enter valid username! \n\n&quot;)
            continue 
        return username

def password_input():
    while True:
        try:
            password = input(&quot;Please enter your password: &quot;)
        except Exception:
            print(&quot;Please enter a valid password! \n\n&quot;)
            continue
        return password

Here we are repeating ourselves, the functions are identical. With a little hack, we can clean up the code
def ask_input(field):
    while True:
        try:
            word = input(f&quot;Please enter your {field}: &quot;)
        except (Exception, KeyboardInterrupt):
            print(f&quot;Please enter a valid {field}! \n\n&quot;)
            continue
        if not word: continue # if field is empty
        return word

def take_input():
    username = ask_input(&quot;username&quot;)
    password = ask_input(&quot;password&quot;)
    return username, password


Split work into functions
In my last point, you must've noticed I moved something into a function
We can really clean up our code if we follow the SRP rule or the Single-responsibility principle

The single-responsibility principle (SRP) is a computer-programming
principle that states that every module, class, or function in a
computer program should have responsibility over a single part of that
program's functionality,

If assig the task of taking input in this program, to let's say take_input()
We can re-use the same function if we would ever want to perform the same task again, without copy-pasting the same segment of code
username, password = take_input()
process_login( username, password ):


take_input() will take care of the input related tasks, and when it has proper valid it returns a username and password
process_login() decides whether the login will be authorized or not, based on the records


Check with key username
if username in username_password:
    if username == &quot;Mert&quot; and password == username_password[username]:
        print(&quot;Welcome Sir&quot;)
    else:
        print(&quot;You're not authorized!&quot;)

This works right now, but only due to the fact that the size of records is 1, any extra records added and your algorithm will fail, if the size was 50 you cannot write 50 if statements, you need to check with the username entered
def process_login(username, password, records):
    try:
        if records[username] == password:
            print(&quot;Authorized&quot;)
            return True
    except KeyError:
        print(&quot;Not authroized&quot;)
        return False

Here, if the username isn't present in records, a KeyError is raised, which I will catch and print Not authorized

Re-written
def ask_input(field):
    while True:
        try:
            word = input(f&quot;Please enter your {field}: &quot;)
        except (Exception, KeyboardInterrupt):
            print(f&quot;Please enter a valid {field}! \n\n&quot;)
            continue
        if not word: continue

        return word

def take_input():
    username = ask_input(&quot;username&quot;)
    password = ask_input(&quot;password&quot;)
    return username,password


def process_login(username, password, records):
    try:
        if records[username] == password:
            print(&quot;Authorized&quot;)

    except KeyError:
        print(&quot;Not authroized&quot;)
        

I have left out the last part, which is the &quot;registration&quot;. Which I will explain next
records = {
    &quot;__Testu&quot; : &quot;__Testp&quot;,
    &quot;__Testu2&quot; : &quot;__Testp2&quot;
}

username, password = take_input()
process_login(username, password, records)

Write records into a file
As it is currently, you have pre-added a record to your dictionary, any record added after this is pointless since the next time you run the program, the dictionary will be initialized again with the same one record you added. 
You need some way to save the users records so that the next time he opens your program, his previous details are saved
The best way to do this is the user the csv module in Python, - csv.DictReader which will automate all reading/writing of the file for you. But is something you would have to write on your own.
Cheers
&gt;&gt;&gt; Please enter your username: (empty input)
&gt;&gt;&gt; Please enter your username: __Testu
&gt;&gt;&gt; Please enter your password: (empty input)
&gt;&gt;&gt; Please enter your password: __Testp

&gt;&gt;&gt; Authorized