For those who are on windows and were struggling to find an working answer here's mine: pynput.

Here is the pynput official "Monitoring the keyboard" source code example:

from pynput.keyboard import Key, Listener

def on_press(key):
    print('{0} pressed'.format(
        key))

def on_release(key):
    print('{0} release'.format(
        key))
    if key == Key.esc:
        # Stop listener
        return False

# Collect events until released
with Listener(
        on_press=on_press,
        on_release=on_release) as listener:
    listener.join()

The function above will print whichever key you are pressing plus start an action as you release the 'esc' key. The keyboard documentation is here for a more variated usage.

Markus von Broady highlighted a potential issue that is: This answer doesn't require you being in the current window to this script be activated, a solution to windows would be:

from win32gui import GetWindowText, GetForegroundWindow
current_window = (GetWindowText(GetForegroundWindow()))
desired_window_name = "Stopwatch" #Whatever the name of your window should be

#Infinite loops are dangerous.
while True: #Don't rely on this line of code too much and make sure to adapt this to your project.
    if current_window == desired_window_name:

        with Listener(
            on_press=on_press,
            on_release=on_release) as listener:
            listener.join()

As per Keyboard documentation:

Other applications, such as some games, may register hooks that swallow all key events. In this case keyboard will be unable to report events.

One way to solve your problem with keyboard module is keyboard.wait('key')

# Blocks until you press esc
keyboard.wait('esc')

Something work around is as below:

import keyboard

keyboard.wait('enter')
print('Enter is pressed')

keyboard.wait('q')
print('Quitting the program')

keyboard.wait('s')
print('Skiping the things')

I took a look at the documentation located here https://github.com/boppreh/keyboard.

I ran and tested this code on python 3.9

import keyboard
from functools import partial 

num = ""

def add(i):
    global num
    num += str(i)
    print(num, end='\r')

for i in range(10): # numbers 0...9
    keyboard.add_hotkey(str(i), partial(add, i)) # add hotkeys

keyboard.wait('enter') # block process until "ENTER" is pressed

print("\nNum:{}".format(num))

You could additionally unhook the hotkeys by calling keyboard.unhook_all_hotkeys() after grabbing the number. I imagine you could wait on "+" and "-" if you wanted to implement addition and subtraction of numbers.

It is because the while is having no delay and the process keep entering in to the while loop. Although, the key press is just one from the user's perspective yet the computer is actually seeing multiple presses of those due to the while loop.

I am not sure how to effectively verify this but if we press the key just once and add a delay as below, we'll be able observe the difference:

import keyboard
import time
while(True):
    if keyboard.is_pressed('ctrl'):
        print("Hello")
        time.sleep(1)