import keyboard
def fun1(r):
    print(r.name)    
keyboard.on_press(fun1)

the code is a simple key logger, what exactly is happening here?

what I understood so far is:

1. import
2. function definition
3. keyboard.on_press is called

Please explain the following things

1. what exactly is keyboard.on_press(fun1) is passing to fun1()
2. why having a parameter is important for fun1
3. what if i dont wanna make a function just wanna put my code in [keyboard.on_press("here")], why would it not be possible.

Few more questions

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

1. whats up with "with" statement here?
2. what does .join() {what it means t have it joined it to main thread}
3. where have we written on_press=on_press {why not just once}

I dont know if this query is dependent on version of python or version of module. I am using all latest versions. So far I read the documentation on https://pynput.readthedocs.io/en/latest/keyboard.html and googles all my questions but could not find the easy explanation.

Here's some code with comments to help explain the syntax:

import keyboard

def somefunc(msg):
    print(msg)  # here

def fun1(r):  # r is keyboard event
    print(type(r))   #  <class 'keyboard._keyboard_event.KeyboardEvent'>
    print(r.name)   # key text
    somefunc("here")   # call some other function
    
keyboard.on_press(fun1) # pass a reference to the other function (on_press will call whatever function we pass), other function must have single parameter

while True: pass  # keep script running

The With keyword just ensures the the object is closed properly even if there is an error.

with keyboard.Listener(
        on_press=on_press,   # named parameter is on_press, we are passing a reference to function on_press
        on_release=on_release) as listener:  # named parameter is on_release, we are passing a reference to function on_release
    listener.join()  # wait for listener thread to finish

# Is shortcut syntax for:

listener = keyboard.Listener(on_press=on_press, on_release=on_release)
try:
    ........
finally:
    listener.stop()  # always do this, even if error
listener.join()

About the double variable. It looks weird, I know. The code is using named parameters.

# sample function
def myfunc(x):   # the parameter name is x
    print(x)
    
# We can call myfunc several ways:
myfunc(1)  # pass value directly
myfunc(x=1)  # specify parameter to assign

x=1  # local variable
myfunc(x)  # pass the local variable as parameter
myfunc(x=x)  # specify parameter to assign, pass local variable # this is the double text you're seeing  

Concerning the callback function, you can pass a function reference to another function.

def MyCallback(a,b):  # custom function
    print(a,b) 
    
def SomeEventHandler(f): # parameter is reference to another function 
    f(1,2)  # call passed in function, it must have these parameters
    
SomeEventHandler(MyCallback)  # pass custom function to handler

I hope this clears things up a bit.