Minor suggestion : add a NO_BUTTON mask for MouseMotion event about libtcod HOT 3 CLOSED

Python conventions would recommend doing this for your example:

if not event.state:
    ...

The above code and your example are unambiguous, but you could add a comment if you needed it to be more verbose.

Realistically, the else clause could be used after checking for a state.

if event.state & tcod.event.BUTTON_LMASK:  # Left button held.
    ...
else:  # Left button not held.
    ...

Bit masks are meant to be used with bit-wise operators, so there isn't much reason to include a named zero. Make sure that you're using & and not == to check if a button is held.

The current names are derived directly from the SDL2 API, and probably won't change very much.

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I like the not event.state, I didn't thought about it.

The else clause is not useful in my case because I need to check specifically if there is no button pressed.
It's related to focus behaviour : an object will gain/lose focus when the mouse move and no button is pressed and is treated before any other event handling basically.

And thanks for the tips on bitwise operators.

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It's related to focus behaviour : an object will gain/lose focus when the mouse move and no button is pressed and is treated before any other event handling basically.

I'd still try to limit that behavior to only the buttons which will interact with those elements:

if not event.state & (tcod.event.BUTTON_LMASK | tcod.event.BUTTON_RMASK):
    ...  # Left and right buttons not held.

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