The module called msvcrt causes Blender crashing even if it does existing in Python list

Question

My goal is to simplify and create my own version of screencast key addon and this is my baby-stepping. What I'm doing here is to display a key on the screen that user pressed. However, the module msvcrt causes Blender crashing.

Some people uses bge or keyboard modules for detecting pressed keys but somewhat reason I have no modules in my Python 3. So I decided using msvcrt instead which is already exists on the module list.

I searched my problem on Google but sadly couldn't get an accepted solutions.

Any ideas?

This is the code:

bl_info = {
    "name" : "KeyPad",
    "author" : "anon",
    "description" : "",
    "blender" : (2, 80, 0),
    "version" : (0, 0, 1),
    "location" : "",
    "warning" : "",
    "category" : "Generic"
}

import bpy, blf, msvcrt

class Ksys:
    defaults = {
        "id": 0,
        "handler": None,
        "count": 3,
        "color": [255, 255, 255],
        "size": [50, 72],
        "position": {
            "left": [2, 80, 0],
            "center": [],
            "right": []
        }
    }
    def __init__(self):
        while True:
            if msvcrt.kbhit():
                key = msvcrt.getch()
                print("Key Pressing detected!:", key)
             
                Ksys.defaults["handler"] = bpy.types.SpaceView3D.draw_handler_add(self.draw(key), (None, None), 'WINDOW', 'POST_PIXEL')
                print("Key Assigning detected!")
                
                print("Waiting for 3 seconds then remove the handler on the window")
                bpy.app.timers.register(self.remove(Ksys.defaults["handler"]), Ksys.defaults["count"])

    def draw(key):
        blf.size(Ksys.defaults["id"], Ksys.defaults["size"][0], Ksys.defaults["size"][1])
        blf.color(Ksys.defaults["id"], Ksys.defaults["color"][0], Ksys.defaults["color"][1], Ksys.defaults["color"][2])
        blf.draw(Ksys.defaults["id"], key)

    def remove(key):
        bpy.types.SpaceView3D.draw_handler_remove(key, 'WINDOW')

# test run before register
o = Ksys()

def register():
    pass  

def unregister():
    pass

Answer 1

There are two ways to go about this in Blender. The recommended portable way is to create a modal operator. Modal Operators is a good free tutorial and this answer gives a minimal implementation that I quote here.

class VIEW3D_OT_process_input(bpy.types.Operator):
    """Process input while Control key is pressed."""
    bl_idname = 'view3d.process_input'
    bl_label = 'Process Input'
    bl_options = {'REGISTER'}

def modal(self, context, event):
    if event.type == 'ESC':
        return {'FINISHED'}
    elif event.ctrl:
        pass # Input processing code.

    return {'PASS_THROUGH'}

def invoke(self, context, event):
    context.window_manager.modal_handler_add(self)
    return {'RUNNING_MODAL'}

There are two obvious limitations of using the keypress event

1. Being modal means that everything else has to wait while the modal operator is running. Sometimes this is a good thing, often it is not.
2. You only get to receive the key press events that the blender interface is willing to expose and the list is far from complete.

The second approach has the disadvantage of being much harder to implement properly and much harder to debug. It also requires writing a version to interface with each OS that the script will run on. See Threading Gotchas for some examples of problems using separate threads. It also doesn't resolve the second problem.

Unless you have a compelling need to dive into one of the harder parts of OS interface, I would strongly advise using the modal approach.