ROS2 python launch file parsing arguments directly to nodes

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Dear ROS community,

im using ros2 and made a launch.py file to start my nodes. I want to do something like this:
ros2 launch launch/launch.py key:=value
but how do i pass it to the nodes?
with paramaters i only can load in yaml files and with arguments i have to use sys.argv to acces it. But if that is the case why do i have to put it like key:=value and not just only a value? so i have to make some uggly code because the launch file wants it in a different format..

i just wan't to use different settings depending on a condition and the condition i wan't to give through the ros2 launch call

sorry for my english