The source code of prompt_toolkit should be readable, concise and efficient. We prefer short functions focusing each on one task and for which the input and output types are clearly specified. We mostly prefer composition over inheritance, because inheritance can result in too much functionality in the same object. We prefer immutable objects where possible (objects don't change after initialisation). Reusability is important. We absolutely refrain from having a changing global state, it should be possible to have multiple independent instances of the same code in the same process. The architecture should be layered: the lower levels operate on primitive operations and data structures giving -- when correctly combined -- all the possible flexibility