example_generated.cmake

find_package(Python COMPONENTS Interpreter) #make sure you have python and populate Python::Interpreter from the system.
add_executable(sometool sometool.cpp)  #the target for the tool that generates the code.

#I assume that somescript.py accepts two arguments: the path to sometool.exe and the path to the output

set(MY_SCRIPT_OUTPUT "${CMAKE_CURRENT_BUILD_DIR}/generated.cpp")
add_custom_command(OUTPUT "${CMAKE_CURRENT_BUILD_DIR}/generated.cpp"   #...additional outputs here if necessary
					COMMAND Python::Interpreter 
					ARGS "${CMAKE_CURRENT_SOURCE_DIR}/somescript.py" "$<TARGET_FILE:sometool>" "${CMAKE_CURRENT_BUILD_DIR}/generated.cpp"
					DEPENDS "${CMAKE_CURRENT_SOURCE_DIR}/somescript.py" #rerun if somescript.py changes.
					)
	
#if you want to do it all at once, this is valid.  The dependency gets added automatically	
add_library(somecode
	"${CMAKE_CURRENT_BUILD_DIR}/generated.cpp"
	othercode.cpp
	othercode.hpp
)
#OR do this:
add_library(somecode
	othercode.cpp
	othercode.hpp
)
target_sources(somecode
	"${CMAKE_CURRENT_BUILD_DIR}/generated.cpp"
)