Question

In CMake I currently have a simple Python script to generate a header, but if I update the script itself CMake won’t re-run the script. Is there a way I can get CMake to do this?

Answer 1

It seems you are directly invoking your code generation script when cmake is run. While it is possible solution but it is definitely not a right way to use code generators with cmake.

I recommend you to use add_custom_command for your case:

add_custom_command(
   OUTPUT generated.h
   COMMAND ${PYTHON_EXECUTABLE} generator.py
   DEPENDS generator.py
   )

And next you can simple put your header to the list of source files passed to add_library/add_executable commands. cmake will automatically track all the dependencies and invoke your script.

Term DEPENDS generator.py informs cmake that it should regenerate header if script is changed.

With this approach file generated.h will be generated only at build time (when you run make or execute a build command in IDE). In contrast if you are running your script at cmake time (with execute_process command) then you have to rerun cmake to regenerate your file. Which is possible but you need to use some tricks to introduce a non-standard dependency.