Import("env") old_uploaderflags = env["UPLOADERFLAGS"] #print("Old uploaderflags: " + str(old_uploaderflags)) index_write_flash = old_uploaderflags.index("write_flash") if index_write_flash != -1: new_uploaderflags = old_uploaderflags[::] new_uploaderflags.insert(index_write_flash + 1, "--erase-all") #print("Replaced with: " + str(new_uploaderflags)) env.Replace(UPLOADERFLAGS=new_uploaderflags)