import os

def show_all_files_directory(dir_path = str, search_endung = False, search_string = False):
    li_all = []
    for root_folder, dirs, files in os.walk(dir_path, topdown=False):       
        for name in files: 
            FullPATH = str(os.path.join(root_folder, name))
            if not search_endung is False:
                if FullPATH.endswith(search_endung):
                    li_all.append(FullPATH)
            elif not search_string is False:
                if not FullPATH.find(search_string) ==  -1:
                     li_all.append(FullPATH)
            else:
                li_all.append(FullPATH)        
    return li_all

    

print(show_all_files_directory(dir_path="/home/jonnybravo/Downloads", search_string=".txt"))