return dir def test_osfiles(temp_dir)os mkdir(os path join(temp_dir' ')os mkdir(os path join(temp_dir' ')dir_contents os listdir(temp_dirassert len(dir_contents= assert 'ain dir_contents assert 'bin dir_contents the funcarg creates new empty temporary directory for files to be created in then it adds finalizer call to remove that directory (using shutil rmtreewhich recursively removes directory and anything inside itafter the test has completed the filesystem is then left in the same state in which it started we can use the request cached_setup method to create function argument variables that last longer than one test this is useful when setting up an expensive operation that can be reused by multiple tests as long as the resource reuse doesn' break the atomic or unit nature of the tests (so that one test does not rely on and is not impacted by previous onefor exampleif we were to test the following echo serverwe may want to run only one instance of the server in separate processand then have multiple tests connect to that instanceimport socket socket socket(socket af_inetsocket sock_streams setsockopt(socket sol_socketsocket so_reuseaddr bind(('localhost', ) listen( while trueclientaddress accept(data client recv( client send(dataclient close(all this code does is listen on specific port and wait for input from client socket when it receives inputit sends the same value back to test thiswe can start the server in separate process and cache the result for use in multiple tests here' how the test code might lookimport subprocess import socket