Why doesn't paramico list files?

Question

linux

import paramiko 

host = '***.***.***.***'
user = '********'
secret = '********'
port = 22

client = paramiko.SSHClient()
client.set_missing_host_key_policy(paramiko.AutoAddPolicy())
client.connect(hostname=host, username=user, password=secret, port=port)
stdin, stdout, stderr = client.exec_command('cd /')
stdin, stdout, stderr = client.exec_command('ls')
data = stdout.read() + stderr.read()
print data
client.close()

An empty string is output. If you do it like this:

stdin, stdout, stderr = client.exec_command('ping localhost -c 3')
data = stdout.read() + stderr.read()
print data

then the result is displayed