Context manager#
Context manager allows specified actions to be performed at the beginning and
end of with block. Two methods are responsible for context manager:
__enter__(self)- indicates what should be done at the beginning ofwithblock. Value that returns method is assigned to variable afteras.__exit__(self, exc_type, exc_value, traceback)- indicates what should be done at the end ofwithblock or when it is interrupted. If there is an exception within block, then exc_type, exc_value, traceback will contain exception information, if there is no exception they will be equal to None.
Examples of context manager usage:
file opening/closing
opening/closing of SSH/Telnet session
transactions handling in database
CiscoSSH class uses paramiko to connect to the equipment:
class CiscoSSH:
def __init__(self, ip, username, password, enable, disable_paging=True):
client = paramiko.SSHClient()
client.set_missing_host_key_policy(paramiko.AutoAddPolicy())
client.connect(
hostname=ip,
username=username,
password=password,
look_for_keys=False,
allow_agent=False)
self.ssh = client.invoke_shell()
self.ssh.send('enable\n')
self.ssh.send(enable + '\n')
if disable_paging:
self.ssh.send('terminal length 0\n')
time.sleep(1)
self.ssh.recv(1000)
def send_show_command(self, command):
self.ssh.send(command + '\n')
time.sleep(2)
result = self.ssh.recv(5000).decode('ascii')
return result
Example of class usage:
In [9]: r1 = CiscoSSH('192.168.100.1', 'cisco', 'cisco', 'cisco')
In [10]: r1.send_show_command('sh clock')
Out[10]: 'sh clock\r\n*12:58:47.523 UTC Sun Jul 28 2019\r\nR1#'
In [11]: r1.send_show_command('sh ip int br')
Out[11]: 'sh ip int br\r\nInterface IP-Address OK? Method Status Protocol\r\nEthernet0/0 192.168.100.1 YES NVRAM up up \r\nEthernet0/1 192.168.200.1 YES NVRAM up up \r\nEthernet0/2 19.1.1.1 YES NVRAM up up \r\nEthernet0/3 192.168.230.1 YES NVRAM up up \r\nLoopback0 4.4.4.4 YES NVRAM up up \r\nLoopback90 90.1.1.1 YES manual up up \r\nR1#'
In order for the class to support work in context manager, it is necessary to
add methods __enter__ and __exit__:
class CiscoSSH:
def __init__(self, ip, username, password, enable, disable_paging=True):
print('Метод __init__')
client = paramiko.SSHClient()
client.set_missing_host_key_policy(paramiko.AutoAddPolicy())
client.connect(
hostname=ip,
username=username,
password=password,
look_for_keys=False,
allow_agent=False)
self.ssh = client.invoke_shell()
self.ssh.send('enable\n')
self.ssh.send(enable + '\n')
if disable_paging:
self.ssh.send('terminal length 0\n')
time.sleep(1)
self.ssh.recv(1000)
def __enter__(self):
print('Метод __enter__')
return self
def __exit__(self, exc_type, exc_value, traceback):
print('Метод __exit__')
self.ssh.close()
def send_show_command(self, command):
self.ssh.send(command + '\n')
time.sleep(2)
result = self.ssh.recv(5000).decode('ascii')
return result
Example of class usage in context manager:
In [14]: with CiscoSSH('192.168.100.1', 'cisco', 'cisco', 'cisco') as r1:
...: print(r1.send_show_command('sh clock'))
...:
Метод __init__
Метод __enter__
sh clock
*13:05:50.677 UTC Sun Jul 28 2019
R1#
Метод __exit__
Even if an exception occurs within block, __exit__ method is executed:
In [18]: with CiscoSSH('192.168.100.1', 'cisco', 'cisco', 'cisco') as r1:
...: result = r1.send_show_command('sh clock')
...: result / 2
...:
Метод __init__
Метод __enter__
Метод __exit__
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-18-b9ff1fa74be2> in <module>
1 with CiscoSSH('192.168.100.1', 'cisco', 'cisco', 'cisco') as r1:
2 result = r1.send_show_command('sh clock')
----> 3 result / 2
4
TypeError: unsupported operand type(s) for /: 'str' and 'int'