Method __init__

For info method to work correctly the instance should have hostname and model variables. If these variables are not available, an error will occur:

In [15]: class Switch:
    ...:     def info(self):
    ...:         print(f'Hostname: {self.hostname}\nModel: {self.model}')
    ...:

In [59]: sw2 = Switch()

In [60]: sw2.info()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-60-5a006dd8aae1> in <module>()
----> 1 sw2.info()

<ipython-input-57-30b05739380d> in info(self)
      1 class Switch:
      2     def info(self):
----> 3         print(f'Hostname: {self.hostname}\nModel: {self.model}')

AttributeError: 'Switch' object has no attribute 'hostname'

Almost always, when an object is created it has some initial data. For example, to create a connection to device with netmiko you have to pass connection parameters.

In Python these initial object data are specified in __init__. Method __init__ is executed after Python has created a new instance and __init__ method is passed arguments with which instance was created:

In [32]: class Switch:
    ...:     def __init__(self, hostname, model):
    ...:         self.hostname = hostname
    ...:         self.model = model
    ...:
    ...:     def info(self):
    ...:         print(f'Hostname: {self.hostname}\nModel: {self.model}')
    ...:

Note that each instance created from this class will have variables: self.model and self.hostname.

Now, when creating an instance of Switch class you have to specify hostname and model:

In [33]: sw1 = Switch('sw1', 'Cisco 3850')

Accordingly, info method works without error:

In [36]: sw1.info()
Hostname: sw1
Model: Cisco 3850

Note

__init__ method is sometimes called a class constructor, although technically in Python __new__ method is executed first and then __init__. In most cases there is no necessety to create __new__ method.

An important feature of __init__ method is that it should not return anything. Python will generate an exception if it tries to do this.