Python Tip - How to Dynamically Assign Attributes

Today I want to share with you a small tip on how to dynamically set class attributes within Python. Before we do, I just want to point out that this is actually something I learnt a few months back after going through the Nornir code base. Therefore true kudos goes out to the Nornir devs! Also if you get a chance, I highly recommend checking out the Nornir code base as it acts as a great learning resource.

Great, so let's dive in...

So let's take a typical example where we statically assign our class attributes. Like so:

class Router:
    def __init__(self, vendor, platform, dc):
        self.vendor = vendor
        self.platform = platform
        self.dc = dc

We can then, of course, create an instance of Router and then access the various attributes (vendor, platform, and dc), like so.

>>> rtr001 = Router(vendor="Juniper", platform="junos", dc="London")

>>> rtr001.vendor
>>> 'Juniper'

>>> rtr001.platform
>>> 'junos'

>>> rtr001.dc
>>> 'London'

This is ok, but what about if we have a number of attributes we need to assign within our constructer (__init__) or what if we want to provide an easy way to extend our Router class in the future without the need of having to adapt the __init__ each time.

This is where we can dynamically assign our instance attributes like so.

class Router:
    def __init__(self, **kwargs):   
        for k, v in kwargs.items():
            setattr(self, k, v)

Ok, so what is actually going on here? Let's go through this line by line:

class Router:
    # Pack all the keyword arguments provided at the point of creating
    # an instance of the class. These keyword args will then be available
    # via the kwargs dict(). 
    def __init__(self, **kwargs):
        # We loop through each of the items in the dictionary and assign
        # the key to the variable 'k', and the value to the variable 'v'. 
        for k, v in kwargs.items():
            # We next assign an attribute to our instance/object using setattr.
            # The syntax is 'setattr(object, name, value)'.
            setattr(self, k, v)

Then as like before we can access our attributes, like so:

>>> rtr002 = Router(vendor="Cisco", platform="ios-xe", dc="Paris")

>>> rtr002.vendor
>>> 'Cisco'

>>> rtr002.platform
>>> 'ios-xe'

>>> rtr002.dc
>>> 'Paris'

Good stuff! Well, I hope you found this short Python tip useful. If you want to learn more about automating your network with Python, check our 5-day Bootcamp over at: