ipdb: How to inspect complex structures

Python comes with a debuger called pdb. The python debugger is very powerful and really facilitates troubleshooting code. You can find the official documentation here. ipdb builds on top of ipython and pdb to give users a more interactive experience.

This how to isn’t very extensive, it’s just a very quick and dirty demo to show some of it’s capabilities and how to use it with nornir. It doesn’t assume previous knowledge but doesn’t spend too much on explanations either so it expects users to give enough material for further investigation.

Installing ipdb

First, you will need to install ipython, follow the official guide to do so, then you will need to install ipdb. You can install the latter via pip:

pip install ipdb

Inspecting results

For the sake of the demo I have written some code that returns a result. The code is not important but what it does is:

1. Read a yaml file containing a bunch of users we want to have configured on our devices
2. Connect to a couple of network devices and get the users configured
3. Check which users are already configured, which ones we want and compute two lists; a list with the users we need to configure and another list with the ones we need to remove.
4. Finally, we pass those lists to a template and we generate some configuration.

Let’s start by insterting a break point right after we get the result (line 56):

If we execute the script, we will get a shell right in that point of the code.

Now we can start using python code to figure out how the object works:

The output above suggests the object is of the type AggregatedResult. The documentation surely will explain how it works but we can keep playing with it a bit. The output also suggests that the object might be a dictionary-like object with keys spine00.bma and spine01.bma. Let’s see keep digging.

Something interesting is that you can press <tab> to get autocompletion. In this case r.<tab> shows us the available methods that the object provides. Seeing methods like keys, items, etc… plus the output of print(r) seems to confirm our theory the object might be a dict-like object.

Let’s put the theory to test:

Ok, looks like we were right. Notice we extracted a MultiResult that belonged to the key spine00.bma and assigned it to a variable for further inspection. This new MultiResult object looks like a list. Let’s use <tab> again to see which methods provide:

With methods like append, extend, etc., this surely looks like a list. Based on the previous output the element 1 seems to be the result of the task napalm_get, let’s see if we can extract it:

Great, now ng_result has the result of running the task napalm_get. Let’s keep digging:

Voila, this new object has attributes like changed, result, diff, etc., so looks like we finally manage to dig down our object and figure out how get what we want.