python

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I’ve been revising my Cisco Live session on IOS XE programmability, and it’s made me think about programming in general, and a particular idea I’ve been embarrassed to admit I loathe: Object Oriented Programming.

Some context:  I started programming on the Apple II+ in BASIC, which shows my age.  Back then programs were input with line numbers and program control was quite simple, consisting of GOTO and GOSUB statements that jumped around the lines of code.  So, you might have something that looked like this:

10 INPUT "Would you like to [C]opy a File or [D]elete a file?"; A$
20 IF A$ = "C" THEN GOTO 100
30 IF A$ = "D" THEN GOTO 200

This was not really an elegant way to build programs, but it was fairly clear.  Given that code was entered directly into the DOS CLI with only line-by-line editing functionality, it could certainly get a bit confusing what happened and where when you had a lot of branches in your code.

In college I took one programming course in Pascal.  Pascal was similar in structure to C, just far more verbose.  Using it required a shift to procedural-style thinking, and while I was able to get a lot of code to work in Pascal, my professor was always dinging me for style mistakes.  I tended to revert to AppleSoft BASIC style coding, using global variables and not breaking things down into procedures/functions enough.  (In Pascal, a procedure is simply a function that returns no value.)  Over time I got used to the new way of thinking, and grew to appreciate it.  BASIC just couldn’t scale, but Pascal could.  I picked up C after college for fun, and then attempted C++, the object-oriented version of C.  I found I had an intense dislike for shifting my programming paradigm yet again, and I felt that the code produced by Object Oriented Programming (OOP) was confusing and hard to read.  At that point I was a full-time network engineer and left programming behind.

When I returned to Cisco in 2015, I was assigned to programmability and had to learn the basics of coding again.  What I teach is really basic coding, scripting really, and I would have no idea how to build or contribute to a large project like many of those being done here at Cisco.  I picked up Python, and I generally like it for scripting.  However, Python has OO functionality, and I was once again annoyed by confusing OO code.

In case you’re not familiar with OOP, here’s how it works.  Instead of writing down (quite intuitively) your program according to the operations it needs to perform, you create objects that have operations associated with them according to the type of object.  An example in C-like pseudocode can help clarify:

Procedural:

rectangle_type: my_rect(20, 10)
print get_area(my_rect)

OOP:

rectangle_type: my_rect(20, 10)
print my_rect.get_area()

Note that in OOP, we create an object of type “rectangle_type” and then is has certain attributes associated with it, including functions.  In the procedural example, we just create a variable and pass it to a function which is not in any way associated to the variable we created.

The problem is, this is counter-intuitive.  Procedural programming follows a logical and clear flow.  It’s easy to read.  It doesn’t have complex class inheritance issues.  It’s just far easier to work with.

I was always ashamed to say this, as I’m more of a scripter than a coder, and a dabbler in the world of programming.  But recently I came across a collection of quotes from people who really do know what they are talking about, and I see many people agree with me.

How should a network engineer, programming neophyte approach this?  My advice is this:  Learn functional/procedural style programming first.  Avoid any course that moves quickly into OOP.

That said, you’ll unfortunately need to learn the fundamentals of OOP.  That’s because many, if not most Python libraries you’ll be using are object oriented.  Even built in Python data-types like strings have a number of OO functions associated with them.  (Want to make a string called “name” lower-case?  Call “name.lower()”) You’ll at least need to understand how to invoke a function associate with a particular object class.

Meanwhile I’ve been programming in AppleSoft quite a bit in my Apple II emulator, and the GOTO’s are so refreshing!

Since I finished my series of articles on the CCIE, I thought I would kick off a new series on my current area of focus:  network programmability.  The past year at Cisco, programmability and automation have been my focus, first on Nexus and now on Catalyst switches.  I did do a two-part post on DCNM, a product which I am no longer covering, but it’s well worth a read if you are interested in learning the value of automation.

One thing I’ve noticed about this topic is that many of the people working on and explaining programmability have a background in software engineering.  I, on the other hand, approach the subject from the perspective of a network engineer.  I did do some programming when I was younger, in Pascal (showing my age here) and C.  I also did a tiny bit of C++ but not enough to really get comfortable with object-oriented programming.  Regardless, I left programming (now known as “coding”) behind for a long time, and the field has advanced in the meantime.  Because of this, when I explain these concepts I don’t bring the assumptions of a professional software engineer, but assume you, the reader, know nothing either.

Thus, it seems logical that in starting out this series, I need to explain what exactly programmability means in the context of network engineering, and what it means to do something programmatically.

Programmability simply means the capacity for a network device to be configured and managed by a computer program, as opposed to being configured and managed directly by humans.  This is a broad definition, but technically using an interface like Expect (really just CLI) or SNMP qualifies as a type of programmability.  Thus, we can qualify this by saying that programmability in today’s world includes the design of interfaces that are optimized for machine-to-machine control.

To manage a network device programmatically really just means using a computer program to control that network device.  However, when we consider a computer program, it has certain characteristics over and above simply controlling a device.  Essential to programming is the design of control structures that make decisions based on certain pieces of information.

Thus, we could use NETCONF to simply push configuration to a router or switch, but this isn’t the most optimal reason to use it.  It would be a far more effective use of NETCONF if we read some piece of data from the device (say interface errors) and took an action based on that data (say, shutting the interface down when the counters got too high.)  The other advantage of programmability is the ability to tie together multiple systems.  For example, we could read a device inventory out of APIC-EM, and then push config to devices based on the device type.  In other words, the decision-making capability of programmability is most important.

Network programmability encompasses a number of technologies:

  • Day 0 technologies to bring network devices up with an appropriate software version and configuration, with little to no human intervention.  Examples:  ZTP, PoAP, PnP.
  • Technologies to push and read configuration and operational data from devices.  Examples:  SNMP, NETCONF.
  • Automation systems such as Puppet, Chef, and Ansible, which are not strictly programming languages, but allow for configuration of numerous devices based on the role of the device.
  • The use of external programming languages, such as Ruby and Python, to interact with network devices.
  • The use of on-box programming technologies, such as on-box Python and EEM, to control network devices.

In this series of articles we will cover all of these topics as well as the mysteries of data models, YANG, YAML, JSON, XML, etc., all within the context of network engineering.  I know when I first encountered YANG and data models, I was quite confused and I hope I clear up some of this confusion.