Interviews are oportunities to learn

Thu 29 January 2015

Over the past month I've had my first experience interviewing for development positions. Its been a somewhat nerve-racking experience but also, at times, enjoyable. I've come to realize that the best way to approach an interview is with an attitude of excitement and an openness to learn.

A couple days before my first 'technical' interview I was feeling pretty overwhelemed. Tom, a Hacker School facilitator, reminded me that its public knowledge that I haven't been programming forever. He continued to explain that its OK I don't know everything, and that I'm not expected to. This was exactly the reminder I needed to hear. Of course I don't know everything. I'm just starting out in this field! Sonali suggested I have fun with my interviews and that I view them as opportunities to learn.

Their words of wisdom have really helped me make the most of my interviews. When I stopped putting pressure on myself to be something I'm not (an experienced programmer who knows everything) I could relax, be open to learning, and be genuinely engaged in conversations about programming. Regardless of whether or not they lead to a job, I am counting my interviews thus far to be a success because of the way I handled them. I was genuine, I had fun, and I learned cool things. Win.

On that note, here's a neat Python thing I learned during a recent technical interview1:

A Python Function's Default Arguments Are Mutable:

Without running the python code, guess what the following lines would return in an interactive Python interpreter. (Actually try to guess them before reading my answer! I've added a blank line where you should be guessing):

>>> def test(a = []):
... a.append(1)
... return a
>>> b = test()
>>> b

>>> c = test()
>>> c 

>>> d = test(c)
>>> d

Have you guessed yet?! ...

My guess was [1], [1], [1,1] but the correct answer is [1], [1,1], [1,1,1].

What does all of this mean and what does it tell us about the way Python handles default arguments?

It means that the default argument binding2 happens at the function's definition not at the function's execution. In other words, the statement a = [] is evaluated when the function definition statement is executed, but not when the function is called and its body is executed.

Lets dig a little deeper. Recall that in Python everything, including functions, is an object. When a function definition is executed a new function object is created. This function object will have an attribute called func_defaults that contains the values of the default arguments.

>>> test.func_defaults

Like usual object attributes, func_defaults can be mutated. In our example, the default argument is mutated when the body of the function is executed. Since the line of code that sets func_defaults to [] is run only when the object is instantiated, these mutations affect the default value for subsequent calls on the function.

I hope this makes sense! Feel free to ask questions if it doesn't. :) Also, some useful resources can be found here and here.

And to anyone out there currently undergoing interviews: Try to have fun. Above all else, be yourself. You are already impressive.

  1. The interviewer agreed that I could write about his question! :) 

  2. Arugment 'binding' is just a fancy way of specifying that a variables name (ex. a) gets assigned to a peice of data (ex. []). 

Category: Blog Tagged: Python