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### Playing around with errors in Python - NameErrors

Let's start with NameErrors, which is one of the more common errors that a newcomer to Python will come across. It is reported when Python can't find a local or global variable in the code. One reason this might pop up is because a variable is being referred to outside of it's namespace. To give you an example

a = 10

def test_f():
a = 20
print a

test_f()
print a


Let's walk through the code. After defining the variable a and the function test_f, you would naively expect the test_f() function call to change the value of a to 20 and print 20. You expect the print statement after the function call to also print 20. Because you expect the function call to have changed the value of a. But, if you try running the code for yourself, you'll notice that the final print statement will print 10. This is where namespaces come into the picture.

Now, let's try this instead

def test_f():
b = 20
print b

test_f()
print b


The call to the test_f function will set and print the variable b but the print statement afterwards will throw a NameError because outside of the test_f function, the variable b isn't defined.

Let's look at another example, this time in the context of classes in Python.

class test_c:
b = 20
def test_f(self):
print b

test_c().test_f()


Let me explain the last statement first and then the rest of the example. test_c() creates an instance of the class test_c and test_c().test_f() calls the test_f method on the instance of the class. Naively, you would expect the code print 20, which is the value of the variable b in the class. But instead, you will get a NameError, telling you that the variable b isn't defined. The solution to this problem is to refer to b as self.b inside any of the methods defined on test_c, which tells Python that this variable belongs to the class on which the method is defined.

There are definitely a lot more ways in which you can make Python throw a NameError at you but I wanted to use the NameError to introduce the concept of namespaces in python. That's all for now. And as always, I am thankful for any feedback on the writing style and/or content. Until next time ...

[1]. hilite.me was used to create the inline code blocks.
[2]. You can refer to the Python official documentation on namespaces for more information.
[3]. A Python Shell can be accessed on the official Python page. A more comprehensive editor can be found here.

### Animation using GNUPlot

Animation using GNUPlotI've been trying to create an animation depicting a quasar spectrum moving across the 5 SDSS pass bands with respect to redshift. It is important to visualise what emission lines are moving in and out of bands to be able to understand the color-redshift plots and the changes in it.
I've tried doing this using the animate function in matplotlib, python but i wasn't able to make it work - meaning i worked on it for a couple of days and then i gave up, not having found solutions for my problems on the internet.
And then i came across this site, where the gunn-peterson trough and the lyman alpha forest have been depicted - in a beautiful manner. And this got me interested in using js and d3 to do the animations and make it dynamic - using sliders etc.
In the meanwhile, i thought i'd look up and see if there was a way to create animations in gnuplot and whoopdedoo, what do i find but nirvana!

In the image, you see 5 static curves and one dynam…

### Pandas download statistics, PyPI and Google BigQuery - Daily downloads and downloads by latest version

Inspired by this blog post : https://langui.sh/2016/12/09/data-driven-decisions/, I wanted to play around with Google BigQuery myself. And the blog post is pretty awesome because it has sample queries. I mix and matched the examples mentioned on the blog post, intent on answering two questions -
1. How many people download the Pandas library on a daily basis? Actually, if you think about it, it's more of a question of how many times was the pandas library downloaded in a single day, because the same person could've downloaded multiple times. Or a bot could've.
This was just a fun first query/question.
2. What is the adoption rate of different versions of the Pandas library? You might have come across similar graphs which show the adoption rate of various versions of Windows.
Answering this question is actually important because the developers should have an idea of what the most popular versions are, see whether or not users are adopting new features/changes they provide…

### Adaptive step size Runge-Kutta method

I am still trying to implement an adaptive step size RK routine. So far, I've been able to implement the step-halving method but not the RK-Fehlberg. I am not able to figure out how to increase the step size after reducing it initially.

To give some background on the topic, Runge-Kutta methods are used to solve ordinary differential equations, of any order. For example, in a first order differential equation, it uses the derivative of the function to predict what the function value at the next step should be. Euler's method is a rudimentary implementation of RK. Adaptive step size RK is changing the step size depending on how fastly or slowly the function is changing. If a function is rapidly rising or falling, it is in a region that we should sample carefully and therefore, we reduce the step size and if the rate of change of the function is small, we can increase the step size. I've been able to implement a way to reduce the step size depending on the rate of change of …