Python your new best friend

1. Disclaimer
Though I’ve extensively programmed in Python, I
have ~0 formal programming training.
So some terminology I use may be total gibberish to
those better taught than I.
My mantra is:
“If it works it works, who cares about the fancy
terminology”.

2. Python version
• Here I will talk about functions within Python
2.6.x / 2.7.x (the JBCA system default)
• The separate development stream of Python
3.x ... Most of what I say won’t work, or will
work very differently.

3. Why Python?
• The current astronomers favourite...
• CASA (interferometric data reduction package
is written in it).
• Its free! (unlike IDL...*)
• *I’m yet to find something IDL can do that
Python can’t...

4. Why Python?

5. • First we need to set up which version of python your linux box
will default to.
• In your home area type:
>emacs –nw .cshrc
• This will open an in terminal text editor.
• Press the down key until you see the line:
#USER MODIFICATIONS
• After this type
alias python2.7 ‘/usr/local/lib/python2.7/bin/python2.7’
• The type ctrl-x ctrl-s. Close your terminal and open another
and we’re good to go.
BEFORE GETTING STARTED

6. Getting started
• From the command line ‘python’ will get you
into the python environment. Within which
you can start some basic work. e.g.
>>> a=3.141*0.005
>>> b=7.0**2.0
>>> c=a+b
>>> print c
49.015705

7. Scripting
• Adding those lines into a file named e.g.
‘test.py’ will then be executable by the
command
>python test.py
Will result in ...
49.015705

8. Dynamic whitespace
• In python whitespace is important, unlike e.g.
Perl. Your left hand indentation matters.
So this will work:
for x in range(len(array)):
print x
y=x**2.7
print y
print y # will print the last
#value of y
This won’t:
for x in range(len(array)):
print x
y=x**2.7
print y
print y # we’ll have crashed
#before we reach here
• Remember for later your ‘if’s, ‘elif’s and
‘else’s need to line up!

9. Importing modules
• A lot of functionality can be imported into your scripts with import
commands e.g.
import numpy
• As python is object orientated you call a ‘numpy’ task as follows:
numpy.sqrt(2.0) #will give us square-root of 2
• But because we’re lazy we don’t want to type numpy over and over so we
can instead use:
import numpy as np
• So the above becomes
np.sqrt(2.0) #will still give us square-root of 2

10. Importing modules 2.
• Some times we only want a couple of functions from a
module for this we can use a ‘from’:
from numpy import sqrt , other_function
• Now:
sqrt(2.0) #will give us what we’re after

11. Why isn’t there a function for this?
• If the function you’re after doesn’t exist... Write your own!
• In your code you can create your own functions, here is an
example:
def my_function(arg1, arg2):
z=np.sqrt(arg1)*np.exp(arg2)
return z
• Which can then be called later in your code simply as:
something=my_function(arg1, arg2)#something will then == z
• With the same number of arguments.

12. Example functions
• Example functions I’ve created:
1. Calculating colour-colour plots from incomplete data lists.
2. Find the peak flux in a spectrum.
3. Finding Zeeman split line pairs and calculating the local magnetic
field strength in ex-OH masers.
4. Calculating the rms noise in an ALMA map... Etc etc
• Functions are good because they mean you don’t have to re-
type code umpteen times throughout a script.

13. Useful python modules for Astronomy
• numpy – array and matrix mathematics, nice
load from txt options...
• scipy – Scientific functions, e.g. correlation,
signal processing, ffts...
• matplotlib – plotting... Makes beautiful plots.
• pyfits – FITS file manipulation.
• astropy - many useful astronomy modules and
packages all in one...
• APLpy for making nice FITS images.

14. Examples
resFOV.py

15. Basic syntax stuff and quick plot
Page 1 of 2
import numpy as np
import matplotlib.pyplot as plt
x=np.arange(1.0,10.0,1.0) #creates an array from 1 to 9
for value in x:
if value ==4.0:
print “wow a 4!”
elif value == 5.0:
print “and now a 5!”
else:
print value
y=np.sqrt(np.exp(x)) #just for something to plot against x!

16. fig1 = plt.figure(1)
ax1 = fig1.add_subplot(111) #sets up a plot environment to
# plot on
ax1.plot(x,y,’bo-’) #plots x v. y , ‘bo-’ sets it to
#plot blue circles with a solid
#line joining them
ax1.set_xlabel(‘x’)
ax1.set_ylabel(‘y’) #take a guess!
plt.show() #shows our plot
Basic syntax stuff and quick plot
Page 2 of 2

17. Challenge
• Using the basics demonstrated in this tutorial
write a script which calculates the
Schwarzschild radius for black holes of mass =
to each of the solar system planets.
• Extra credit, plot mass vs. radii and label each
planet.