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Python: Your new best friend print “Adam Avison”

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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”.

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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.

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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...

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Why Python?

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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

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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

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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

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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!

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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

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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

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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.

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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.

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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.

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Examples resFOV.py

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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!

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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

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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.

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