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+.. 9.3 LO: using python modules (3)
+.. ---------------------------------
+.. * executing python scripts from command line
+.. * import
+.. * scipy
+.. * pylab
+.. * sys
+.. * STDLIB modules show off
+
+====================
+Using Python modules
+====================
+{{{ show the welcome slide }}}
+
+Welcome to the spoken tutorial on using python modules.
+
+{{{ switch to next slide, outline slide }}}
+
+In this tutorial, we will see how to run python scripts from command
+line, importing modules, importing scipy and pylab modules.
+
+{{{ switch to next slide on executing python scripts from command line }}}
+
+Let us create a simple python script to print hello world. Open your
+text editor and type the following,
+
+{{{ open the text editor and type the following }}}
+::
+
+ print "Hello world!"
+ print
+
+and save the script as hello.py,
+
+{{{ save the script as hello.py }}}
+
+Till now we saw how to run a script using the IPython interpreter
+using the
+::
+
+ %run -i hello.py
+
+option, but that is not the correct way of running a python
+script.
+
+The correct method is to run it using the Python interpreter. Open the
+terminal and navigate to the directory where hello.py is,
+
+{{{ open terminal and navigate to directory where hello.py was saved }}}
+
+now run the Python script as,
+::
+
+ python hello.py
+
+It executed the script and we got the output ``Hello World!``.
+
+{{{ highlight ``python filename`` syntax on slide while narrating }}}
+
+The syntax is python space filename.
+
+Now recall the four plot problem where we plotted four plots in a single
+figure. Let us run that script from command line.
+
+If you don't have the script,
+
+{{{ open the four_plot.py file in text editor }}}
+
+just pause here and create a python script with the following lines
+and save it as four_plot.py.
+
+Now let us run four_plot.py as a python script.
+::
+
+ python four_plot.py
+
+Oops! even though it was supposed to work, it didn't. It gave an error
+``linspace()`` is not defined, which means that the function
+``linspace()`` is not available in the current name-space.
+
+But if you try to run the same script using ``%run -i four_plot.py``
+in your IPython interpreter started with the option ``-pylab`` it will
+work, because the ``-pylab`` option does some work for us by importing
+the required modules to our name-space when ipython interpreter
+starts. And thus we don't have to explicitly import modules.
+
+So now let us try to fix the problem and run the script in command
+line,
+
+add the following line as the first line in the script,
+{{{ add the line as first line in four_plot.py and save }}}
+::
+
+ from scipy import *
+
+Now let us run the script again,
+::
+
+ python four_plot.py
+
+Now it gave another error plot not defined, let us edit the file again
+and add the line below the line we just added,
+{{{ add the line as second line in four_plot.py and save }}}
+::
+
+ from pylab import *
+
+And run the script,
+::
+
+ python four_plot.py
+
+Yes! it worked. So what did we do?
+
+We actually imported the required modules using the keyword ``import``.
+It could have also be done as,
+
+{{{ highlight the following in slide and say it loud }}}
+::
+
+ from scipy import linspace
+
+instead of,
+::
+
+ from scipy import *
+
+So in practice it is always good to use function names instead of
+asterisk or star. As if we use asterisk to import from a particular
+module then it will replace any existing functions with the same name
+in our name-space.
+
+So let us modify four_plot.py as,
+{{{ delete the first two lines and add the following }}}
+::
+
+ from scipy import linspace, pi, sin
+ from pylab import plot, legend, annotate, title, show
+ from pylab import xlim, ylim
+
+{{{ switch to next slide }}}
+it could also be done as,
+
+.. import scipy
+.. import pylab
+.. x = scipy.linspace(-5*scipy.pi, 5*scipy.pi, 500)
+.. pylab.plot(x, x, 'b')
+.. pylab.plot(x, -x, 'b')
+.. pylab.plot(x, scipy.sin(x), 'g', linewidth=2)
+.. pylab.plot(x, x*scipy.sin(x), 'r', linewidth=3)
+.. pylab.legend(['x', '-x', 'sin(x)', 'xsin(x)'])
+.. pylab.annotate('origin', xy = (0, 0))
+.. pylab.xlim(-5*scipy.pi, 5*scipy.pi)
+.. pylab.ylim(-5*scipy.pi, 5*scipy.pi)
+
+
+Notice that we use ``scipy.pi`` instead of just ``pi`` as in the
+previous method, and the functions are called as ``pylab.plot()`` and
+``pylab.annotate()`` and not as ``plot()`` and ``annotate()``.
+
+{{{ switch to next slide, problem statement }}}
+
+Write a script to plot a sine wave from minus two pi to two pi.
+
+Pause here and try to solve the problem yourself before looking at the
+solution.
+
+It can solved as,
+
+{{{ open sine.py and show it }}}
+
+the first line we import the required functions ``linspace()`` and
+``sin()`` and constant ``pi`` from the module scipy. the second and
+third line we import the functions ``plot()``, ``legend()``,
+``show()``, ``title()``, ``xlabel()`` and ``ylabel()``. And the rest
+the code to generate the plot.
+
+We can run it as,
+{{{ now switch focus to terminal and run the script }}}
+::
+
+ python sine.py
+
+{{{ switch to next slide, What is a module? }}}
+
+So till now we have been learning about importing modules, now what is
+a module?
+
+A module is simply a file containing Python definitions and
+statements. Definitions from a module can be imported into other
+modules or into the main module.
+
+{{{ switch to next slide, Python standard library }}}
+
+Python has a very rich standard library of modules
+
+Python's standard library is very extensive, offering a wide range of
+facilities. Some of the standard modules are,
+
+for Math: math, random
+for Internet access: urllib2, smtplib
+for System, Command line arguments: sys
+for Operating system interface: os
+for regular expressions: re
+for compression: gzip, zipfile, tarfile
+And there are lot more.
+
+Find more information at Python Library reference,
+``http://docs.python.org/library/``
+
+The modules pylab, scipy, Mayavi are not part of the standard python
+library.
+
+{{{ switch to next slide, recap }}}
+
+This brings us to the end of this tutorial, in this tutorial we
+learned running scripts from command line, learned about modules, saw
+the python standard library.
+
+{{{ switch to next slide, thank you slide }}}
+
+Thank you!
+
+.. Author: Anoop Jacob Thomas <anoop@fossee.in>
+ Reviewer 1:
+ Reviewer 2:
+ External reviewer: