.. 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: