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Using Python modules

====================

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Welcome to the spoken tutorial on using python modules.

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In this tutorial, we will see how to run python scripts from command

line, importing modules, importing scipy and pylab modules. And also

see the Python standard library.

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Let us create a simple python script to print hello world. Open your

text editor and type the following,

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::

    print "Hello world!"

    print

and save the script as hello.py,

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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,

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now run the Python script as,

::

    python hello.py

It executed the script and we got the output ``Hello World!``.

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The syntax is python space filename.

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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, 

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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,

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add the following line as the first line in the script,

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::

    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,

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::

    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,

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::

    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.

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So let us modify four_plot.py as,

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::

    from scipy import linspace, pi, sin

    from pylab import plot, legend, annotate

    from pylab import xlim, ylim, title, show

Now let us try running the code again as,

::

    python four_plot.py

It works! In this method we actually imported the functions to the

current name-space, and there is another method of doing it. And that

is,

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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()``.

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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,

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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,

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::

    python sine.py

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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.

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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.

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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.

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Thank you!