1 .. Objectives

     2 .. ----------

     3 

     4 .. At the end of this tutorial, you will be able to 

     5 

     6 .. 1. Execute python scripts from command line.

     7 .. #. Use import in scripts.

     8 .. #. Import scipy and pylab modules

     9 .. #. Use python standard modules and 3rd party modules.

    10 

    11 

    12 .. Prerequisites

    13 .. -------------

    14 

    15 ..   1. should have ``pylab`` installed. 

    16 ..   #. using plot command interactively.

    17 ..   #. embellishing a plot.

    18 ..   #. saving plots.

    19      

    20 .. Author              : Anoop Jacob Thomas <anoop@fossee.in>

    21    Internal Reviewer   : Puneeth

    22    External Reviewer   :

    23    Language Reviewer   : Bhanukiran

    24    Checklist OK?       : <11-11-2010, Anand, OK> [2010-10-05]

    25 

    26 

    27 ====================

    28 Using Python modules

    29 ====================

    30 {{{ show the welcome slide }}}

    31 

    32 Welcome to the spoken tutorial on Using Python Modules.

    33 

    34 {{{ switch to next slide, outline slide }}}

    35 

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

    37 line. We'll see how to import modules, importing scipy and pylab

    38 modules and have a look at the Python standard library.

    39 

    40 {{{ switch to next slide on executing python scripts from command line }}}

    41 

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

    43 text editor and type the following,

    44 

    45 {{{ open the text editor and type the following }}}

    46 ::

    47 

    48     print "Hello world!"

    49     print

    50 

    51 and save the script as ``hello.py``,

    52 

    53 {{{ save the script as hello.py }}}

    54 

    55 Till now we saw how to run a script using the IPython interpreter

    56 using the

    57 ::

    58 

    59     %run -i hello.py

    60 

    61 option, but that is not the correct way of running a python

    62 script. 

    63 

    64 The correct method is to run it using the Python interpreter. Open the

    65 terminal and navigate to the directory where hello.py is,

    66 

    67 {{{ open terminal and navigate to directory where hello.py was saved }}}

    68 

    69 {{{ switch to next slide }}}

    70 

    71 now run the Python script as,

    72 ::

    73 

    74     python hello.py

    75 

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

    77 

    78 {{{ highlight ``python filename`` syntax on slide while narrating }}}

    79 

    80 The syntax is python space filename.

    81 

    82 {{{ switch to next slide, four plot problem }}}

    83 

    84 Now recall the four plot problem where we plotted four plots in a single

    85 figure. Let us run that script from command line.

    86 

    87 If you don't have the script, 

    88 

    89 {{{ open the four_plot.py file in text editor }}}

    90 

    91 just pause here and create a python script with the following lines

    92 and save it as four_plot.py.

    93 

    94 Now let us run four_plot.py as a python script.

    95 ::

    96 

    97     python four_plot.py

    98 

    99 Oops! even though it was supposed to work, it didn't. It gave an error

   100 ``linspace()`` is not defined, which means that the function

   101 ``linspace()`` is not available in the current name-space.

   102 

   103 But if you try to run the same script using ``%run -i four_plot.py``

   104 in your IPython interpreter started with the option ``-pylab`` it will

   105 work, because the ``-pylab`` option does some work for us by importing

   106 the required modules to our name-space when ipython interpreter

   107 starts. And thus we don't have to explicitly import modules.

   108 

   109 So now let us try to fix the problem and run the script in command

   110 line,

   111 

   112 {{{ switch to next slide, fix ``linspace`` problem }}}

   113 

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

   115 {{{ add the line as first line in four_plot.py and save }}}

   116 ::

   117 

   118     from scipy import *

   119 

   120 Now let us run the script again,

   121 ::

   122 

   123     python four_plot.py

   124 

   125 Now it gave another error -- plot not defined, let us edit the file

   126 again and add the line below the line we just added,

   127 

   128 {{{ switch to next slide, fix ``plot`` problem }}}

   129 

   130 {{{ add the line as second line in four_plot.py and save }}}

   131 ::

   132 

   133     from pylab import *

   134 

   135 And run the script,

   136 ::

   137 

   138     python four_plot.py

   139 

   140 Yes! it worked. So what did we do?

   141 

   142 We actually imported the required modules using the keyword ``import``.

   143 It could have also be done as,

   144 

   145 {{{ switch to next slide, better way of fixing }}}

   146 

   147 {{{ highlight the following in slide and say it loud }}}

   148 ::

   149 

   150     from scipy import linspace

   151 

   152 instead of,

   153 ::

   154 

   155     from scipy import *

   156 

   157 So in practice it is always good to use function names instead of

   158 asterisk or star. If we use asterisk to import from a particular

   159 module then it will replace any existing functions with the same name

   160 in our name-space.

   161 

   162 {{{ switch to next slide, Instead of ``*`` }}}

   163 

   164 So let us modify four_plot.py as,

   165 {{{ delete the first two lines and add the following }}}

   166 ::

   167 

   168     from scipy import linspace, pi, sin

   169     from pylab import plot, legend, annotate

   170     from pylab import xlim, ylim, title, show

   171 

   172 Now let us try running the code again as,

   173 ::

   174 

   175     python four_plot.py

   176 

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

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

   179 is,

   180 

   181 {{{ switch to next slide }}}

   182 

   183 Notice that we use ``scipy.pi`` instead of just ``pi`` as in the

   184 previous method, and the functions are called as ``pylab.plot()`` and

   185 ``pylab.annotate()`` and not as ``plot()`` and ``annotate()``.

   186 

   187 {{{ switch to next slide, problem statement }}}

   188 

   189 %% %% Write a script to plot a sine wave from minus two pi to two pi.

   190 

   191 Pause here and try to solve the problem yourself before looking at the

   192 solution.

   193 

   194 It can solved as,

   195 

   196 {{{ open sine.py and show it }}}

   197 

   198 the first line we import the required functions ``linspace()`` and

   199 ``sin()`` and constant ``pi`` from the module scipy. the second and

   200 third line we import the functions ``plot()``, ``legend()``,

   201 ``show()``, ``title()``, ``xlabel()`` and ``ylabel()``. And the rest

   202 the code to generate the plot.

   203 

   204 We can run it as,

   205 {{{ now switch focus to terminal and run the script }}}

   206 ::

   207 

   208     python sine.py

   209 

   210 {{{ switch to next slide, What is a module? }}}

   211 

   212 Until now we have been learning about importing modules, now what is a

   213 module?

   214 

   215 A module is simply a file containing Python definitions and

   216 statements. Definitions from a module can be imported into other

   217 modules or into the main module.

   218 

   219 {{{ switch to next slide, Python standard library }}}

   220 

   221 Python has a very rich standard library of modules. It is very

   222 extensive, offering a wide range of facilities. Some of the standard

   223 modules are,

   224 

   225 for Math: math, random

   226 for Internet access: urllib2, smtplib

   227 for System, Command line arguments: sys

   228 for Operating system interface: os

   229 for regular expressions: re

   230 for compression: gzip, zipfile, tarfile

   231 And there are lot more.

   232 

   233 Find more information at Python Library reference,

   234 ``http://docs.python.org/library/``

   235 

   236 There are a lot of other modules like pylab, scipy, Mayavi, etc which

   237 are not part of the standard python library.

   238 

   239 {{{ switch to next slide, summary }}}

   240 

   241 This brings us to the end of this tutorial, in this tutorial we

   242 learned running scripts from command line, learned about modules, saw

   243 the python standard library.

   244 

   245 {{{ switch to next slide, thank you slide }}}

   246 

   247 Thank you!