1 .. Objectives

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

     3 

     4 .. Prerequisites

     5 .. -------------

     6      

     7 .. Author              : Nishanth Amuluru

     8    Internal Reviewer   : 

     9    External Reviewer   :

    10    Checklist OK?       : <put date stamp here, if OK> [2010-10-05]

    11 

    12 Script

    13 ------

    14 

    15 Hello friends and welcome to the tutorial on "Writing Python scripts"

    16 

    17 {{{ Show the slide containing title }}}

    18 

    19 {{{ Show the slide containing the outline slide }}}

    20 

    21 In this tutorial, we shall learn

    22 

    23  * How write Python scripts 

    24 

    25 Often we will have to reuse the code that we haave written. We do that by

    26 writing functions. Functions are bundled into packages and are imported as and

    27 required in the script.

    28 

    29 Let us first write a function that computes the gcd of two numbers and save it

    30 in a script.

    31 

    32 {{{ Open an editor and start typing out the following code }}}

    33 ::

    34 

    35     def gcd(a, b):

    36 

    37         while b:

    38             a, b = b, a%b

    39 

    40         return a

    41 

    42 We shall write an test function in the script that tests the gcd function every

    43 time the script is run.

    44 

    45 {{{ Add to the script }}}

    46 

    47 ::

    48 

    49     if gcd(40, 12) == 4:

    50         print "Everything OK"

    51     else:

    52         print "The GCD function is wrong"

    53 

    54 Let us save the file as script.py in /home/fossee/gcd_script.py

    55 

    56 We shall run the script by doing

    57 ::

    58 

    59     $ python /home/fossee/gcd_script.py

    60 

    61 We can see that the script is executed and everything is fine.

    62 

    63 What if we want to use the gcd function in some of our later scripts. This

    64 is also possible since every python file can be used as a module.

    65 

    66 But first, we shall understand what happens when you import a module.

    67 

    68 Open IPython and type

    69 ::

    70 

    71     import sys

    72     sys.path

    73 

    74 This is a list of locations where python searches for a module when it

    75 encounters an import statement.

    76 

    77 hence when we just did =import sys=, python searches for a file named sys.py or

    78 a folder named sys in all these locations one by one, until it finds one.

    79 

    80 We can place our script in any one of these locations and can import it.

    81 

    82 The first item in the list is an empty string which means the current working

    83 directory is also searched. 

    84 

    85 Alternatively, we can also import the module if we are working in same 

    86 directory where the script exists.

    87 

    88 Since we are in /home/fossee, we can simply do

    89 ::

    90 

    91     import gcd_script

    92     

    93 We can see that the gcd_script is imported. But the test code that we added at

    94 the end of the file is also executed.

    95 

    96 But we want the test code to be executed only when the file is run as a python 

    97 script and not when it is imported.

    98 

    99 This is possible by using =__name__= variable.

   100 

   101 First we shall look at how to use the idiom and then understand how it works.

   102 

   103 Go to the file and add

   104 ::

   105 

   106     if __name__ == "__main__":

   107         

   108 before the test code and indent the test code.

   109 

   110 Let us first run the code.

   111 ::

   112 

   113     $ python gcd_script.py

   114 

   115 We can see that the test runs successfully.

   116 

   117 Now we shall import the file

   118 ::

   119     

   120     import gcd_script

   121 

   122 We see that now the test code is not executed.

   123 

   124 The __name__ variable is local to every module and it is equal to __main__ only

   125 when the file is run as a script.

   126 

   127 hence all the code that goes after __name__ == "__main__" is executed only when

   128 the file is run as a python script.

   129 

   130 {{{ Show summary slide }}}

   131 

   132 This brings us to the end of the tutorial.

   133 we have learnt

   134 

   135  * What happens when we import a module

   136  * How to use a script as a module

   137  * How to write test functions using the __name__ idiom 

   138 

   139 {{{ Show the "sponsored by FOSSEE" slide }}}

   140 

   141 #[Nishanth]: Will add this line after all of us fix on one.

   142 This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India

   143 

   144 Hope you have enjoyed and found it useful.

   145 Thankyou

   146