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diff --git a/lecture_notes/advanced_python/modules.rst b/lecture_notes/advanced_python/modules.rst deleted file mode 100644 index f3df676..0000000 --- a/lecture_notes/advanced_python/modules.rst +++ /dev/null @@ -1,278 +0,0 @@ -Using Python modules -==================== - -We shall, in this section, see how to run Python scripts from the command -line, and more details about importing modules. - -Let us create a simple python script to print hello world. Open your text -editor and type the following, and save the script as ``hello.py``. - -:: - - print "Hello world!" - -Until now we have been running scripts from inside IPython, using - -:: - - %run -i hello.py - -But, as we know, IPython is just an advanced interpreter and it is not -mandatory that every one who wants to run your program has IPython -installed. - -So, the right method to run the script, would be to run it using the Python -interpreter. Open the terminal and navigate to the directory where -``hello.py`` is saved. - -Then run the script by saying, -:: - - python hello.py - -The script is executed and the string ``Hello World!`` has been printed. - -Now let us run a script that plots a simple sine curve in the range -2pi to -2pi, using Python, instead of running it from within IPython. - -Type the following lines and save it in ``sine_plot.py`` file. - -:: - - x = linspace(-2*pi, 2*pi, 100) - plot(x, sin(x)) - show() - -Now let us run sine_plot.py as a python script. - -:: - - python sine_plot.py - -Do we get the plot? No! All we get is error messages. Python complains that -``linspace`` isn't defined. What happened? Let us try to run the same -script from within IPython. Start IPython, with the ``-pylab`` argument and -run the script. It works! - -What is going on, here? IPython when started with the ``-pylab`` option, is -importing a whole set of functionality for us to work with, without -bothering about importing etc. - -Let us now try to fix the problem by importing ``linspace``. - -Let's add the following line as the first line in the script. - -:: - - from scipy import * - -Now let us run the script again, - -:: - - python sine_plot.py - -Now we get an error, that says ``plot`` is not defined. Let's edit the file -and import this from pylab. Let's add the following line, as the second -line of our script. - -:: - - from pylab import * - -And run the script, - -:: - - python sine_plot.py - -Yes! it worked. So what did we do? - -We actually imported the required functions and keywords, using ``import``. -By using the * , we are asking python to import everything from the -``scipy`` and ``pylab`` modules. This isn't a good practice, as -1. it imports a lot of unnecessary things -2. the two modules may have functions with the same name, which would - cause a conflict. - -One of the ways out is to import only the stuff that we need, explicitly. - -Let us modify sine_plot.py by replacing the import statements with the -following lines. - -:: - - from scipy import linspace, pi, sin - from pylab import plot, show - -Now let us try running the code again as, - -:: - - python show_plot.py - -As expected, this works. But, once the number of functions that you need to -import increases, this is slightly inconvenient. Also, you cannot use -functions with the same name, from different modules. To overcome this, -there's another method. - -We could just import the modules as it is, and use the functions or other -objects as a part of those modules. Change the import lines, to the -following. - -:: - - import scipy - import pylab - -Now, replace ``pi`` with ``scipy.pi``. Similarly, for ``sin`` and -``linspace``. Replace ``plot`` and ``show`` with ``pylab.plot`` and -``pylab.show``, respectively. - -Now, run the file and see that we get the output, as expected. - -We have learned how to import from modules, but what exactly is a module? -How is one written? - -A module is simply a Python script containing the definitions and -statements, which can be imported. So, it is very easy to create your own -modules. You just need to stick in all your definitions into your python -file and put the file in your current directory. - -When importing, Python searches the locations present in a variable called -the Python path. So, our module file should be present in one of the -locations in the Python path. The first location to be searched is the -current directory of the script being run. So, having our module file in -the current working directory, is the simplest way to allow it to be used -as a module and import things from it. - -Python has a very rich standard library of modules. It 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/`` - -There are a lot of other modules like pylab, scipy, Mayavi, etc which -are not part of the standard Python library. - -This brings us to the end of our discussion on modules and running scripts -from the command line. - -Writing modules -=============== - -In this section we shall look at writing modules, in some more detail. -Often we will have to reuse the code that we have previously written. We do -that by writing functions. Functions can then be put into modules, and -imported as and when required. - -Let us first write a function that computes the gcd of two numbers and save it in a script. - -:: - - def gcd(a, b): - while b: - a, b = b, a%b - return a - -Now, we shall write a test function in the script that tests the gcd function, to see if it works. - -:: - - if gcd(40, 12) == 4 and gcd(12, 13) == 1: - print "Everything OK" - else: - print "The GCD function is wrong" - -Let us save the file as gcd_script.py in ``/home/fossee/gcd_script.py`` and -run it. - -:: - - $ python /home/fossee/gcd_script.py - -We can see that the script is executed and everything is fine. - -What if we want to use the gcd function in some of our other scripts. This -is also possible since every python file can be used as a module. - -But first, we shall understand what happens when you import a module. - -Open IPython and type - -:: - - import sys - sys.path - -This is a list of locations where python searches for a module when it -encounters an import statement. Hence, when we just did ``import sys``, -python searches for a file named ``sys.py`` or a folder named ``sys`` in -all these locations one by one, until it finds one. We can place our script -in any one of these locations and import it. - -The first item in the list is an empty string which means the current -working directory is also searched. - -Alternatively, we can also import the module if we are working in same -directory where the script exists. - -Since we are in /home/fossee, we can simply do - -:: - - import gcd_script - -We can see that the gcd_script is imported. But the test code that we added -at the end of the file is also executed. - -But we want the test code to be executed only when the file is run as a -python script and not when it is imported. - -This is possible by using ``__name__`` variable. - - -Go to the file and add - -:: - - if __name__ == "__main__": - -before the test code and indent the test code. - -Let us first run the code. - -:: - - $ python gcd_script.py - -We can see that the test runs successfully. - -Now we shall import the file - -:: - - import gcd_script - -We see that now the test code is not executed. - -The ``__name__`` variable is local to every module and it is equal to -``__main__`` only when the file is run as a script. Hence, all the code -that goes in to the if block, ``if __name__ == "__main__":`` is executed -only when the file is run as a python script. - -.. - Local Variables: - mode: rst - indent-tabs-mode: nil - sentence-end-double-space: nil - fill-column: 75 - End: |