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-% Created 2011-04-18 Mon 03:20
-\documentclass[presentation]{beamer}
-\usepackage[utf8]{inputenc}
-\usepackage[T1]{fontenc}
-\usepackage{fixltx2e}
-\usepackage{graphicx}
-\usepackage{longtable}
-\usepackage{float}
-\usepackage{wrapfig}
-\usepackage{soul}
-\usepackage{textcomp}
-\usepackage{marvosym}
-\usepackage{wasysym}
-\usepackage{latexsym}
-\usepackage{amssymb}
-\usepackage{hyperref}
-\tolerance=1000
-\usepackage[english]{babel} \usepackage{ae,aecompl}
-\usepackage{mathpazo,courier,euler} \usepackage[scaled=.95]{helvet}
-\usepackage{listings}
-\lstset{language=Python, basicstyle=\ttfamily\bfseries,
-commentstyle=\color{red}\itshape, stringstyle=\color{darkgreen},
-showstringspaces=false, keywordstyle=\color{blue}\bfseries}
-\providecommand{\alert}[1]{\textbf{#1}}
-
-\title{modules.rst.org}
-\author{}
-\date{\today}
-
-\usetheme{Warsaw}\usecolortheme{default}\useoutertheme{infolines}\setbeamercovered{transparent}
-\begin{document}
-
-\maketitle
-
-\begin{frame}
-\frametitle{Outline}
-\setcounter{tocdepth}{3}
-\tableofcontents
-\end{frame}
-
-
-
-
-\section{Using Python modules}
-\label{sec-1}
-
-
-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 \texttt{hello.py}.
-
-\begin{verbatim}
-print "Hello world!"
-\end{verbatim}
-
-Until now we have been running scripts from inside IPython, using
-
-\begin{verbatim}
-%run -i hello.py
-\end{verbatim}
-
-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 \texttt{hello.py} is saved.
-
-Then run the script by saying,
-
-\begin{verbatim}
-python hello.py
-\end{verbatim}
-
-The script is executed and the string \texttt{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 \texttt{sine\_plot.py} file.
-
-\begin{verbatim}
-x = linspace(-2*pi, 2*pi, 100)
-plot(x, sin(x))
-show()
-\end{verbatim}
-
-Now let us run sine\_plot.py as a python script.
-
-\begin{verbatim}
-python sine_plot.py
-\end{verbatim}
-
-Do we get the plot? No! All we get is error messages. Python complains
-that \texttt{linspace} isn't defined. What happened? Let us try to run the same
-script from within IPython. Start IPython, with the \texttt{-pylab} argument
-and run the script. It works!
-
-What is going on, here? IPython when started with the \texttt{-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 \texttt{linspace}.
-
-Let's add the following line as the first line in the script.
-
-\begin{verbatim}
-from scipy import *
-\end{verbatim}
-
-Now let us run the script again,
-
-\begin{verbatim}
-python sine_plot.py
-\end{verbatim}
-
-Now we get an error, that says \texttt{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.
-
-\begin{verbatim}
-from pylab import *
-\end{verbatim}
-
-And run the script,
-
-\begin{verbatim}
-python sine_plot.py
-\end{verbatim}
-
-Yes! it worked. So what did we do?
-
-We actually imported the required functions and keywords, using
-\texttt{import}. By using the *, we are asking python to import everything from
-the \texttt{scipy} and \texttt{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.
-
-\begin{verbatim}
-from scipy import linspace, pi, sin
-from pylab import plot, show
-\end{verbatim}
-
-Now let us try running the code again as,
-
-\begin{verbatim}
-python show_plot.py
-\end{verbatim}
-
-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.
-
-\begin{verbatim}
-import scipy
-import pylab
-\end{verbatim}
-
-Now, replace \texttt{pi} with \texttt{scipy.pi}. Similarly, for \texttt{sin} and \texttt{linspace}.
-Replace \texttt{plot} and \texttt{show} with \texttt{pylab.plot} and \texttt{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,
-\texttt{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.
-\section{Writing modules}
-\label{sec-2}
-
-
-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.
-
-\begin{verbatim}
-def gcd(a, b):
-    while b:
-        a, b = b, a%b
-    return a
-\end{verbatim}
-
-Now, we shall write a test function in the script that tests the gcd
-function, to see if it works.
-
-\begin{verbatim}
-if gcd(40, 12) == 4 and gcd(12, 13) == 1:
-    print "Everything OK"
-else:
-    print "The GCD function is wrong"
-\end{verbatim}
-
-Let us save the file as gcd\_script.py in \texttt{/home/fossee/gcd\_script.py}
-and run it.
-
-\begin{verbatim}
-$ python /home/fossee/gcd_script.py
-\end{verbatim}
-
-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
-
-\begin{verbatim}
-import sys
-sys.path
-\end{verbatim}
-
-This is a list of locations where python searches for a module when it
-encounters an import statement. Hence, when we just did \texttt{import sys},
-python searches for a file named \texttt{sys.py} or a folder named \texttt{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
-
-\begin{verbatim}
-import gcd_script
-\end{verbatim}
-
-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 \texttt{\_\_name\_\_} variable.
-
-Go to the file and add
-
-\begin{verbatim}
-if __name__ == "__main__":
-\end{verbatim}
-
-before the test code and indent the test code.
-
-Let us first run the code.
-
-\begin{verbatim}
-$ python gcd_script.py
-\end{verbatim}
-
-We can see that the test runs successfully.
-
-Now we shall import the file
-
-\begin{verbatim}
-import gcd_script
-\end{verbatim}
-
-We see that now the test code is not executed.
-
-The \texttt{\_\_name\_\_} variable is local to every module and it is equal to
-\texttt{\_\_main\_\_} only when the file is run as a script. Hence, all the code
-that goes in to the if block, \texttt{if \_\_name\_\_ =} ``__main__'':= is executed
-only when the file is run as a python script.
-
-\end{document}