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\section{Using Python modules}
\begin{frame}[fragile]
\frametitle{\texttt{hello.py}}
\begin{itemize}
\item Script to print `hello world' -- \texttt{hello.py}
\end{itemize}
\begin{lstlisting}
print "Hello world!"
\end{lstlisting}
\begin{itemize}
\item We have been running scripts from IPython
\end{itemize}
\begin{lstlisting}
In[]: %run -i hello.py
\end{lstlisting}
\begin{itemize}
\item Now, we run from the shell using python
\end{itemize}
\begin{lstlisting}
$ python hello.py
\end{lstlisting} %$
\end{frame}
\begin{frame}[fragile]
\frametitle{Simple plot}
\begin{itemize}
\item Save the following in \texttt{sine\_plot.py}
\end{itemize}
\begin{lstlisting}
x = linspace(-2*pi, 2*pi, 100)
plot(x, sin(x))
show()
\end{lstlisting}
\begin{itemize}
\item Now, let us run the script
\end{itemize}
\begin{lstlisting}
$ python sine_plot.py
\end{lstlisting} % $
\begin{itemize}
\item What's wrong?
\end{itemize}
\end{frame}
\begin{frame}[fragile]
\frametitle{Importing}
\begin{itemize}
\item \texttt{-pylab} is importing a lot of functionality
\item Add the following to the top of your file
\end{itemize}
\begin{lstlisting}
from scipy import *
\end{lstlisting}
\begin{lstlisting}
$ python sine_plot.py
\end{lstlisting} % $
\begin{itemize}
\item Now, plot is not found
\item Add the following as the second line of your script
\end{itemize}
\begin{lstlisting}
from pylab import *
\end{lstlisting}
\begin{lstlisting}
$ python sine_plot.py
\end{lstlisting} % $
\begin{itemize}
\item It works!
\end{itemize}
\end{frame}
\begin{frame}[fragile]
\frametitle{Importing \ldots}
\begin{itemize}
\item \texttt{*} imports everything from \texttt{scipy} and
\texttt{pylab}
\item But, It imports lot of unnecessary stuff
\item And two modules may contain the same name, causing a conflict
\item There are two ways out
\end{itemize}
\begin{lstlisting}
from scipy import linspace, pi, sin
from pylab import plot, show
\end{lstlisting}
\begin{itemize}
\item OR change the imports to following and
\item Replace \texttt{pi} with \texttt{scipy.pi}, etc.
\end{itemize}
\begin{lstlisting}
import scipy
import pylab
\end{lstlisting}
\end{frame}
\section{Writing modules}
\begin{frame}[fragile]
\frametitle{GCD script}
\begin{itemize}
\item Function that computes gcd of two numbers
\item Save it as \texttt{gcd\_script.py}
\end{itemize}
\begin{lstlisting}
def gcd(a, b):
while b:
a, b = b, a%b
return a
\end{lstlisting}
\begin{itemize}
\item Also add the tests to the file
\end{itemize}
\begin{lstlisting}
if gcd(40, 12) == 4 and gcd(12, 13) == 1:
print "Everything OK"
else:
print "The GCD function is wrong"
\end{lstlisting}
\begin{lstlisting}
$ python gcd_script.py
\end{lstlisting} % $
\end{frame}
\begin{frame}[fragile]
\frametitle{Python path}
\begin{itemize}
\item In IPython type the following
\end{itemize}
\begin{lstlisting}
import sys
sys.path
\end{lstlisting}
\begin{itemize}
\item List of locations where python searches for a module
\item \texttt{import sys} -- searches for file \texttt{sys.py} or
dir \texttt{sys} in all these locations
\item So, our own modules can be in any one of the locations
\item Current working directory is one of the locations
\end{itemize}
\end{frame}
\begin{frame}[fragile]
\frametitle{\texttt{\_\_name\_\_}}
\begin{lstlisting}
import gcd_script
\end{lstlisting}
\begin{itemize}
\item The import is successful
\item But the test code, gets run
\item Add the tests to the following \texttt{if} block
\end{itemize}
\begin{lstlisting}
if __name__ == "__main__":
\end{lstlisting}
\begin{itemize}
\item Now the script runs properly
\item As well as the import works; test code not executed
\item \texttt{\_\_name\_\_} is local to every module and is equal
to \texttt{\_\_main\_\_} only when the file is run as a script.
\end{itemize}
\end{frame}
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