\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}
      %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}