source-code.tex

\documentclass{article}
\usepackage{listings}
\begin{document}
\section{Binary Tree Diagram} 

% display source code
\lstset
{
    language=Python,
    basicstyle=\footnotesize,
    numbers=left,
    stepnumber=1,
    showstringspaces=false,
    tabsize=1,
    breaklines=true,
    breakatwhitespace=false,
}
\lstinputlisting[language=Python]{source.py}

\end{document}