documentclass[]{article}
usepackage{sagetex}
usepackage{xcolor}

lstset{language=Sage,
commentstyle={ttfamilycolor{green}},
keywordstyle={ttfamilycolor{blue}bfseries},
stringstyle ={ttfamilycolor{dgraycolor}bfseries},
tabsize = 4,
basicstyle={small ttfamily},
backgroundcolor= color{orange!20},
}

begin{document}
The following function computes the square of a number:
begin{sageblock}
def f(x):
    return x**2
end{sageblock}

Testing in the code gives for $x=2$ the value sage{f(2)}.

Commandline Version:
begin{sagecommandline}
sage: x = 2
sage: f(2)
end{sagecommandline}

end{document}