The listings package divides all characters in 3 categories: letters, digits and others.
This is a screenshot of its documentation where these categories are listed:
Now, if you use both breaklines=true and breakatwhitespace=true then line breaks are allowed only when a whitespace is encountered.
If you omit breakatwhitespace=true then all characters listed as "other" are good points to break (including whitespaces).
If you want to remove some characters from this list, you have to declare them as letters with the option alsoletter.
The following is a modified version of your MWE where I declared that the characters ()[].= cannot be used for line breaking since they are now letters:
\documentclass[a4paper,twoside]{book}
\usepackage{listings}
\usepackage{xcolor}
\definecolor{mygreen}{rgb}{0,0.6,0}
\lstset{ %
basicstyle=\footnotesize\ttfamily, % the size of the fonts that are used for the code
% breakatwhitespace=true, % sets if automatic breaks should only happen at whitespace
breakindent=1em,
breaklines=true, % sets automatic line breaking
commentstyle=\color{mygreen}, % comment style
keepspaces=true, % keeps spaces in text, useful for keeping indentation of code (possibly needs columns=flexible)
keywordstyle=\color{blue}, % keyword style
linewidth=\textwidth,
morekeywords={*,...}, % if you want to add more keywords to the set
numbers=left, % where to put the line-numbers; possible values are (none, left, right)
numbersep=5pt, % how far the line-numbers are from the code
numberstyle=\tiny\color{gray}, % the style that is used for the line-numbers
showlines=true,
stepnumber=1, % the step between two line-numbers. If it's 1, each line will be numbered
stringstyle=\color{red}, % string literal style
tabsize=2, % sets default tabsize to 2 spaces
title=\lstname % show the filename of files included with \lstinputlisting; also try caption instead of title
}
\begin{document}
\begin{lstlisting}[language=Python,caption={},alsoletter={()[].=}]
plt.contourf(xi,yi,Ti,50,cmap=palette,norm=colors.Normalize(vmin=max([0,np.nanmin(Ti)]),vmax=np.
\end{lstlisting}
\end{document}
Note alsoletter={()[].=} in the options of the lstlisting environment.
As you can see, the result is that the line is broken at a comma:
If you want lines to break only at commas, add the remaining characters in the "other" list.
Beware anyway that this also affects keywords and other things... So, my suggestion is to simply use breaklines without breakatwhitespace.
Best Answer
You can add a bit of glue to allow more stretch: