Temporarily turn off a global option

Question

See this minimal example

\documentclass{article}
\usepackage{polyglossia}
\setdefaultlanguage{marathi}
\usepackage[no-math]{fontspec}
\setmainfont[Script=Devanagari,Mapping=devanagarinumerals]{Shobhika}

\begin{document}
    \ensuremath{\frac{१}{२}}
\end{document}

I need no-math option for a purpose which is not necessary in this MWE, but I also want to type \frac command once. How to turn off no-math option (which was initially declared globally) for a short scope in the document.

Answer 1

Use \text:

\documentclass{article}
\usepackage[no-math]{fontspec}
\usepackage{polyglossia}
\setdefaultlanguage{marathi}
\setmainfont[Script=Devanagari,Mapping=devanagarinumerals]{Shobhika}
\usepackage{amsmath}

\begin{document}

$\frac{\text{१}}{\text{२}}$

\end{document}

Answer 2

You ask how to temporarily turn off a global option.

In general you have to look at the code (in the class or package) that sets the global option, create a macro that negates that code and also a macro that reinstates the option code. Something along the lines:

\documentclass[opta]{...}
\newcommand{\killopta}{...}
\newcommand{\enableopta}{...}
%
\begin{document}
bla bla bla
\killopta % disable option opta
more blas
\enableopta % reenable option opta
yet more blas
\end{document}

ADDITION

In a comment the OP asked how to deal the the nomath option in the fontspec package.

I have never used fontspec but looking at the fonspec code as given in fontspec-luatex.sty there is the following code:

\DeclareOption {math}    {\bool_gset_true:N \g__fontspec_math_bool }
\DeclareOption {no-math} { \bool_gset_false:N \g__fontspec_math_bool }

I would guess that creating two new commands, say \mathon and \mathoff whose definitions are the same as for the math and no-math options would enable you to switch between them in the document.