Here is a simplied rule for my problem

rule test:
    output:
        output_tex = expand("TPA_{region}.tex", region = [0,1]),
    shell:
        """
        for((i=0;i<2;i++));
        do echo '"{output.output_tex['$i']}"';
        done
        """

My expected output is

"TPA_0.tex"
"TPA_1.tex"

Normally, to quote a variable inside single quotation I can use ''$i''. however, with Snakemake shell placeholder, it’s ambiguous. I wonder whether this could be done.