Sorry. That was my mistake.

I think that this is clever but not best practices at all.
E.g. you have chars = ['A', 'B', 'D']
Instead of 2-3 loop cycles you need to make 26 cycles for lowercase letter and then 2-3 cycles for uppercase letters.

Thus we do not have to maintain counters of chars at all. I think your comment is very useful one.

I think that it is pretty clever because in case of numbers=None exception will be raised. But if we are waiting integer to be returned maybe returning 0 is more suitable here. But nevermind.

Sorry, but I do not think that this silution is pythonic way. So it is neither best pratices nor clever. But it is my own opinion.

I think that the script would be much simplier, e.g.
if [[ -z "$1" ]]; then
echo "Nothing to do"
exit 1
fi

the rest code here...

I think this is not a problem here.

Quotation of var $1 is mandatory. In the other case this script fails when file name contains spaces.