[Znafon]

You could modify it easily, for example the user could register an additional function to the mapping. That would not be as easy to do with a module and getattr().

You can also list the registered function with math_exprs.keys() and that might not be as easy with a module either.

[anentropic]

if all the keys are unknown until runtime then I guess the dict makes sense

I don't think it makes much sense in the example code given