This is totally crazy. With a slightly different syntax it's really not so bad: {a:b} for mappings from a to b (a,b) for tuples of a and b [a] for iterables

This is totally crazy. With a slightly different syntax it's really not so bad: {a:b} for mappings from a to b (a,b) for tuples of a and b [a] for iterables of a a | b for unions of a and b Then factor out that nasty common thing and you end up with: Optional[ {basestring: StringThing} | [(basestring, StringThing)] ] StringThing = (basestring, Optional[basestring | file]]) | (basestring, Optional[basestring | file]], Optional[basestring]) | (basestring, Optional[basestring | file]], Optional[basestring], Optional[Headers]) Headers = {basestring: basestring} | [(basestring, basestring)] It's not inherently complicated nor does it have anything to do with duck typing, it's just that the syntax chosen is awful.