Question

I have a method that returns either a list or a tuple. What is the most pythonic way of denoting the return type in the argument?

def names(self, section, as_type=()):
    return type(as_type)(([m[0] for m in self.items(section)]))

Answer 1

The pythonic way would be not to care about the type at all. Return a tuple, and if the calling function needs a list, then let it call list() on the result. Or vice versa, whichever makes more sense as a default type.

Even better, have it return a generator expression:

def names(self, section):
    return (m[0] for m in self.items(section))

Now the caller gets an iterable that is evaluated lazily. He then can decide to iterate over it:

for name in obj.names(section):
    ...

or create a list or tuple from it from scratch – he never has to change an existing list into a tuple or vice versa, so this is efficient in all cases:

mylist = list(obj.names(section))
mytuple = tuple(obj.names(section))