Why is the name of the containing class not recognized as a return value function annotation? [duplicate]

Answer 1

PEP 484 provides an official solution to this in the form of forward references.

When a type hint contains names that have not been defined yet, that definition may be expressed as a string literal, to be resolved later.

In the case of the question code:

class Trie:
    @staticmethod
    def from_mapping(mapping) -> Trie:
        # docstrings and initialization ommitted
        trie = Trie()
        return trie

Becomes:

class Trie:
    @staticmethod
    def from_mapping(mapping) -> 'Trie':
        # docstrings and initialization ommitted
        trie = Trie()
        return trie