Mutually recursive types in Python 3.6, using named tuple syntax

Question

I'm trying to implement Nodes and Edges for a graph. Here is my code:

from typing import NamedTuple, List

class Node(NamedTuple):
    name: str
    edges: List[Edge]

class Edge(NamedTuple):
    src: Node
    dest: Node

This raises an error because the Edge type is not defined when Node is created.

NameError: name 'Edge' is not defined

Switching the definitions around doesn't work because Edge also refers to Node.

How can I make it work?

Answer 1

Use string literals.

from typing import NamedTuple, List

class Node(NamedTuple):
    name: str
    edges: List['Edge']

class Edge(NamedTuple):
    src: Node
    dest: Node

The details are in PEP-484, under the "Forward References" section:

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.

Answer 2

You can use string literals; they'll be evaluated later:

class Node(NamedTuple):
    name: str
    edges: List['Edge']

See the Forward References section of PEP 484 -- Type Hints:

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.

For NamedTuple objects, forward references are stored explicitly for later dereferencing:

>>> Node._field_types
{'name': <class 'str'>, 'edges': typing.List[_ForwardRef('Edge')]}

which a type checker can later dereference from the locals and globals:

>>> typing._eval_type(Node._field_types['edges'], globals(), locals())
typing.List[__main__.Edge]