Recursive Typing in Python 3.5+

Question

In Python 3.5, type annotations were added (see here).

Is there a way of defining recursive type annotations, such as for a tree-like structure?

class Employee(object):
    def __init__(self, name: str, reports: List[Employee]):
       self.name = name
       self.reports = reports

In the above, it doesn’t seem as though the annotation List[Employee] works. Running the code results in this error:

NameError: name 'Employee' is not defined

Asked By: nimda

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Source

Answer 1

You can use Forward References as defined in PEP 484

A situation where this occurs commonly is the definition of a
container class, where the class being defined occurs in the signature
of some of the methods. For example, the following code (the start of
a simple binary tree implementation) does not work:
class Tree:
    def __init__(self, left: Tree, right: Tree):
        self.left = left
        self.right = right
To address this, we write:
class Tree:
    def __init__(self, left: 'Tree', right: 'Tree'):
        self.left = left
        self.right = right
It is allowable to use string literals as part of a type hint, for
example:
class Tree:
    ...
    def leaves(self) -> List['Tree']:

Answered By: Brendan Abel

Recursive Typing in Python 3.5+

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