Type hints: solve circular dependency

python type-hinting python-3.5
Tamriel picture Tamriel · Nov 21, 2015 · Viewed 11.2k times · Source

The following produces NameError: name 'Client' is not defined. How can I solve it?

class Server():
    def register_client(self, client: Client)
        pass


class Client():
    def __init__(self, server: Server):
        server.register_client(self)

Answer

Martijn Pieters picture Martijn Pieters · Nov 21, 2015

You can use a forward reference by using a string name for the not-yet-defined Client class:

class Server():
    def register_client(self, client: 'Client')
        pass

As of Python 3.7, you can also postpone all runtime parsing of annotations by adding the following __future__ import at the top of your module:

from __future__ import annotations

at which point the annotations are stored as string representations of the abstract syntax tree for the expression; you can use typing.get_type_hints() to resolve those (and resolve forward references as used above).

See PEP 563 -- Postponed Evaluation of Annotations for details; this behaviour will be the default in Python 4.0.

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