Python Type Hinting without Cyclic Imports

Problem:

Importing modules with cyclic dependencies introduces runtime exceptions when type hinting is used in conjunction with mixin classes.

Details:

In Python 3.4, a class is split into two files (main.py and mymixin.py), where mymixin.py contains a mixin class that inherits from the main class in main.py (i.e., class Main(object, MyMixin):). Type hinting in MyMixin's methods requires specifying the return type as 'Main', leading to a cyclic import issue.

Python 3.4 Solution:

To bypass the cyclic import while type hinting, a custom approach using a forward declaration is employed.通过使用“键入”模块的“ type_checking”常数，在运行时忽略了类型注释块中的导入语句。 The 'Main' type annotation is also converted into a string to forward declare it.

Python 3.7 Solution (PEP 563):

Using the 'from future import annotations' import statement, all type annotations become strings and are skipped during runtime evaluation, making the forward declaration syntax清洁剂。