Unpacking Python's Type Annotations

Question

I\'m trying to generate some JavaScript based on the type annotations I have provided in on some Python functions by using the signature() function in the

Answer 1

Take note, this applies to Python 3.5.1

For Python 3.5.2 take a look at phillip's answer.

You shouldn't be checking with the identity operator as Phillip stated, use equality to get this right.

To check if a hint is a subclass of a list you could use issubclass checks (even though you should take note that this can be quirky in certain cases and is currently worked on):

issubclass(List[int], list)  # True

To get the members of a type hint you generally have two watch out for the cases involved.

If it has a simple type, as in List[int] the value of the argument is located in the __parameters__ value:

signature.return_annotation.__parameters__[0] # int

Now, in more complex scenarios i.e a class supplied as an argument with List[User] you must again extract the __parameter__[0] and then get the __forward_arg__. This is because Python wraps the argument in a special ForwardRef class:

d = signature.return_annotation.__parameter__[0]
d.__forward_arg__ # 'User'

Take note, you don't need to actually use inspect here, typing has a helper function named get_type_hints that returns the type hints as a dictionary (it uses the function objects __annotations__ attribute).