Create Direction type to allow more type-safe usage of direction

[johnthagen]

When writing an extension, I found an area that could be improved to help down stream developers that use type checking or IDEs (such as PyCharm) that support type annotations:

drf-spectacular/drf_spectacular/extensions.py

Line 61 in 59f9749

def map_serializer(self, auto_schema: 'AutoSchema', direction):

drf-spectacular/drf_spectacular/plumbing.py

Line 113 in 59f9749

def is_patched_serializer(serializer, direction):

Usage of direction throughout the codebase could be augmented with the use of a dedicated Direction Enum or Literal type:

@unique
class Direction(Enum):
    Request = "request"
    Response = "response"

Or

from typing import Literal

Direction = Literal["request", "response"]

The Literal type was added in Python 3.8, so for supporting older versions, they typing-extensions package backports these in a PyPI package.

Resulting code base would look like:

def map_serializer(self, auto_schema: 'AutoSchema', direction: Direction): 

[tfranzel]

Hey @johnthagen,

very good point. Unfortunately using an Enum could break existing users. I therefore opted for the Literal solution. We recently added typing-extensions as explicit dep anyway.