Example semi-strongly typed pipeline

pipeline.py

from sys import Type
from typing import List

class Policy:
    ...


class RetryPolicy(Policy):

    def __init__(self, max_retries=4, timeout=None):
        self.max_retries = max_retries
        self.timeout = timeout

class LoggingPolicy(Policy):
    ...

class TracingPolicy(Policy):
    ...


class Pipeline:

    def __init__(self, policies: "List[Policy]"):
        self.policies = policies
        self.prepare_request_handlers = []
        self.sending_request_handlers = []

    def send(self, request):
        """ Send the given request through the pipeline"""

        # Each prepare_request_handler is called in the order they were registered
        for handler in self.prepare_request_handlers:
            request = handler(request)

        request = self.policies[0].send(request)

        # Each sending_request_handler is called in the order they were registered
        for handler in self.prepare_request_handlers:
            request = handler(request)

        # The last policy is the "transport" policy
        response = self.policies[-1].send(request)

        return response

    def add_prepare_request_handler(self, handler):
        self.prepare_request_handlers.add(handler)
        
    def get_policy(self, policy_type: "Type") -> "Policy":
        """ Get the policy of the given type

        Will return the policy if there is one and only one policy of the given type in the
        pipeline. Will throw otherwise.
        """
        matches = list([policy for policy in self.policies if isinstance(policy, policy_type)])
        if len(matches) != 1:
            raise ValueError('Found {} matches for policy type {}'.format(len(matches), policy_type.__name__))
        return matches[0]

    def set_policy(self, policy: "Policy", *, policy_type: "Type"=None):
        """ Set the policy of the given type to a new instance.
        """
        # Ensure that there is ona and only one policy of the type to replace/set
        policy_type_to_replace = policy_type or type(policy)
        self.get_policy(policy_type_to_replace)
        self.policies = list([policy if isinstance(existing_policy, policy_type_to_replace)
                              else existing_policy
                              for existing_policy in self.policies])


# Usage example

# Somehow the default pipeline is constructed for a given client
pipeline = Pipeline([
    LoggingPolicy(),
    RetryPolicy(),
    TracingPolicy()
])

retry_policy = pipeline.get_policy(RetryPolicy)
retry_policy.max_retries = 1

@lmazuel, I was playing around with some options on how to access parts of the pipeline without having to use indexes/reconstruct the whole thing in the 90+% case of just minor tweaks.

Thoughts?

@johanste, how do you envision this being used in clients? i.e. does the client create pipelines or the users of the client create pipelines?