Make session type pluggable

Update readme

README.md

@@ -31,22 +31,26 @@ the extremely likely circumstance that these two clocks are even slightly out of
 pip install fredio
 ```
 
+### Development Status
+Alpha. Breaking changes should be expected for minor releases until v1, so please pin versions!
+
 ### Examples
 
 #### Standard synchronous usage
 
 ```python
+"""Pipeline to process series batches on-demand"""
+
 import fredio
 
 # Pass an api_key here, or set as FRED_API_KEY environment variable
-# This will also start a background Task for rate limiting
 client = fredio.configure()
 
-# open documentation for the /fred/series endpoint in the default browser
+# Open documentation for the /fred/series endpoint in the default browser
 client.series.observations.docs.open()
 
-# create a data pipeline to request US GDP data from the /series/observations
-# endpoint, clean the results, and write a csv to the local filesystem
+# Request US GDP data from the /series/observations endpoint, clean 
+# the results, and write a csv to the local filesystem
 (client.series.observations
 .get_pandas(
     series_id="GDP",
@@ -62,6 +66,8 @@ the main configuration function. The request Session will queue all successful H
 in the form of `(name, response)`, where `name` corresponds to the final path in the URL endpoint.
 
 ```python
+"""Pipeline to process series updates in near-real time"""
+
 import asyncio
 import datetime
 import fredio
@@ -120,3 +126,6 @@ if __name__ == "__main__":
     with fredio.configure(enable_events=True) as fred:
         asyncio.run(main(fred))
 ```
+
+### Related Projects
+[fred-fdw](https://github.com/bgrams/fred-fdw) - a PostgreSQL Foreign Data Wrapper for FRED

fredio/client.py

@@ -6,7 +6,7 @@
 import os
 import webbrowser
 from datetime import datetime
-from typing import Optional, TYPE_CHECKING
+from typing import Optional, Type, TYPE_CHECKING
 
 from aiohttp import ClientSession, ClientResponse, ClientResponseError
 from yarl import URL
@@ -53,7 +53,11 @@ def setdefault(obj, key, value):
 
         return instance
 
-    def __init__(self, api_key: str, **session_kws):
+    def __init__(self,
+                 api_key: str,
+                 *,
+                 session_cls: Type[ClientSession] = ClientSession,
+                 **session_kws):
         """
         :param api_key: FRED API key
         :param session_kws: Keyword arguments passed to ClientSession
@@ -66,7 +70,7 @@ def __init__(self, api_key: str, **session_kws):
 
         # Lazy instantiation to be called within an async function
         # ClientSession is cached
-        self._session_cls = ClientSession
+        self._session_cls = session_cls
         self._session_kws = frozenset(session_kws.items())
         self._session: Optional[ClientSession] = None