Merge pull request #528 from JWCook/backtesting-example

Add documentation example for backtesting with time-machine

docs/examples.md

@@ -133,3 +133,17 @@ The following scripts can also be found in the
 :lines: 1,17-
 ```
 :::
+
+
+### Backtesting with time-machine
+```{include} ../examples/time_machine_backtesting.py
+:start-line: 2
+:end-line: 4
+```
+
+:::{admonition} Example: custom_request_matcher.py
+:class: toggle
+```{literalinclude} ../examples/time_machine_backtesting.py
+:lines: 1,6-
+```
+:::

examples/time_machine_backtesting.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python3
+"""
+An example of using the [time-machine](https://github.com/adamchainz/time-machine) library for backtesting,
+e.g., testing with cached responses that were available at an arbitrary time in the past.
+"""
+from datetime import datetime
+
+import requests
+import time_machine
+from requests_cache import CachedSession, set_response_defaults
+
+
+class BacktestCachedSession(CachedSession):
+    def request(self, method: str, url: str, **kwargs):
+        response = super().request(method, url, **kwargs)
+
+        # Response was cached after the (simulated) current time, so ignore it and send a new request
+        if response.created_at and response.created_at > datetime.utcnow():
+            new_response = requests.request(method, url, **kwargs)
+            return set_response_defaults(new_response)
+        else:
+            return response
+
+
+def demo():
+    session = BacktestCachedSession()
+    response = session.get('https://httpbin.org/get')
+    response = session.get('https://httpbin.org/get')
+    assert response.from_cache is True
+
+    # Response was not cached yet at this point, so we should get a fresh one
+    with time_machine.travel(datetime(2020, 1, 1)):
+        response = session.get('https://httpbin.org/get')
+        assert response.from_cache is False
+
+
+if __name__ == '__main__':
+    demo()