feat: add fastapi integration

posthog/integrations/fastapi.py

@@ -0,0 +1,56 @@
+from typing import TYPE_CHECKING
+import posthog
+from posthog import contexts
+
+if TYPE_CHECKING:
+    from starlette.requests import Request
+
+
+async def inject_posthog_tags(request: Request):
+    """
+    Inject some common tags into posthog events for this request. This is helpful for linking frontend events to backend events.
+
+    The Django implementation pulls a lot of data from the request. However, this data should already be in place from the frontend
+    library. If it's not, something else is wrong and we shouldn't hack around it by including more data here.
+
+    Instead, let's primary just tag the distinct_id and session_id from the request headers. These are injected in all HeyAPI
+    requests, but are *not* guaranteed to be present on all requests.
+
+    https://github.com/PostHog/posthog-python/blob/master/posthog/integrations/django.py
+    """
+
+    # by default, there is not a context, so we must define one
+    # `capture_exceptions=False` https://github.com/PostHog/posthog-python/issues/353
+    with posthog.new_context(capture_exceptions=False):
+        # intentionally overwrite any other session or distinct id set previously
+        # it's up to the user to add dependencies in the correct order
+        if distinct_id := request.headers.get("X-Posthog-Distinct-Id"):
+            contexts.identify_context(distinct_id)
+
+        if session_id := request.headers.get("X-Posthog-Session-Id"):
+            contexts.set_context_session(session_id)
+
+        tags = {}
+
+        if absolute_url := str(request.url):
+            tags["$current_url"] = absolute_url
+
+        if request.method:
+            tags["$request_method"] = request.method
+
+        if request.url.path:
+            tags["$request_path"] = request.url.path
+
+        # TODO adding another dependency for IP extraction seems heavy-handed, but using the default IP is mostly useless
+        # if request_ip := fastapi_access_logger.client_ip_from_request(request):
+        #     # django uses $ip_address, but posthog web library uses $ip
+        #     tags["$ip"] = request_ip
+
+        # although absolute_url contains the host, we must set it separately
+        if request.url.hostname:
+            tags["$host"] = request.url.hostname
+
+        for k, v in tags.items():
+            contexts.tag(k, v)
+
+        yield