feat: Make instrumentator work with just Starlette (#288)

* Replace fastapi with Starlette

* build(deps): Move fastapi to dev dependencies

* fix: Make it work

* docs: Add entry to changelog

---------

Co-authored-by: Tim Schwenke <[email protected]>

CHANGELOG.md

@@ -9,6 +9,16 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0).
 
 ### Added
 
+- Instrumentator now works without FastAPI. This is possible because every
+  FastAPI app is also a Starlette app (but not the other way around). Or to be
+  more specific: FastAPI uses Starlette for things like routing and middleware
+  this package relies on. The change is backwards compatible, even type checkers
+  like mypy should continue working. Thanks to
+  [@mvanderlee](https://github.com/mvanderlee) for proposing this in
+  [#280](https://github.com/trallnag/prometheus-fastapi-instrumentator/issues/280)
+  and implementing it in
+  [#288](https://github.com/trallnag/prometheus-fastapi-instrumentator/pull/288).
+
 - Relaxed type of `get_route_name` argument to `HTTPConnection`. This allows
   developers to use the `get_route_name` function for getting the name of
   websocket routes as well. Thanks to [@pajowu](https://github.com/pajowu) for

pyproject.toml

@@ -14,7 +14,7 @@ keywords = ["prometheus", "instrumentation", "fastapi", "exporter", "metrics"]
 
 [tool.poetry.dependencies]
 python = ">= 3.8.1, < 4.0.0"
-fastapi = ">= 0.38.1, < 1.0.0"
+starlette  = ">= 0.30.0, < 1.0.0"
 prometheus-client = ">= 0.8.0, < 1.0.0"
 
 [tool.poetry.group.dev.dependencies]
@@ -32,6 +32,7 @@ asgiref = "^3.7.2"
 uvicorn = ">=0.28.0"
 gunicorn = "^21.2.0"
 pytest-asyncio = ">=0.23.5.post1"
+fastapi = "^0.110.0"
 
 [tool.black]
 line-length = 90

src/prometheus_fastapi_instrumentator/instrumentation.py

@@ -1,19 +1,20 @@
 import asyncio
 import gzip
+import importlib.util
 import os
 import re
 import warnings
 from enum import Enum
 from typing import Any, Awaitable, Callable, List, Optional, Sequence, Union, cast
 
-from fastapi import FastAPI
 from prometheus_client import (
     CONTENT_TYPE_LATEST,
     REGISTRY,
     CollectorRegistry,
     generate_latest,
     multiprocess,
 )
+from starlette.applications import Starlette
 from starlette.requests import Request
 from starlette.responses import Response
 
@@ -40,7 +41,7 @@ def __init__(
         inprogress_labels: bool = False,
         registry: Union[CollectorRegistry, None] = None,
     ) -> None:
-        """Create a Prometheus FastAPI Instrumentator.
+        """Create a Prometheus FastAPI (and Starlette) Instrumentator.
 
         Args:
             should_group_status_codes (bool): Should status codes be grouped into
@@ -149,7 +150,7 @@ def __init__(
 
     def instrument(
         self,
-        app: FastAPI,
+        app: Starlette,
         metric_namespace: str = "",
         metric_subsystem: str = "",
         should_only_respect_2xx_for_highr: bool = False,
@@ -183,10 +184,11 @@ def instrument(
         The middleware iterates through all `instrumentations` and executes them.
 
         Args:
-            app (FastAPI): FastAPI app instance.
+            app: Starlette app instance. Note that every FastAPI app is a
+                Starlette app.
 
         Raises:
-            e: Only raised if FastAPI itself throws an exception.
+            e: Only raised if app itself throws an exception.
 
         Returns:
             self: Instrumentator. Builder Pattern.
@@ -222,7 +224,7 @@ def instrument(
 
     def expose(
         self,
-        app: FastAPI,
+        app: Starlette,
         should_gzip: bool = False,
         endpoint: str = "/metrics",
         include_in_schema: bool = True,
@@ -232,7 +234,9 @@ def expose(
         """Exposes endpoint for metrics.
 
         Args:
-            app: FastAPI app instance. Endpoint will be added to this app.
+            app: App instance. Endpoint will be added to this app. This can be
+            a Starlette app or a FastAPI app. If it is a Starlette app, `tags`
+            `kwargs` will be ignored.
 
             should_gzip: Should the endpoint return compressed data? It will
                 also check for `gzip` in the `Accept-Encoding` header.
@@ -245,9 +249,9 @@ def expose(
             include_in_schema: Should the endpoint show up in the documentation?
 
             tags (List[str], optional): If you manage your routes with tags.
-                Defaults to None.
+                Defaults to None. Only passed to FastAPI app.
 
-            kwargs: Will be passed to FastAPI route annotation.
+            kwargs: Will be passed to app. Only passed to FastAPI app.
 
         Returns:
             self: Instrumentator. Builder Pattern.
@@ -256,7 +260,6 @@ def expose(
         if self.should_respect_env_var and not self._should_instrumentate():
             return self
 
-        @app.get(endpoint, include_in_schema=include_in_schema, tags=tags, **kwargs)
         def metrics(request: Request) -> Response:
             """Endpoint that serves Prometheus metrics."""
 
@@ -277,6 +280,21 @@ def metrics(request: Request) -> Response:
 
             return resp
 
+        route_configured = False
+        if importlib.util.find_spec("fastapi"):
+            from fastapi import FastAPI
+
+            if isinstance(app, FastAPI):
+                fastapi_app: FastAPI = app
+                fastapi_app.get(
+                    endpoint, include_in_schema=include_in_schema, tags=tags, **kwargs
+                )(metrics)
+                route_configured = True
+        if not route_configured:
+            app.add_route(
+                path=endpoint, route=metrics, include_in_schema=include_in_schema
+            )
+
         return self
 
     def add(

src/prometheus_fastapi_instrumentator/metrics.py

@@ -117,7 +117,7 @@ def latency(
     buckets: Sequence[Union[float, str]] = Histogram.DEFAULT_BUCKETS,
     registry: CollectorRegistry = REGISTRY,
 ) -> Optional[Callable[[Info], None]]:
-    """Default metric for the Prometheus FastAPI Instrumentator.
+    """Default metric for the Prometheus Starlette Instrumentator.
 
     Args:
         metric_name (str, optional): Name of the metric to be created. Must be

src/prometheus_fastapi_instrumentator/middleware.py

@@ -6,8 +6,8 @@
 from timeit import default_timer
 from typing import Awaitable, Callable, Optional, Sequence, Tuple, Union
 
-from fastapi import FastAPI
 from prometheus_client import REGISTRY, CollectorRegistry, Gauge
+from starlette.applications import Starlette
 from starlette.datastructures import Headers
 from starlette.requests import Request
 from starlette.responses import Response
@@ -19,7 +19,7 @@
 class PrometheusInstrumentatorMiddleware:
     def __init__(
         self,
-        app: FastAPI,
+        app: Starlette,
         *,
         should_group_status_codes: bool = True,
         should_ignore_untemplated: bool = False,

tests/test_instrumentation_expose.py → tests/test_expose.py

@@ -2,6 +2,9 @@
 from typing import Any, Dict, Optional
 
 from fastapi import FastAPI, HTTPException
+from starlette.applications import Starlette
+from starlette.responses import PlainTextResponse
+from starlette.routing import Route
 from starlette.testclient import TestClient
 
 from prometheus_fastapi_instrumentator import Instrumentator
@@ -10,7 +13,7 @@
 # Setup
 
 
-def create_app() -> FastAPI:
+def create_fastapi_app() -> FastAPI:
     app = FastAPI()
 
     @app.get("/")
@@ -45,6 +48,13 @@ def create_item(item: Dict[Any, Any]):
     return app
 
 
+def create_starlette_app() -> Starlette:
+    async def homepage(request):
+        return PlainTextResponse("Homepage")
+
+    return Starlette(routes=[Route("/", endpoint=homepage)])
+
+
 def reset_prometheus() -> None:
     from prometheus_client import REGISTRY
 
@@ -70,7 +80,7 @@ def reset_prometheus() -> None:
 
 def test_expose_default_content_type():
     reset_prometheus()
-    app = create_app()
+    app = create_fastapi_app()
     Instrumentator().instrument(app).expose(app)
     client = TestClient(app)
 
@@ -80,3 +90,25 @@ def test_expose_default_content_type():
         "text/plain; version=0.0.4; charset=utf-8; charset=utf-8"
         not in response.headers.values()
     )
+
+
+def test_fastapi_app_expose():
+    reset_prometheus()
+    app = create_fastapi_app()
+    Instrumentator().instrument(app).expose(app)
+    client = TestClient(app)
+
+    response = client.get("/metrics")
+
+    assert response.status_code == 200
+
+
+def test_starlette_app_expose():
+    reset_prometheus()
+    app = create_starlette_app()
+    Instrumentator().instrument(app).expose(app)
+    client = TestClient(app)
+
+    response = client.get("/metrics")
+
+    assert response.status_code == 200