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FastAPI's Test Client

This is my first blog post! 🥳

Please enjoy, and let me know if you have any feedback. 🤓

Abstract

If you are new to FastAPI, you might benefit from reading the following:

If you already know stuff about FastAPI, you might jump to:

Today, we'll talk about the main tool for testing FastAPI applications: the TestClient.

TestClient origin and features

The TestClient is a feature from Starlette (one of the two main dependencies of FastAPI). On which, FastAPI only does a reimport on the testclient module, as we can see here.

We can use the TestClient to test our WebSocket and HTTP endpoints.

The TestClient weird behavior

Although documented on both FastAPI and Starlette's documentation, most of the people are not aware of the TestClient's behavior when it comes to events. To put it simple, there are two ways of creating a TestClient object, and in one of those ways, the events are not executed.

Let's see the behavior with the following FastAPI application:

main.py
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from fastapi import FastAPI

app = FastAPI()
started = False

@app.on_event("startup") # (1)!
def startup():
    global started
    started = True

@app.get("/")
def home():
    if started:
        return {"message": "STARTED"}
    else:
        return {"message": "NOT STARTED"}

  1. There are only two events available: startup and shutdown.

    Read more about it on the ASGI documentation.

As you can see, there's a single endpoint, which gives us a different message depending on the value of the started variable. The started variable is set to True on the startup event.

Now, let's test it with the TestClient:

test.py
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from fastapi.testclient import TestClient

from main import app

def test_home():
    client = TestClient(app)
    response = client.get("/")
    assert response.status_code == 200
    assert response.json() == {"message": "NOT STARTED"}

Install the dependencies:

python -m pip install "fastapi[all]" pytest

Then run pytest: 

pytest test.py

As you can see, the above test passes. Which means the startup event was not triggered. 😱

On the other hand, if we run the following test, we'll get a different result:

test.py
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from fastapi.testclient import TestClient

from main import app

def test_home():
    with TestClient(app):
        response = client.get("/")
        assert response.status_code == 200
        assert response.json() == {"message": "STARTED"}

Install the dependencies:

python -m pip install "fastapi[all]" pytest

Then run pytest: 

pytest test.py

When used as context manager, the TestClient will trigger the startup event.

The Future of the TestClient

By the moment I'm writing this blog, the latest FastAPI version is 0.83.0 with Starlette pinned on 0.19.1. Starlette is already on version 0.20.3, and the next release will change the internals of the TestClient. To be more specific, the HTTP client will be changed from requests to httpx.

As there are some differences between the two clients, the TestClient will reflect the same differences.

This change will be in Starlette on version 0.21.0, and I'm unsure when it will land on FastAPI.

Let's see the changes you should be aware:

  1. allow_redirects will be now called follow_redirects.
  2. cookies parameter will be deprecated under method calls (it should be used on the client instantiation).
  3. data parameter will be called content when sending bytes or text.
  4. content_type will default to "text/plain" when sending file instead of empty string.
  5. The HTTP methods DELETE, GET, HEAD and OPTIONS will not accept content, data, json and files parameters.

  6. data parameter doesn't accept list of tuples, instead it should be a dictionary.

    client.post(..., data=[("key1", "1"), ("key1", "2"), ("key2", "3")])

    client.post(..., data={"key1": ["1", "2"], "key2": "3"})

Those changes will likely impact your test suite. Having this in mind, I've created a codemod that will help you to migrate your tests: bump-testclient. 🎉

Here is the list of what the codemod will do:

  1. Replace allow_redirects with follow_redirects.
  2. Replace data with content when sending bytes or text.
  3. Replace client.<method>(..., <parameter>=...) by client.request("<method>", ..., <parameter>=...) when parameter is either content, data, json or files.

In case you want to read more about the differences between the underneath clients, you can check the httpx documentation.

Thanks for reading till here! 🤓