TestClient basics in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this FastAPI TestClient code?

Consider this FastAPI app and test code using TestClient. What will be the printed output?

FastAPI

from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()

@app.get('/hello')
def read_hello():
    return {'message': 'Hello World'}

client = TestClient(app)
response = client.get('/hello')
print(response.json())

A {'message': 'Hello World'}

B {"error": "Not Found"}

C None

D SyntaxError

💡 Hint

Think about what the endpoint returns and how TestClient calls it.

The endpoint '/hello' returns a JSON with key 'message' and value 'Hello World'. TestClient calls this endpoint and response.json() returns that dictionary.

📝 Syntax

intermediate

Which option will cause a syntax error in this TestClient usage?

Identify which code snippet will cause a syntax error when using FastAPI's TestClient.

FastAPI

from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()

@app.get('/items')
def get_items():
    return {'items': [1, 2, 3]}

client = TestClient(app)

# Which of these calls is invalid?

A response = client.get('/items')

B response = client.post('/items')

C response = client.get('/items' # missing closing parenthesis

D response = client.get('/items', params={'q': 'test'})

💡 Hint

Look carefully at the parentheses in each call.

Option C is missing a closing parenthesis, causing a syntax error.

❓ state_output

advanced

What is the status code after this TestClient POST request?

Given this FastAPI app and test code, what will be the status code of the response?

FastAPI

from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()

@app.post('/submit')
def submit_data():
    return {'result': 'success'}

client = TestClient(app)
response = client.post('/submit')
status = response.status_code
print(status)

A 405

B 200

C 500

D 404

💡 Hint

Check the HTTP method and the route defined.

The POST request to '/submit' matches the route and returns a 200 OK status by default.

🔧 Debug

advanced

Why does this TestClient call raise a KeyError?

This FastAPI app returns a JSON. The test code tries to access a key from the response JSON but raises a KeyError. Why?

FastAPI

from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()

@app.get('/data')
def get_data():
    return {'name': 'Alice', 'age': 30}

client = TestClient(app)
response = client.get('/data')
print(response.json()['email'])

A The TestClient was not initialized properly.

B The response is not JSON, so .json() fails.

C The endpoint '/data' does not exist.

D The key 'email' does not exist in the returned JSON.

💡 Hint

Check the keys in the returned dictionary.

The JSON returned has keys 'name' and 'age' but no 'email', so accessing 'email' causes KeyError.

🧠 Conceptual

expert

Which statement about FastAPI TestClient is true?

Select the correct statement about FastAPI's TestClient behavior.

A TestClient allows calling FastAPI endpoints without starting an actual server.

B TestClient runs the FastAPI app in a separate server process during tests.

C TestClient requires the app to be running on localhost before tests.

D TestClient can only test GET requests, not POST or others.

💡 Hint

Think about how TestClient interacts with the FastAPI app internally.

TestClient runs the app in the same process and calls endpoints directly without needing a server.