Why query parameters filter data in FastAPI - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a query parameter in FastAPI.

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/items/")
async def read_items(q: [1] = None):
    return {"query": q}

Drag options to blanks, or click blank then click option'

Alist

Bint

Cstr

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using an integer type for a text query parameter.

The query parameter q is defined as a string type to accept text filters.

2fill in blank

medium

Complete the code to filter items by a query parameter.

FastAPI

items = ["apple", "banana", "cherry"]

@app.get("/items/")
async def read_items(q: str = None):
    if q:
        return [item for item in items if [1] in item]
    return items

Drag options to blanks, or click blank then click option'

Aq

Bitems

Citem

Dread_items

Attempts:

3 left

💡 Hint

Common Mistakes

Using the list name instead of the query parameter for filtering.

The code checks if the query string q is part of each item to filter the list.

3fill in blank

hard

Fix the error in the query parameter default value to make it optional.

FastAPI

@app.get("/users/")
async def read_users(name: [1]):
    return {"name": name}

Drag options to blanks, or click blank then click option'

Astr = None

Bstr = ''

Cint = None

Dbool = False

Attempts:

3 left

💡 Hint

Common Mistakes

Using empty string as default makes parameter required with empty value.

Setting the default to None makes the query parameter optional.

4fill in blank

hard

Fill both blanks to filter items by minimum and maximum price query parameters.

FastAPI

items = [{"name": "apple", "price": 5}, {"name": "banana", "price": 3}, {"name": "cherry", "price": 7}]

@app.get("/items/")
async def read_items(min_price: int = None, max_price: int = None):
    filtered = [item for item in items if (min_price is None or item[[1]] >= min_price) and (max_price is None or item[[2]] <= max_price)]
    return filtered

Drag options to blanks, or click blank then click option'

A"price"

B"name"

C"cost"

D"value"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong keys like "name" or "cost" which do not exist.

Both blanks should access the "price" key to filter by price range.

5fill in blank

hard

Fill all three blanks to create a filtered dictionary of items with names and prices above a threshold.

FastAPI

items = [{"name": "apple", "price": 5}, {"name": "banana", "price": 3}, {"name": "cherry", "price": 7}]

@app.get("/items/")
async def read_items(min_price: int = 4):
    result = {item[[1]]: item[[2]] for item in items if item[[3]] > min_price}
    return result

Drag options to blanks, or click blank then click option'

A"name"

B"price"

D"cost"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong keys like "cost" or mixing keys incorrectly.

The dictionary keys are item names and values are prices filtered by price greater than min_price.

Practice

1. What is the main purpose of query parameters in a FastAPI endpoint?

easy

A. To filter or customize the data returned by the API

B. To define the HTTP method used by the endpoint

C. To specify the database connection details

D. To set the server port number

Solution

1. Step 1: Understand what query parameters do

   Query parameters are used to send extra information in the URL to control what data the API returns.

2. Step 2: Identify the purpose in FastAPI

   In FastAPI, query parameters let users filter or customize the response without changing the endpoint itself.

3. Final Answer:

   To filter or customize the data returned by the API -> Option A

4. Quick Check:

   Query parameters = filter/customize data [OK]

Hint: Query parameters control what data you get back [OK]

Common Mistakes:

• Confusing query parameters with HTTP methods
• Thinking query parameters set server or database settings
• Assuming query parameters change endpoint URLs

2. Which of the following is the correct way to declare an optional query parameter named category in FastAPI?

easy

A. def read_items(category: str):

B. def read_items(category: str = None):

C. def read_items(category):

D. def read_items(category: int = 0):

Solution

1. Step 1: Recall how to declare optional query parameters

   In FastAPI, optional query parameters are declared by giving a default value, often None.

2. Step 2: Check the options for correct syntax

   def read_items(category: str = None): uses category: str = None, which means the parameter is optional and of type string.

3. Final Answer:

   def read_items(category: str = None): -> Option B

4. Quick Check:

   Optional query param = default None [OK]

Hint: Optional query params have default values like None [OK]

Common Mistakes:

• Not providing a default value for optional parameters
• Using wrong data types for query parameters
• Omitting type annotations

3. Given this FastAPI endpoint:

from fastapi import FastAPI
app = FastAPI()

items = [
  {"name": "apple", "category": "fruit"},
  {"name": "carrot", "category": "vegetable"}
]

@app.get("/items")
async def read_items(category: str = None):
    if category:
        return [item for item in items if item["category"] == category]
    return items

What will be the output of a GET request to /items?category=fruit?

medium

A. [{"name": "apple", "category": "fruit"}]

B. [{"name": "carrot", "category": "vegetable"}]

C. []

D. [{"name": "apple", "category": "fruit"}, {"name": "carrot", "category": "vegetable"}]

Solution

1. Step 1: Understand the query parameter usage

   The endpoint checks if category is given, then filters items matching that category.

2. Step 2: Apply the filter for category='fruit'

   Only the item with category 'fruit' is the apple, so the returned list contains only that item.

3. Final Answer:

   [{"name": "apple", "category": "fruit"}] -> Option A

4. Quick Check:

   Filter by category 'fruit' returns apple [OK]

Hint: Filter returns only items matching query parameter value [OK]

Common Mistakes:

• Returning all items ignoring the filter
• Returning empty list when filter matches
• Confusing category values

4. Identify the error in this FastAPI endpoint that tries to filter items by a query parameter color:

from fastapi import FastAPI
app = FastAPI()

items = [
  {"name": "shirt", "color": "blue"},
  {"name": "pants", "color": "black"}
]

@app.get("/items")
def read_items(color):
    return [item for item in items if item["color"] == color]

medium

A. The items list is empty

B. The list comprehension syntax is incorrect

C. The endpoint should use POST instead of GET

D. The query parameter 'color' is missing a type annotation and default value

Solution

1. Step 1: Check function parameter declaration

   The parameter 'color' lacks a type annotation and default value, so FastAPI treats it as a required path parameter, causing errors.

2. Step 2: Correct declaration for query parameter

   Adding a type (e.g., str) and a default (e.g., None) makes it an optional query parameter, fixing the error.

3. Final Answer:

   The query parameter 'color' is missing a type annotation and default value -> Option D

4. Quick Check:

   Query params need type and default [OK]

Hint: Query params must have type and default to work [OK]

Common Mistakes:

• Omitting type annotations for query parameters
• Confusing query parameters with path parameters
• Using wrong HTTP methods for filtering

5. You want to create a FastAPI endpoint that returns products filtered by optional query parameters category and max_price. Which code snippet correctly implements this filtering?

hard

A. async def get_products(category: str, max_price: float):
return products

B. async def get_products(category, max_price):
return [p for p in products if p['category'] == category and p['price'] <= max_price]

C. async def get_products(category: str = None, max_price: float = None):
filtered = products
if category:
filtered = [p for p in filtered if p['category'] == category]
if max_price is not None:
filtered = [p for p in filtered if p['price'] <= max_price]
return filtered

D. async def get_products():
return products

Solution

1. Step 1: Understand optional query parameters usage

   Both category and max_price are optional, so they need default values (None) to allow filtering only when provided.

2. Step 2: Check filtering logic

   async def get_products(category: str = None, max_price: float = None): filtered = products if category: filtered = [p for p in filtered if p['category'] == category] if max_price is not None: filtered = [p for p in filtered if p['price'] <= max_price] return filtered filters products step-by-step, applying each filter only if the parameter is given, which is correct and safe.

3. Final Answer:

   async def get_products(category: str = None, max_price: float = None): filtered = products if category: filtered = [p for p in filtered if p['category'] == category] if max_price is not None: filtered = [p for p in filtered if p['price'] <= max_price] return filtered -> Option C

4. Quick Check:

   Optional filters with defaults and conditions = correct [OK]

Hint: Use defaults None and conditionally filter inside function [OK]

Common Mistakes:

• Not providing default values for optional parameters
• Filtering without checking if parameters exist
• Returning unfiltered data ignoring query parameters