Default values in FastAPI - Mini Project: Build & Apply

Using Default Values in FastAPI Query Parameters

📖 Scenario: You are building a simple web API that returns information about books. Sometimes users want to filter books by genre, but if they don't specify a genre, the API should return all books.

🎯 Goal: Create a FastAPI app with a GET endpoint /books that accepts an optional query parameter genre with a default value of "all". The endpoint should return a list of books filtered by the genre if specified, or all books if the genre is "all".

📋 What You'll Learn

Create a list variable called books containing dictionaries with keys title and genre.

Create a variable called default_genre with the value "all".

Create a GET endpoint /books with a query parameter genre that defaults to default_genre.

Filter the books list by genre if it is not "all", otherwise return all books.

💡 Why This Matters

🌍 Real World

APIs often need to handle optional filters with default values to provide flexible data retrieval for users.

💼 Career

Understanding default values in query parameters is essential for backend developers building user-friendly and robust APIs.

1

Create the initial books data

Create a list called books with these exact dictionaries: {"title": "The Hobbit", "genre": "fantasy"}, {"title": "1984", "genre": "dystopian"}, and {"title": "Python 101", "genre": "education"}.

FastAPI

# Create the books list with the given dictionaries
# Your code here

Hint

Use a list with dictionaries. Each dictionary has keys title and genre.

2

Add a default genre variable

Create a variable called default_genre and set it to the string "all".

FastAPI

books = [
    {"title": "The Hobbit", "genre": "fantasy"},
    {"title": "1984", "genre": "dystopian"},
    {"title": "Python 101", "genre": "education"}
]
# Create default_genre variable with value "all"
# Your code here

Hint

Just assign the string "all" to default_genre.

3

Create the FastAPI app and GET endpoint with default query parameter

Import FastAPI and Query from fastapi. Create a FastAPI app called app. Define a GET endpoint /books with a query parameter genre that defaults to default_genre using Query(default_genre).

FastAPI

books = [
    {"title": "The Hobbit", "genre": "fantasy"},
    {"title": "1984", "genre": "dystopian"},
    {"title": "Python 101", "genre": "education"}
]
default_genre = "all"

# Import FastAPI and Query, create app, and define GET /books endpoint with genre query parameter defaulting to default_genre
# Your code here

Hint

Use Query(default_genre) to set the default value for the query parameter.

4

Filter books by genre or return all

Inside the get_books function, return a list of books filtered by genre if genre is not "all". Otherwise, return the full books list.

FastAPI

from fastapi import FastAPI, Query

books = [
    {"title": "The Hobbit", "genre": "fantasy"},
    {"title": "1984", "genre": "dystopian"},
    {"title": "Python 101", "genre": "education"}
]
default_genre = "all"

app = FastAPI()

@app.get("/books")
async def get_books(genre: str = Query(default_genre)):
    # Return filtered books if genre is not "all", else return all books
    # Your code here

Hint

Use a list comprehension to filter books by genre.

Practice

1. In FastAPI, what is the main purpose of setting a default value for a function parameter in an endpoint?

easy

A. To automatically convert the parameter to a string

B. To make the parameter optional and provide a fallback value if not supplied

C. To disable the endpoint if the parameter is missing

D. To force the client to always send a value for that parameter

Solution

1. Step 1: Understand default values in FastAPI

   Default values allow parameters to be optional by providing a fallback if the client does not send a value.

2. Step 2: Identify the effect on endpoint behavior

   When a default is set, the endpoint works even if the parameter is missing, using the default instead.

3. Final Answer:

   To make the parameter optional and provide a fallback value if not supplied -> Option B

4. Quick Check:

   Default value = optional parameter [OK]

Hint: Default values make parameters optional with fallback [OK]

Common Mistakes:

• Thinking default forces client to send value
• Assuming default disables endpoint if missing
• Confusing default with type conversion

2. Which of the following is the correct way to set a default value of 10 for a query parameter named limit in a FastAPI endpoint?

easy

A. def read_items(limit: int = 10):

B. def read_items(limit: int = Query(10)):

C. def read_items(limit: Query = 10):

D. def read_items(limit: int Query(10)):

Solution

1. Step 1: Recall Python default parameter syntax

   Default values are set by assigning a value with = after the parameter name and type.

2. Step 2: Check FastAPI query parameter defaults

   For simple defaults, just use limit: int = 10. Using Query() is optional for extra validation.

3. Final Answer:

   def read_items(limit: int = 10): -> Option A

4. Quick Check:

   Default value syntax = param: type = value [OK]

Hint: Use param: type = default for simple defaults [OK]

Common Mistakes:

• Misplacing Query() without import or wrong syntax
• Using Query without type annotation
• Missing = sign for default assignment

3. Consider this FastAPI endpoint code:

from fastapi import FastAPI
app = FastAPI()

@app.get("/items/")
async def read_items(skip: int = 0, limit: int = 5):
    return {"skip": skip, "limit": limit}

What will be the JSON response when a client calls /items/ without any query parameters?

medium

A. {"skip": "0", "limit": "5"}

B. {"skip": null, "limit": null}

C. Error: missing required parameters

D. {"skip": 0, "limit": 5}

Solution

1. Step 1: Identify default values for parameters

   Both skip and limit have default values: 0 and 5 respectively.

2. Step 2: Understand behavior when no query parameters sent

   FastAPI uses default values when parameters are missing, so skip=0 and limit=5 are used.

3. Final Answer:

   {"skip": 0, "limit": 5} -> Option D

4. Quick Check:

   Defaults used when no query params [OK]

Hint: No query params? Defaults fill in automatically [OK]

Common Mistakes:

• Expecting null instead of default values
• Confusing string vs integer in response
• Assuming error if parameters missing

4. What is wrong with this FastAPI endpoint code?

from fastapi import FastAPI
app = FastAPI()

@app.get("/users/")
def get_users(limit: int = Query(10)):
    return {"limit": limit}

medium

A. Missing import for Query

B. Default value must be a constant, not Query()

C. Function must be async

D. Query() cannot be used for default values

Solution

1. Step 1: Check imports for Query

   The code uses Query(10) but does not import Query from fastapi.

2. Step 2: Understand effect of missing import

   Without importing Query, Python raises a NameError because Query is undefined.

3. Final Answer:

   Missing import for Query -> Option A

4. Quick Check:

   Use from fastapi import Query if using Query() [OK]

Hint: Always import Query when using Query() defaults [OK]

Common Mistakes:

• Forgetting to import Query
• Thinking Query() is invalid default
• Assuming async required for all endpoints

5. You want to create a FastAPI endpoint that accepts an optional query parameter category with a default value of "all". However, if the client sends an empty string "", you want to treat it as None (no category filter). Which approach correctly implements this behavior?

hard

A. def read_items(category: Optional[str] = None):
if category is None:
category = "all"
return {"category": category}

B. def read_items(category: str = Query(default="all", min_length=1)):
return {"category": category}

C. def read_items(category: str = "all"):
if category == "":
category = None
return {"category": category}

D. def read_items(category: str = Query(default="all")):
if category == "all":
category = None
return {"category": category}

Solution

1. Step 1: Understand the requirement for empty string handling

   The parameter should default to "all" but treat empty string "" as None.

2. Step 2: Analyze each option's logic

   def read_items(category: str = "all"): if category == "": category = None return {"category": category} sets default to "all" and explicitly checks if category is "", then sets it to None, matching the requirement.

3. Final Answer:

   def read_items(category: str = "all"): if category == "": category = None return {"category": category} -> Option C

4. Quick Check:

   Check empty string and convert to None manually [OK]

Hint: Check for empty string and convert to None inside function [OK]

Common Mistakes:

• Relying on Query() to handle empty string conversion
• Setting default None but not handling empty string
• Confusing default "all" with None logic