Basic query parameter declaration in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a query parameter named 'q' in a FastAPI path operation.

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/items/")
async def read_items(q: str = [1]):
    return {"q": q}

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

A"default"

BTrue

CNone

D...

Attempts:

3 left

💡 Hint

Common Mistakes

Using '...' without importing it

Setting default to True which is not a string or None

Forgetting to set a default value

Setting the default value to None makes the query parameter optional.

2fill in blank

medium

Complete the code to declare an integer query parameter named 'page' with default 1.

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/users/")
async def list_users(page: [1] = 1):
    return {"page": page}

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

Aint

Bstr

Cfloat

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using string type for numeric parameters

Forgetting to set a default value

Using float instead of int

Use int to declare an integer query parameter.

3fill in blank

hard

Fix the error in the code by completing the query parameter declaration for 'limit' as an optional integer.

FastAPI

from typing import Optional
from fastapi import FastAPI
app = FastAPI()

@app.get("/products/")
async def get_products(limit: [1] = None):
    return {"limit": limit}

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

Aint

BOptional[str]

Cstr

DOptional[int]

Attempts:

3 left

💡 Hint

Common Mistakes

Using just int without Optional when default is None

Using Optional[str] when expecting int

Not importing Optional

Use Optional[int] to declare an optional integer query parameter.

4fill in blank

hard

Fill both blanks to declare a query parameter 'search' as a string with default empty string and 'limit' as an integer with default 10.

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/items/")
async def search_items(search: [1] = "", limit: [2] = 10):
    return {"search": search, "limit": limit}

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

Astr

Bint

Cfloat

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing types between parameters

Using float or bool incorrectly

Not matching default values to types

search is a string query parameter with default empty string, and limit is an integer with default 10.

5fill in blank

hard

Fill all three blanks to declare query parameters: 'q' as optional string with default None, 'page' as int default 1, and 'size' as int default 20.

FastAPI

from typing import Optional
from fastapi import FastAPI
app = FastAPI()

@app.get("/search/")
async def search(q: [1] = None, page: [2] = 1, size: [3] = 20):
    return {"q": q, "page": page, "size": size}

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

AOptional[str]

Bint

Cstr

DOptional[int]

Attempts:

3 left

💡 Hint

Common Mistakes

Using str instead of Optional[str] for optional parameters

Using Optional[int] unnecessarily for page and size

Mixing types between parameters

q is an optional string, page and size are integers with default values.

Practice

1. In FastAPI, how do you declare a query parameter that is optional with a default value?

easy

A. By importing Query from fastapi and not setting a default value

B. By using a special decorator like @query_param

C. By defining the parameter inside the request body

D. By adding a function parameter with a default value, e.g., def read_item(q: str = None)

Solution

1. Step 1: Understand query parameter declaration

   In FastAPI, query parameters are declared as function parameters in path operation functions.

2. Step 2: Recognize optional parameters

   If a parameter has a default value (like None), it becomes optional and is treated as a query parameter.

3. Final Answer:

   By adding a function parameter with a default value, e.g., def read_item(q: str = None) -> Option D

4. Quick Check:

   Optional query param = default value [OK]

Hint: Optional query params have default values in function signature [OK]

Common Mistakes:

• Thinking a decorator is needed for query params
• Confusing query params with request body
• Not setting a default value makes param required

2. Which of the following is the correct syntax to declare a required query parameter named item_id of type int in FastAPI?

easy

A. def read_item(item_id: int):

B. def read_item(item_id):

C. def read_item(item_id: str = 'default'):

D. def read_item(item_id: int = None):

Solution

1. Step 1: Identify required query parameter syntax

   A required query parameter has no default value and must have a type annotation.

2. Step 2: Check each option

   def read_item(item_id: int): declares item_id as an int without a default, making it required.

3. Final Answer:

   def read_item(item_id: int): -> Option A

4. Quick Check:

   Required param = no default value [OK]

Hint: No default means required query param [OK]

Common Mistakes:

• Setting default None makes param optional
• Using wrong type annotation
• Omitting type annotation makes param a path param

3. What will be the output if you call the FastAPI endpoint defined as below without any query parameters?

from fastapi import FastAPI
app = FastAPI()

@app.get('/items/')
def read_items(q: str = 'default'):
    return {'q': q}

medium

A. {'q': None}

B. Error: missing required query parameter

C. {'q': 'default'}

D. {'q': ''}

Solution

1. Step 1: Analyze the function parameter

   The parameter q has a default value of 'default', so it is optional.

2. Step 2: Understand the call without query params

   When no query parameter is provided, FastAPI uses the default value 'default'.

3. Final Answer:

   {'q': 'default'} -> Option C

4. Quick Check:

   Default value used when query param missing [OK]

Hint: Default param value is returned if query missing [OK]

Common Mistakes:

• Expecting None instead of default
• Thinking missing param causes error
• Assuming empty string is default

4. Identify the error in this FastAPI query parameter declaration:

from fastapi import FastAPI
app = FastAPI()

@app.get('/users/')
def get_user(user_id: int = None):
    return {'user_id': user_id}

medium

A. Default value None is invalid for int type without Optional

B. No error, this is a valid optional query parameter

C. Type hint should be Optional[int] from typing

D. Query parameter must be declared with Query()

Solution

1. Step 1: Check parameter declaration

   Declaring user_id: int = None makes it optional with default None.

2. Step 2: Verify FastAPI behavior

   FastAPI accepts this and treats user_id as an optional query parameter.

3. Final Answer:

   No error, this is a valid optional query parameter -> Option B

4. Quick Check:

   Default None with int type is allowed [OK]

Hint: Default None with int type is valid in FastAPI [OK]

Common Mistakes:

• Assuming Optional[int] is required
• Thinking Query() is mandatory
• Confusing type hints with runtime errors

5. You want to declare two query parameters in FastAPI: q which is optional with default None, and limit which is required and an integer. Which is the correct function signature?

hard

A. def read_items(limit: int, q: str = None):

B. def read_items(q: str = None, limit: int):

C. def read_items(q: str, limit: int = None):

D. def read_items(q: str = None, limit: int = 10):

Solution

1. Step 1: Understand parameter order rules

   In Python, parameters without defaults must come before those with defaults.

2. Step 2: Apply to FastAPI query params

   limit is required (no default), so it must come before optional q with default None.

3. Final Answer:

   def read_items(limit: int, q: str = None): -> Option A

4. Quick Check:

   Required params before optional ones [OK]

Hint: Put required params before optional ones in function signature [OK]

Common Mistakes:

• Placing optional param before required causes syntax error
• Assigning default to required param
• Mixing parameter order incorrectly