Default values in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a default value for the query parameter.

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'

ANone

B"default"

C0

DTrue

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to use quotes for string default values.

Using None without typing Optional[str].

The default value for the query parameter q is set by assigning it in the function signature. Here, "default" is the default string value.

2fill in blank

medium

Complete the code to set a default integer value for the path parameter.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/users/{user_id}")
async def read_user(user_id: int = [1]):
    return {"user_id": user_id}

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

ANone

B"1"

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using string values for integer parameters.

Setting None as default for non-optional parameters.

The default value for the path parameter user_id must be an integer. Here, 1 is the correct default integer value without quotes.

3fill in blank

hard

Fix the error in setting a default value for an optional query parameter.

FastAPI

from typing import Optional
from fastapi import FastAPI

app = FastAPI()

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

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

ATrue

B""

C0

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using empty string instead of None for optional parameters.

Not importing Optional from typing.

When using Optional[str], the default value should be None to indicate the parameter is optional and may be omitted.

4fill in blank

hard

Fill both blanks to set a default value and type for a query parameter.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/products/")
async def get_products(limit: [1] = [2]):
    return {"limit": limit}

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

Aint

B10

Cstr

D"10"

Attempts:

3 left

💡 Hint

Common Mistakes

Using string default value for an integer type.

Swapping type and default value positions.

The query parameter limit is typed as int and has a default value of 10 (an integer).

5fill in blank

hard

Fill all three blanks to create a default value for a query parameter with a description.

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/items/")
async def read_items(q: str = Query([1], description=[2], min_length=[3])):
    return {"q": q}

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

A"default"

B"Search query string"

C3

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Putting None as default when a string default is expected.

Using quotes incorrectly around integers.

The query parameter q uses Query to set a default value "default", a description, and a minimum length of 3 characters.

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