Default values in FastAPI - Step-by-Step Execution

Concept Flow - Default values

Define endpoint with parameter

↓

Request received

↓

Check if parameter provided?

No→Use default value

Yes↓

Use provided value

↓

Process request with value

↓

Send response

FastAPI checks if a request parameter is given; if not, it uses the default value defined in the endpoint.

Execution Sample

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/items/")
async def read_item(q: str = "default"):
    return {"q": q}

Defines a GET endpoint with a query parameter 'q' that defaults to 'default' if not provided.

Execution Table

Step	Request Query Parameter 'q'	Parameter Value Used	Action	Response
1	None	"default"	Use default value	{"q": "default"}
2	"hello"	"hello"	Use provided value	{"q": "hello"}
3	""	""	Use provided empty string	{"q": ""}
4	None	"default"	Use default value	{"q": "default"}

💡 Execution stops after sending response for each request.

Variable Tracker

Variable	Start	Request 1	Request 2	Request 3	Request 4
q	undefined	"default"	"hello"	""	"default"

Key Moments - 2 Insights

What happens if the query parameter 'q' is not included in the request?

FastAPI uses the default value 'default' as shown in execution_table row 1 and 4 where 'q' is None.

If the query parameter 'q' is an empty string, does FastAPI use the default?

No, it uses the empty string provided, as shown in execution_table row 3 where 'q' is "".

Visual Quiz - 3 Questions

Look at the execution table, what value of 'q' is used at step 2?

A "hello"

B None

C "default"

D ""

💡 Hint

Check the 'Parameter Value Used' column at step 2 in the execution_table.

At which step does FastAPI use the default value for 'q'?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Parameter Value Used' column where it shows "default".

If the default value for 'q' was changed to 'none', what would be the response at step 1?

A {"q": "default"}

B {"q": "none"}

C {"q": null}

D Error

💡 Hint

Default value changes affect the response when no parameter is provided, see execution_table row 1.

Concept Snapshot

FastAPI default values:
- Define default in function param: q: str = "default"
- If client omits 'q', default is used
- If client sends empty string, it's used as is
- Default helps avoid errors and sets fallback
- Works for query, path, body parameters

Full Transcript

This visual execution shows how FastAPI handles default values for endpoint parameters. When a request comes in, FastAPI checks if the parameter 'q' is provided. If not, it uses the default value defined in the function signature, here "default". If the client sends a value, even an empty string, FastAPI uses that value instead. The execution table traces four requests: two without 'q' using the default, one with 'hello', and one with an empty string. The variable tracker shows how 'q' changes per request. Key moments clarify common confusions about defaults and empty strings. The quiz tests understanding by referencing the execution steps. This helps beginners see how default values work in FastAPI endpoints step-by-step.

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