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.