Boolean query parameters in FastAPI - Step-by-Step Execution

Concept Flow - Boolean query parameters

Client sends HTTP GET request

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FastAPI receives request

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Extract query parameters

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Parse boolean query parameter

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True value

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Use in endpoint logic

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Return response based on boolean

FastAPI reads the query parameter as a string, converts it to a boolean, then uses it in the endpoint logic to decide the response.

Execution Sample

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/items")
async def read_items(show_details: bool = Query(False)):
    return {"show_details": show_details}

This code defines a GET endpoint that reads a boolean query parameter 'show_details' and returns its value.

Execution Table

Step	Request URL	Query Parameter Value	Parsed Boolean	Action	Response
1	/items?show_details=true	true	True	Use True in logic	{"show_details": true}
2	/items?show_details=false	false	False	Use False in logic	{"show_details": false}
3	/items?show_details=1	1	True	Use True in logic	{"show_details": true}
4	/items?show_details=0	0	False	Use False in logic	{"show_details": false}
5	/items	None (default)	False (default)	Use default False	{"show_details": false}
6	/items?show_details=unexpected	unexpected	Error (validation)	Reject request	{"detail": [{"loc": ["query", "show_details"], "msg": "value is not a valid boolean", "type": "type_error.bool"}]}

💡 Execution stops after response is returned or validation error occurs.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6
show_details	False (default)	True	False	True	False	False	Validation Error

Key Moments - 2 Insights

Why does 'show_details=1' become True but 'show_details=unexpected' causes an error?

FastAPI accepts '1' and 'true' as valid True values but rejects strings that don't match boolean formats, causing validation error as shown in execution_table row 6.

What happens if the query parameter is missing?

The default value False is used automatically, as shown in execution_table row 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the parsed boolean value when the URL is '/items?show_details=0'?

A True

B False

C Validation Error

D None

💡 Hint

Check execution_table row 4 under 'Parsed Boolean' column.

At which step does the request get rejected due to invalid boolean?

A Step 6

B Step 3

C Step 1

D Step 5

💡 Hint

Look for 'Error (validation)' in the 'Action' column in execution_table.

If the default value in the code changed to True, what would be the response for '/items' with no query parameter?

A Validation error

B {"show_details": false}

C {"show_details": true}

D Empty response

💡 Hint

Refer to variable_tracker and execution_table row 5 for default behavior.

Concept Snapshot

FastAPI boolean query parameters:
- Use bool type in function signature
- Use Query() for defaults and metadata
- Accepts true/false, 1/0 as valid inputs
- Invalid strings cause validation error
- Default used if parameter missing

Full Transcript

In FastAPI, boolean query parameters are read as strings from the URL and converted to Python booleans. The parameter is defined in the endpoint function with type bool and optionally a default using Query(). When a request comes in, FastAPI tries to parse the query parameter value into a boolean. Values like 'true', '1' become True; 'false', '0' become False. If the parameter is missing, the default value is used. If the value cannot be parsed as a boolean, FastAPI returns a validation error. This behavior lets you control endpoint logic based on simple true/false flags sent in the URL.