File download responses in FastAPI - Step-by-Step Execution

Concept Flow - File download responses

Client sends GET request for file

↓

FastAPI receives request

↓

Server locates file on disk

↓

FastAPI creates StreamingResponse

↓

Response headers set (Content-Disposition)

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File data streamed to client

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Client browser prompts to save or open file

The client asks for a file, FastAPI finds it, prepares a streaming response with headers, and sends it so the client can download.

Execution Sample

FastAPI

from fastapi import FastAPI
from fastapi.responses import FileResponse

app = FastAPI()

@app.get("/download")
async def download_file():
    return FileResponse("./example.txt", media_type="text/plain", filename="example.txt")

This code defines a FastAPI route that sends a file named example.txt to the client for download.

Execution Table

Step	Action	Evaluation	Result
1	Client sends GET /download	Request received by FastAPI	FastAPI starts processing
2	Locate file './example.txt'	File exists and accessible	File path confirmed
3	Create FileResponse	Set media_type='text/plain', filename='example.txt'	Response object ready with headers
4	Send response headers	Content-Type: text/plain, Content-Disposition: attachment; filename="example.txt"	Browser knows it's a file download
5	Stream file content	File data sent in chunks	Client receives file data
6	Client browser prompts save/open	User sees download dialog	Download completes
7	End	No more data to send	Response finished

💡 File fully sent and client download dialog completed

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	Final
file_path	None	'./example.txt'	'./example.txt'	'./example.txt'	'./example.txt'
response	None	None	FileResponse object with headers	FileResponse streaming data	Response finished

Key Moments - 3 Insights

Why do we set the filename in FileResponse?

Setting filename in FileResponse adds Content-Disposition header so the browser knows to download and suggest that filename, as shown in step 4 of execution_table.

What happens if the file path is wrong or file missing?

FastAPI will raise an error before creating the response (step 2), so no file is sent and client gets an error response.

Why use FileResponse instead of reading file content manually?

FileResponse streams the file efficiently without loading whole file in memory, shown in step 5 streaming file data.

Visual Quiz - 3 Questions

Look at the execution_table, what headers are sent in step 4?

A Only Content-Disposition without filename

B Only Content-Type

C Content-Type and Content-Disposition with filename

D No headers are sent

💡 Hint

Check step 4 in execution_table for headers sent

At which step does FastAPI confirm the file exists?

A Step 2

B Step 1

C Step 3

D Step 5

💡 Hint

Look for file path confirmation in execution_table

If the file was very large, which step ensures efficient sending?

A Step 4 sending headers

B Step 5 streaming file content

C Step 3 creating response

D Step 6 client prompt

💡 Hint

Streaming large files happens in step 5

Concept Snapshot

FastAPI File Download Responses:
- Use FileResponse to send files
- Set filename for download prompt
- Streams file efficiently
- Headers: Content-Type and Content-Disposition
- Client receives file download dialog

Full Transcript

When a client requests a file download, FastAPI receives the request and locates the file on the server. It then creates a FileResponse object that sets the correct headers including Content-Type and Content-Disposition with the filename. The file content is streamed efficiently to the client. The client browser then shows a prompt to save or open the file. If the file is missing, FastAPI returns an error before sending any data. This process ensures smooth and memory-efficient file downloads.