Multiple file uploads in FastAPI - Step-by-Step Execution

Concept Flow - Multiple file uploads

Client sends POST request with multiple files

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FastAPI receives request

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Extract files list from request

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Process each file in the list

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Return response with file details

This flow shows how FastAPI handles multiple files sent in one request by extracting and processing each file before responding.

Execution Sample

FastAPI

from fastapi import FastAPI, File, UploadFile
from typing import List

app = FastAPI()

@app.post("/uploadfiles/")
async def upload_files(files: List[UploadFile] = File(...)):
    return {"filenames": [file.filename for file in files]}

This FastAPI endpoint accepts multiple uploaded files and returns their filenames.

Execution Table

Step	Action	Files List State	Processing File	Output
1	Receive POST request with 3 files	files = [file1, file2, file3]	None yet	None yet
2	Extract files list from request	files = [file1, file2, file3]	None yet	None yet
3	Process first file	files = [file1, file2, file3]	file1 (filename: 'a.txt')	Add 'a.txt' to output list
4	Process second file	files = [file1, file2, file3]	file2 (filename: 'b.jpg')	Add 'b.jpg' to output list
5	Process third file	files = [file1, file2, file3]	file3 (filename: 'c.pdf')	Add 'c.pdf' to output list
6	Return response	files = [file1, file2, file3]	None	{"filenames": ["a.txt", "b.jpg", "c.pdf"]}
7	End	No more files to process	None	Response sent to client

💡 All files processed and response returned with filenames list

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
files	None	[file1, file2, file3]	[file1, file2, file3]	[file1, file2, file3]	None (after response)
output list	[]	["a.txt"]	["a.txt", "b.jpg"]	["a.txt", "b.jpg", "c.pdf"]	["a.txt", "b.jpg", "c.pdf"]

Key Moments - 3 Insights

Why do we use List[UploadFile] instead of just UploadFile?

Because the endpoint expects multiple files, List[UploadFile] tells FastAPI to collect all uploaded files into a list, as shown in execution_table rows 1 and 2.

How does FastAPI know to extract multiple files from the request?

Using File(...) with a List type signals FastAPI to parse multiple files from the form data, demonstrated in execution_table step 2.

What happens if no files are sent in the request?

FastAPI will raise a validation error because File(...) requires at least one file, so the code won't reach processing steps (rows 3-6).

Visual Quiz - 3 Questions

Look at the execution_table at step 4, which file is being processed?

A file2 (b.jpg)

B file1 (a.txt)

C file3 (c.pdf)

D No file is processed at step 4

💡 Hint

Check the 'Processing File' column at step 4 in the execution_table

At which step does the response with filenames get returned?

A Step 5

B Step 6

C Step 3

D Step 7

💡 Hint

Look at the 'Output' column for the step where the JSON response is formed

If the client sends 5 files instead of 3, how does the 'output list' variable change?

A It will contain 3 filenames only

B It will be empty

C It will contain 5 filenames after processing

D It will cause an error

💡 Hint

Refer to variable_tracker row for 'output list' and how it grows with each processed file

Concept Snapshot

FastAPI multiple file uploads:
- Use List[UploadFile] with File(...) to accept many files
- FastAPI extracts all files into a list
- Process files by looping over the list
- Return response with file info
- Validation requires at least one file

Full Transcript

This example shows how FastAPI handles multiple file uploads. The client sends a POST request with several files. FastAPI receives the request and extracts all files into a list using List[UploadFile] with File(...). Then, it processes each file one by one, for example, collecting their filenames. Finally, it returns a JSON response listing all uploaded filenames. If no files are sent, FastAPI raises a validation error. This flow helps beginners understand how to accept and work with multiple files in FastAPI endpoints.