Multiple file uploads in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when multiple files are uploaded using FastAPI's UploadFile list?

Consider this FastAPI endpoint that accepts multiple files:

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

app = FastAPI()

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

What will be the output if a client uploads three files named a.txt, b.txt, and c.txt?

FastAPI

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

app = FastAPI()

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

A {"filenames": ["a.txt", "b.txt", "c.txt"]}

B {"filenames": "a.txt,b.txt,c.txt"}

C {"filenames": ["file1", "file2", "file3"]}

D {"filenames": []}

💡 Hint

Think about how FastAPI collects multiple files into a list and how the filenames are accessed.

The endpoint receives a list of UploadFile objects. Each UploadFile has a filename attribute. The list comprehension collects all filenames into a list, so the output JSON contains the list of uploaded filenames.

📝 Syntax

intermediate

Which option correctly defines a FastAPI endpoint to accept multiple files?

Which of the following FastAPI endpoint definitions correctly accepts multiple files for upload?

A

@app.post('/upload')
async def upload(files: UploadFile = File(None)):
    return {'count': 0}

B

@app.post('/upload')
async def upload(files: UploadFile = File(...)):
    return {'count': 1}

C

@app.post('/upload')
async def upload(files: List[str] = File(...)):
    return {'count': len(files)}

D

@app.post('/upload')
async def upload(files: List[UploadFile] = File(...)):
    return {'count': len(files)}

💡 Hint

Remember that to accept multiple files, the parameter should be a list of UploadFile.

Option D correctly uses List[UploadFile] with File(...) to accept multiple files. Option D accepts only one file. Option D uses List[str] which is invalid for files. Option D sets a default None and accepts only one file.

🔧 Debug

advanced

Why does this FastAPI code raise a TypeError when uploading multiple files?

Given this FastAPI endpoint:

from fastapi import FastAPI, UploadFile, File

app = FastAPI()

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

Uploading multiple files causes a TypeError: 'UploadFile' object is not iterable. Why?

FastAPI

from fastapi import FastAPI, UploadFile, File

app = FastAPI()

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

A Because File(...) does not accept multiple files.

B Because files is a single UploadFile, not a list, so it cannot be iterated.

C Because UploadFile does not have a filename attribute.

D Because the endpoint is missing async keyword.

💡 Hint

Check the type of the parameter files and how it is used in the list comprehension.

The parameter files is declared as a single UploadFile, not a list. Trying to iterate over it causes a TypeError. To accept multiple files, it should be List[UploadFile].

❓ state_output

advanced

What is the value of 'total_size' after uploading multiple files in this FastAPI endpoint?

Consider this FastAPI endpoint that calculates total size of uploaded files:

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

app = FastAPI()

@app.post('/upload')
async def upload(files: List[UploadFile] = File(...)):
    total_size = 0
    for file in files:
        content = await file.read()
        total_size += len(content)
    return {'total_size': total_size}

If a client uploads two files, one with 5 bytes and another with 10 bytes, what will be the returned total_size?

FastAPI

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

app = FastAPI()

@app.post('/upload')
async def upload(files: List[UploadFile] = File(...)):
    total_size = 0
    for file in files:
        content = await file.read()
        total_size += len(content)
    return {'total_size': total_size}

A {"total_size": 15}

B {"total_size": 2}

C {"total_size": 0}

D {"total_size": 5}

💡 Hint

Think about how the code reads each file's content and sums their lengths.

The code reads each file's content asynchronously and sums the length of bytes. For files of 5 and 10 bytes, total_size is 15.

🧠 Conceptual

expert

Which statement about FastAPI multiple file uploads is TRUE?

Choose the correct statement about handling multiple file uploads in FastAPI.

A FastAPI automatically saves all uploaded files to disk unless manually read.

B The <code>UploadFile</code> objects are synchronous file objects and block the event loop when read.

C Using <code>List[UploadFile]</code> with <code>File(...)</code> allows receiving multiple files without blocking the event loop.

D FastAPI requires a separate endpoint for each file to handle multiple uploads.

💡 Hint

Consider how FastAPI handles file uploads asynchronously and the role of UploadFile.

Option C is true because UploadFile uses async file handling, allowing multiple files to be processed without blocking. Option C is false; files are not saved automatically. Option C is false; UploadFile is async. Option C is false; one endpoint can handle multiple files.