File validation (size, type) in FastAPI - Step-by-Step Execution

Concept Flow - File validation (size, type)

Receive file upload request

↓

Check file size <= max allowed?

No→Reject: File too large

Yes↓

Check file type in allowed types?

No→Reject: Invalid file type

Yes↓

Accept file and process/save

↓

Response sent

The server receives a file, checks its size and type, rejects if invalid, otherwise accepts and processes it.

Execution Sample

FastAPI

from fastapi import FastAPI, File, UploadFile, HTTPException
app = FastAPI()

@app.post("/upload")
async def upload(file: UploadFile = File(...)):
    if file.content_type not in ["image/png", "image/jpeg"]:
        raise HTTPException(status_code=400, detail="Invalid file type")
    contents = await file.read()
    if len(contents) > 1024 * 1024:
        raise HTTPException(status_code=400, detail="File too large")
    return {"filename": file.filename, "size": len(contents)}

This code checks if the uploaded file is PNG or JPEG and smaller than 1MB, else rejects it.

Execution Table

Step	Action	Check	Result	Next Step
1	Receive file upload	N/A	File received	Check file type
2	Check file type	file.content_type in ['image/png', 'image/jpeg']	True	Read file contents
3	Read file contents	N/A	File bytes loaded	Check file size
4	Check file size	len(contents) <= 1048576	True	Accept file
5	Accept file	N/A	Return filename and size	End
6	Receive file upload	N/A	File received	Check file type
7	Check file type	file.content_type in ['image/png', 'image/jpeg']	False	Reject: Invalid file type
8	Reject file	N/A	Raise HTTPException 400	End
9	Receive file upload	N/A	File received	Check file type
10	Check file type	file.content_type in ['image/png', 'image/jpeg']	True	Read file contents
11	Read file contents	N/A	File bytes loaded	Check file size
12	Check file size	len(contents) <= 1048576	False	Reject: File too large
13	Reject file	N/A	Raise HTTPException 400	End

💡 Execution stops when file is accepted or rejected due to type or size check failure.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
file.content_type	N/A	image/png or image/jpeg or other	Same	Same
contents (bytes)	N/A	Loaded bytes of file	Same	Same
len(contents)	N/A	File size in bytes	Compared to 1048576	Same

Key Moments - 3 Insights

Why do we check file type before reading contents?

Checking file type first (see execution_table step 2) avoids unnecessary reading of large files if type is invalid.

What happens if file size exceeds the limit?

At step 12 in execution_table, if size is too big, the code raises an HTTPException rejecting the file.

Why do we use await when reading file contents?

Because reading file is asynchronous in FastAPI, await ensures we get the full content before size check (step 3).

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 7 if the file type is not allowed?

A The file is accepted and processed

B The file is rejected with HTTP 400 error

C The file size is checked next

D The file contents are read

💡 Hint

Check row with Step 7 in execution_table for action on invalid file type

At which step does the code check if the file size is within the allowed limit?

A Step 2

B Step 11

C Step 12

D Step 4

💡 Hint

Look for size check condition in execution_table rows 4 and 12

If the file size is 2MB, what will be the result according to the execution flow?

A File rejected due to size too large

B File rejected due to invalid type

C File accepted and filename returned

D File size check skipped

💡 Hint

Refer to execution_table step 12 where size check fails and file is rejected

Concept Snapshot

FastAPI file validation:
- Use UploadFile and File(...) in endpoint
- Check file.content_type for allowed types
- Read file bytes asynchronously with await
- Check size with len(contents) <= max bytes
- Raise HTTPException(400) if invalid
- Return info if valid

Full Transcript

This visual execution shows how FastAPI handles file validation by first receiving the file upload request. It checks the file type against allowed types like image/png or image/jpeg. If the type is invalid, it rejects immediately with an error. If valid, it reads the file contents asynchronously. Then it checks the file size to ensure it is within the allowed limit (1MB in this example). If the file is too large, it rejects with an error. Otherwise, it accepts the file and returns its filename and size. Variables like file.content_type and contents change during execution. Key moments include why type is checked before reading contents, and why await is used for reading. The quiz questions help reinforce understanding of the flow and checks.