File size limits in Flask - Step-by-Step Execution

Concept Flow - File size limits

Start Flask app

↓

User uploads file

↓

Check file size <= limit?

No→Reject upload with error

Yes↓

Process and save file

↓

Send success response

The app starts, user uploads a file, the app checks if the file size is within the limit, rejects if too big, otherwise processes it.

Execution Sample

Flask

from flask import Flask, request
app = Flask(__name__)
app.config['MAX_CONTENT_LENGTH'] = 2 * 1024 * 1024  # 2MB

@app.route('/upload', methods=['POST'])
def upload():
    file = request.files.get('file')
    return 'File accepted' if file else ('No file', 400)

This Flask app limits uploads to 2MB and accepts a file from a POST request.

Execution Table

Step	Action	File Size (bytes)	Check Result	Outcome
1	User sends POST with file	1,500,000	1,500,000 <= 2,097,152	File accepted
2	User sends POST with file	2,500,000	2,500,000 <= 2,097,152	Upload rejected (413 error)
3	User sends POST without file	N/A	No file present	Upload rejected (400 error)

💡 Uploads stop when file size exceeds 2MB or no file is sent.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
file.size	N/A	1,500,000	N/A	N/A
MAX_CONTENT_LENGTH	2,097,152	2,097,152	2,097,152	2,097,152
upload_status	None	Accepted	Rejected	Rejected

Key Moments - 2 Insights

Why does the app reject the upload before reaching the route code when the file is too large?

Flask automatically rejects requests exceeding MAX_CONTENT_LENGTH with a 413 error before calling the route function, as shown in step 2 of the execution_table.

What happens if no file is included in the upload request?

The route function checks if 'file' exists in request.files; if not, it returns an error, as shown in step 3 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the outcome when the file size is 1,500,000 bytes?

A Upload rejected (400 error)

B File accepted

C Upload rejected (413 error)

D No file present

💡 Hint

Check row 1 in the execution_table under Outcome column.

At which step does the file size exceed the limit causing rejection?

A Step 2

B Step 1

C Step 3

D None

💡 Hint

Look at the Check Result column in the execution_table where file size is greater than MAX_CONTENT_LENGTH.

If MAX_CONTENT_LENGTH was increased to 3MB, what would happen at Step 2?

A Upload rejected (400 error)

B Upload rejected (413 error)

C File accepted

D No file present

💡 Hint

Refer to variable_tracker for MAX_CONTENT_LENGTH and compare with file.size at Step 2.

Concept Snapshot

Flask file size limits:
- Set MAX_CONTENT_LENGTH in app.config (bytes)
- Flask auto rejects requests exceeding limit with 413 error
- Check request.files for uploaded file
- Handle missing file cases
- Limits protect server from large uploads

Full Transcript

This visual trace shows how Flask handles file size limits during uploads. The app sets MAX_CONTENT_LENGTH to 2MB. When a user uploads a file, Flask checks the file size before running the route. If the file is larger than 2MB, Flask rejects the request with a 413 error automatically. If the file is smaller, the route processes it and returns acceptance. If no file is sent, the route returns an error. Variables like file.size and MAX_CONTENT_LENGTH track the size and limit. This prevents large files from overloading the server.