File uploads handling in Flask - Step-by-Step Execution

Concept Flow - File uploads handling

User selects file in browser

↓

Browser sends POST request with file

↓

Flask receives request

↓

Check if file part exists

Yes↓

Validate file (e.g., extension)

Yes↓

Save file to server folder

↓

Send response to user

↓

END

This flow shows how a file selected by the user is sent to the Flask server, validated, saved, and then a response is sent back.

Execution Sample

Flask

from flask import Flask, request
app = Flask(__name__)

@app.route('/upload', methods=['POST'])
def upload_file():
    file = request.files.get('file')
    if file:
        file.save(f"uploads/{file.filename}")
        return 'File saved'
    return 'No file uploaded'

This Flask route handles a POST request to upload a file, saves it if present, and returns a confirmation.

Execution Table

Step	Action	Request Data	Check	Result	Response
1	Receive POST request	file=example.txt	file part exists?	Yes	Continue
2	Validate file	filename=example.txt	Filename allowed?	Yes	Continue
3	Save file	Save to uploads/example.txt	File saved?	Yes	Return 'File saved'
4	End	No further action	-	-	Response sent to user
5	Receive POST request	no file part	file part exists?	No	Return 'No file uploaded'

💡 Execution stops after sending response to user.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
file	None	<FileStorage 'example.txt'>	<FileStorage 'example.txt'>	Saved to disk	Saved
request.files	{}	{'file': <FileStorage>}	{'file': <FileStorage>}	{'file': <FileStorage>}	{'file': <FileStorage>}

Key Moments - 2 Insights

Why do we check if 'file' exists in request.files before saving?

Because if the user submits the form without selecting a file, 'file' will be None. Checking prevents errors when calling save(). See execution_table step 1 and 5.

What happens if the filename is empty or not allowed?

The code should validate the filename before saving. If invalid, it should not save and return an error. In the sample, we assume validation passes (step 2).

Visual Quiz - 3 Questions

Look at the execution table, what is the response when no file is uploaded?

A 'No file uploaded'

B Empty response

C 'File saved'

D 'Error saving file'

💡 Hint

Check execution_table row 5 for the response when file part does not exist.

At which step is the file actually saved to the server?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table row 3 where the action is 'Save file'.

If the file part exists but filename is empty, what should happen?

A Return 'File saved'

B Skip saving and return an error or message

C Save the file anyway

D Crash the server

💡 Hint

Refer to key_moments about validating filename before saving.

Concept Snapshot

Flask file upload handling:
- Use request.files.get('file') to get uploaded file
- Check if file exists before saving
- Validate filename and file type
- Save file with file.save('path')
- Return response to user
- Protect server by validating inputs

Full Transcript

This example shows how Flask handles file uploads. When a user selects a file and submits the form, the browser sends a POST request with the file data. Flask receives this request and checks if the file part exists in request.files. If it does, the file is validated and saved to a server folder. Finally, Flask sends a response confirming the file was saved. If no file is uploaded, Flask returns a message indicating that. This process prevents errors by checking for the file before saving and encourages validating filenames to keep the server safe.