Email with attachments in Flask - Step-by-Step Execution

Concept Flow - Email with attachments

Start

↓

Create Flask app

↓

Define email route

↓

Receive form data + file

↓

Create email message object

↓

Attach file to message

↓

Send email via SMTP

↓

Return success response

↓

End

This flow shows how a Flask app receives form data and a file, attaches the file to an email, sends it, and responds.

Execution Sample

Flask

from flask import Flask, request
from flask_mail import Mail, Message

app = Flask(__name__)

# Configure mail settings here
app.config.update(
    MAIL_SERVER='smtp.example.com',
    MAIL_PORT=587,
    MAIL_USE_TLS=True,
    MAIL_USERNAME='your_username',
    MAIL_PASSWORD='your_password'
)

mail = Mail(app)

@app.route('/send', methods=['POST'])
def send_email():
    file = request.files['file']
    msg = Message('Hello', recipients=['[e-mail]'])
    msg.body = 'See attached file.'
    msg.attach(file.filename, file.content_type, file.read())
    mail.send(msg)
    return 'Email sent!'

This Flask route receives a file upload, attaches it to an email, and sends it.

Execution Table

Step	Action	Input/State	Output/State Change
1	Start Flask app	No input	App ready to receive requests
2	Receive POST /send	Form data + file uploaded	File object available in request.files
3	Create Message object	Subject='Hello', recipients=['[e-mail]']	Message created with subject and recipients
4	Set message body	Body text set	Message body updated
5	Attach file	File name, content type, file bytes	File attached to message
6	Send email	Message with attachment	Email sent via SMTP
7	Return response	Email sent	Response 'Email sent!' returned
8	End	Request handled	Ready for next request

💡 Request completes after sending email and returning response

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	Final
file	None	File object from request.files	File object	File object	File object
msg	None	None	Message object created	Message with attachment	Message with attachment

Key Moments - 3 Insights

Why do we use file.read() when attaching the file?

file.read() reads the file content bytes needed to attach the file. See execution_table step 5 where file bytes are input to attach.

What happens if no file is uploaded?

request.files['file'] will be missing or empty, causing an error before step 3. You should check file presence before attaching.

Why do we set msg.body separately?

msg.body sets the email text content. Without it, the email would have no message text. See step 4 in execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of 'msg' after step 3?

A Message with attachment

B Message object created with subject and recipients

C None

D File object

💡 Hint

Check the 'Output/State Change' column for step 3 in execution_table

At which step is the file content read and attached to the email?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look for 'Attach file' action in execution_table

If the file was not read with file.read(), what would happen?

A The email would send without attachment content

B The email would fail to send

C The file would be attached correctly anyway

D The Flask app would crash immediately

💡 Hint

Refer to key_moments about why file.read() is needed

Concept Snapshot

Flask Email with Attachments:
- Use Flask-Mail to create Message
- Receive file from request.files
- Attach file with msg.attach(filename, content_type, data)
- Send email with mail.send(msg)
- Always read file content with file.read()
- Return response after sending

Full Transcript

This example shows how a Flask app sends an email with an attachment. The app waits for a POST request with a file upload. It creates an email message, sets the subject and body, then attaches the uploaded file by reading its content. The email is sent using Flask-Mail's mail.send function. Finally, the app returns a success message. Key points include reading the file content before attaching and setting the message body. The execution table traces each step from receiving the file to sending the email and returning the response.