Email with attachments in Flask

Introduction

Sending emails with attachments lets you share files like pictures or documents easily through your app.

You want to send a report PDF from your web app to users.

You need to email images uploaded by users to an admin.

Your app should send invoices as attached files automatically.

You want to share logs or data files via email for support.

Syntax

Flask

from flask_mail import Mail, Message

mail = Mail(app)

msg = Message(subject, sender=sender_email, recipients=[receiver_email])
msg.body = 'Email text here'
msg.attach(filename, content_type, data)

mail.send(msg)

Use msg.attach() to add files. It needs the filename, content type (like 'application/pdf'), and the file data in bytes.

Make sure Flask-Mail is configured with your email server settings before sending.

Examples

Attach a PDF file named 'report.pdf' by reading it in binary mode.

Flask

msg.attach('report.pdf', 'application/pdf', open('report.pdf', 'rb').read())

Attach an image using image data stored in a variable.

Flask

msg.attach('image.png', 'image/png', image_data)

Attach a simple text file created from bytes directly.

Flask

msg.attach('notes.txt', 'text/plain', b'Hello, this is a text file')

Sample Program

This Flask app sends an email with a text file attached when you visit /send-email. It reads 'example.txt' and attaches it to the email.

Flask

from flask import Flask
from flask_mail import Mail, Message

app = Flask(__name__)

# Configure mail server (example with Gmail SMTP)
app.config['MAIL_SERVER'] = 'smtp.gmail.com'
app.config['MAIL_PORT'] = 587
app.config['MAIL_USE_TLS'] = True
app.config['MAIL_USERNAME'] = '[e-mail]'
app.config['MAIL_PASSWORD'] = 'your_password'

mail = Mail(app)

@app.route('/send-email')
def send_email():
    msg = Message('Hello with Attachment',
                  sender='[e-mail]',
                  recipients=['[e-mail]'])
    msg.body = 'Hi there! Please find the attached file.'
    with open('example.txt', 'rb') as f:
        msg.attach('example.txt', 'text/plain', f.read())
    mail.send(msg)
    return 'Email sent with attachment!'

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

Always keep your email credentials safe and do not hardcode them in production.

Attachments must be read in binary mode to avoid corruption.

Test sending emails with small files first to confirm setup works.

Summary

Use Flask-Mail's attach() method to add files to emails.

Configure your mail server settings before sending emails.

Read files in binary mode to attach them correctly.

Practice

1. What is the main purpose of the attach() method in Flask-Mail when sending emails?

easy

A. To set the email subject

B. To configure the mail server settings

C. To specify the recipient's email address

D. To add files as attachments to the email

Solution

1. Step 1: Understand Flask-Mail's attach() method

   The attach() method is designed to add files as attachments to an email message.

2. Step 2: Differentiate from other email settings

   Setting subject, recipient, or server configuration are done by other methods or configurations, not attach().

3. Final Answer:

   To add files as attachments to the email -> Option D

4. Quick Check:

   attach() adds files to email [OK]

Hint: attach() means adding files to email [OK]

Common Mistakes:

• Confusing attach() with setting email subject
• Thinking attach() configures server
• Using attach() to set recipient address

2. Which of the following is the correct way to attach a file named report.pdf using Flask-Mail's attach() method?

easy

A. msg.attach('report.pdf', 'application/pdf', open('report.pdf', 'r').read())

B. msg.attach('report.pdf', 'application/pdf', open('report.pdf', 'rb').read())

C. msg.attach('report.pdf', 'text/plain', open('report.pdf', 'rb').read())

D. msg.attach('report.pdf', 'application/pdf', 'report.pdf')

Solution

1. Step 1: Check file mode for reading attachments

   Files must be read in binary mode ('rb') to attach correctly, especially PDFs.

2. Step 2: Verify MIME type correctness

   The MIME type for PDF files is 'application/pdf', so it must match the file type.

3. Final Answer:

   msg.attach('report.pdf', 'application/pdf', open('report.pdf', 'rb').read()) -> Option B

4. Quick Check:

   Binary read + correct MIME = msg.attach('report.pdf', 'application/pdf', open('report.pdf', 'rb').read()) [OK]

Hint: Read files in 'rb' mode and use correct MIME type [OK]

Common Mistakes:

• Reading file in text mode ('r') instead of binary ('rb')
• Using wrong MIME type like 'text/plain' for PDFs
• Passing filename string instead of file content

3. Given the code snippet below, what will be the output behavior when sending the email?

from flask_mail import Message
msg = Message('Hello', sender='[e-mail]', recipients=['[e-mail]'])
with open('image.png', 'rb') as f:
    msg.attach('image.png', 'image/png', f.read())
mail.send(msg)

medium

A. The code will raise a FileNotFoundError if 'image.png' is missing

B. The email will send but without any attachments

C. The email will be sent with 'image.png' attached correctly

D. The code will raise a TypeError due to wrong attach parameters

Solution

1. Step 1: Analyze file opening behavior

   The code opens 'image.png' in binary mode. If the file does not exist, Python raises FileNotFoundError.

2. Step 2: Consider normal execution

   If the file exists, the email sends with the attachment. If missing, the error stops execution.

3. Final Answer:

   The code will raise a FileNotFoundError if 'image.png' is missing -> Option A

4. Quick Check:

   Missing file causes FileNotFoundError [OK]

Hint: Missing file causes FileNotFoundError, not silent send [OK]

Common Mistakes:

• Assuming email sends without attachment if file missing
• Thinking attach() parameters cause TypeError here
• Ignoring file existence check

4. Identify the error in the following Flask-Mail code snippet that tries to attach a text file:

msg = Message('Report', sender='[e-mail]', recipients=['[e-mail]'])
file = open('report.txt', 'r')
msg.attach('report.txt', 'text/plain', file.read())
file.close()
mail.send(msg)

medium

A. The attach method requires a filename only, not content

B. The MIME type 'text/plain' is incorrect for text files

C. File should be opened in binary mode ('rb') to attach properly

D. The file should be closed after sending the email

Solution

1. Step 1: Check file open mode for attachments

   Even for text files, Flask-Mail expects file content in bytes, so 'rb' mode is recommended.

2. Step 2: Validate MIME type and file handling

   'text/plain' is correct for text files, and closing file before sending is fine.

3. Final Answer:

   File should be opened in binary mode ('rb') to attach properly -> Option C

4. Quick Check:

   Always open files in 'rb' for attach() [OK]

Hint: Open files in 'rb' mode even for text attachments [OK]

Common Mistakes:

• Opening files in text mode ('r') causes attachment issues
• Misunderstanding MIME type for text files
• Closing file after reading is correct, not an error

5. You want to send an email with multiple attachments using Flask-Mail. Which approach correctly attaches both a PDF and an image file before sending?

msg = Message('Files', sender='[e-mail]', recipients=['[e-mail]'])
# Attach PDF
with open('doc.pdf', 'rb') as pdf:
    msg.attach('doc.pdf', 'application/pdf', pdf.read())
# Attach Image
with open('photo.jpg', 'rb') as img:
    msg.attach('photo.jpg', 'image/jpeg', img.read())
mail.send(msg)

hard

A. This code correctly attaches both files and sends the email

B. You must call mail.send() after each attach() call separately

C. Files must be concatenated into one before attaching

D. attach() only supports one file per email, so this fails

Solution

1. Step 1: Review multiple attach() calls

   Flask-Mail allows multiple attach() calls on the same Message object to add multiple files.

2. Step 2: Confirm file reading and sending

   Files are read in binary mode with correct MIME types, and mail.send() is called once after all attachments.

3. Final Answer:

   This code correctly attaches both files and sends the email -> Option A

4. Quick Check:

   Multiple attach() calls before one send() is correct [OK]

Hint: Call attach() multiple times, then send once [OK]

Common Mistakes:

• Calling mail.send() multiple times unnecessarily
• Trying to merge files before attaching
• Believing attach() supports only one file