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.

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