Why does MATLAB receive py.NoneType when calling my Python email function?

Below is a common culprit for this kind of behavior. In your function the control flow does not “fall‐through” to the expected output value because of an early (and misplaced) return statement. Notice that after successfully attaching the file, you immediately do

PYTHONCopied
with open(attachment_path, 'rb') as attachment_file:
    …
    return 'yes'

Because the return is inside the with‐block it exits the function immediately. (In fact, even if that were intended to be an intermediate status message, MATLAB may not capture it as you expect.)

In contrast, your “Email sent!” return is never reached. This unintentional early return is the likely cause for MATLAB’s Python integration to end up with a py.NoneType. (MATLAB’s engine then sees no value from the function—hence the None.)

To fix the issue, adjust the flow so that you complete all operations before returning a final result. For example, you might want to build the email (attaching the file) and then call the SMTP routine and finally return a status message. For instance:

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import smtplib
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart
from email.mime.base import MIMEBase
from email import encoders
import os

def send_email(gmail_user, gmail_password, to_email, subject, body, attachment_path):
    """
    Send an email with an attachment using Gmail's SMTP server.

    Parameters:
      gmail_user (str): Your Gmail email address
      gmail_password (str): Your Gmail app password
      to_email (str): Recipient's email address
      subject (str): Subject of the email
      body (str): Body text of the email
      attachment_path (str): Full path to the attachment file
    """
    try:
        # Create the email
        email = MIMEMultipart()
        email['From'] = gmail_user
        email['To'] = to_email
        email['Subject'] = subject

        # Add the email body
        email.attach(MIMEText(body, 'plain'))

        # Normalize the file path
        attachment_path = os.path.normpath(attachment_path)
        print(f"Processed attachment path: {attachment_path}")

        # Check if the attachment file exists
        if not os.path.exists(attachment_path):
            print(f"Error: The file {attachment_path} does not exist.")
            return 'Name in Email not correct!'

        # Open the file in binary mode and encode it
        with open(attachment_path, 'rb') as attachment_file:
            attachment = MIMEBase('application', 'octet-stream')
            attachment.set_payload(attachment_file.read())
            encoders.encode_base64(attachment)
            filename = os.path.basename(attachment_path)
            print(f"Attachment found: {filename}")

        # Add header for the attachment and attach it to the email
        attachment.add_header('Content-Disposition', 'attachment', filename=filename)
        email.attach(attachment)
        print(f"Attachment '{filename}' added successfully.")

        # Send the email using Gmail's SMTP server
        with smtplib.SMTP('smtp.gmail.com', 587) as server:
            server.starttls()  # Secure the connection
            server.login(gmail_user, gmail_password)
            server.sendmail(gmail_user, to_email, email.as_string())
            print("Email sent successfully!")
            return 'Email sent!'

    except Exception as e:
        print(f"Failed to send email: {e}")
        return 'Email failed fatal!'

    return "Unknown error occurred."

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Key changes:

1. The return that was inside the file open block is removed so that the attachment is added and then the SMTP connection is used to send the email.
2. Only after email is sent (or when catching an exception) is a return issued.

One more tip: when calling this function from MATLAB, you might need to convert the Python string to a MATLAB character array. For example:

MATLABCopied
result = py.send_email.send_email(gmail_user, gmail_password, to_email, subject, body, attachment_path);
resultStr = char(result);  % convert Python str to MATLAB char array

This ensures MATLAB handles the Python string correctly.

In summary, the main problem is the misplaced return inside the context manager. Adjusting the control flow lets MATLAB receive the proper non-None return value.