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The latest version of Open Event Android App Generator, v2 lacked the feature of mailing the generated APK to the email ID that is entered at the start of the app generation process. This also included mailing the error logs in case of APK failure.

This is an important feature for app generator because the process of app generation is a time taking one. The users have to wait for the app to be generated so that they can download the generated APK. To avoid this, the generator can automatically email the APK as soon as it is generated.

I took up this issue a few days back and started working on it. I started with thinking about the ways through which it will be implemented. This required some discussions with the mentors and co-developers. We finalised on the following ways:

• Using Sendgrid
• Using SMTP

I will be discussing the implementation of both of them in this blog. The code for APK mailing starts with the function call Notification.send in generator.py

if completed and apk_path and not error:
   Notification.send(
       to=self.creator_email,
       subject='Your android application for %s has been generated ' % self.event_name,
       message='Hi,<br><br>'
               'Your android application for the \'%s\' event has been generated. '
               'And apk file has been attached along with this email.<br><br>'
               'Thanks,<br>'
               'Open Event App Generator' % self.event_name,
       file_attachment=apk_path,
       via_api=self.via_api
   )
else:
   Notification.send(
       to=self.creator_email,
       subject='Your android application for %s could not generated ' % self.event_name,
       message='Hi,<br><br> '
               'Your android application for the \'%s\' event could not generated. '
               'The error message has been provided below.<br><br>'
               '<code>%s</code><br><br>'
               'Thanks,<br>'
               'Open Event App Generator' % (self.event_name, str(error) if error else ''),
       file_attachment=apk_path,
       via_api=self.via_api
   )

This leads me to the class Notification.py. It has three functions:-

1. send(to, subject, message, file_attachment, via_api)

2. send_mail_via_smtp(payload):

3. send_email_via_sendgrid(payload):

As the name suggests, the first function:

send(to, subject, message, file_attachment, via_api)

mainly decides which service (out of smtp and sendgrid) should be used to send the email, on the basis of the input parameters (especially, the ‘EMAIL_SERVICE’ parameter that has to be set in config.py).
The function looks like as follows:

send(to, subject, message, file_attachment, via_api)

It is in the send() that the other two functions are called. If the email_service is smtp, it calls the Notification.send_mail_via_smtp(payload). Otherwise, the Notification.send_email_via_sendgrid(payload) is called.
The sendgrid function is pretty straightforward:

@staticmethod
def send_email_via_sendgrid(payload):

   key = current_app.config['SENDGRID_KEY']
   if not key:
       logger.info('Sendgrid key not defined')
       return
   headers = {
       "Authorization": ("Bearer " + key)
   }
   requests.post(
       "https://api.sendgrid.com/api/mail.send.json",
       data=payload,
       headers=headers
   )

It requires a personalised sendgrid key which is accessed from the config.py file. Apart from that it handles some errors by giving logs in celery tasks. The main line in the function that initiates the email is a POST request made using the python library ‘requests’. The request is made as follows:

 requests.post(
       "https://api.sendgrid.com/api/mail.send.json",
       data=payload,
       headers=headers
   )

The send_mail_via_smtp(payload): function looks for some configurations before sending the mail:

@staticmethod
def send_mail_via_smtp(payload):
   """
   Send email via SMTP
   :param config:
   :param payload:
   :return:
   """
   smtp_encryption = current_app.config['SMTP_ENCRYPTION']
   if smtp_encryption == 'tls':
       smtp_encryption = 'required'
   elif smtp_encryption == 'ssl':
       smtp_encryption = 'ssl'
   elif smtp_encryption == 'tls_optional':
       smtp_encryption = 'optional'
   else:
       smtp_encryption = 'none'
   config = {
       'host': current_app.config['SMTP_HOST'],
       'username': current_app.config['SMTP_USERNAME'],
       'password': current_app.config['SMTP_PASSWORD'],
       'encryption': smtp_encryption,
       'port': current_app.config['SMTP_PORT'],
   }
   mailer_config = {
       'transport': {
           'use': 'smtp',
           'host': config['host'],
           'username': config['username'],
           'password': config['password'],
           'tls': config['encryption'],
           'port': config['port']
       }
   }

   mailer = Mailer(mailer_config)
   mailer.start()
   message = Message(author=payload['from'], to=payload['to'])
   message.subject = payload['subject']
   message.plain = strip_tags(payload['message'])
   message.rich = payload['message']
   message.attach(payload['attachment'])
   mailer.send(message)
   mailer.stop()

It is using the Marrow Mailer Python library to email with attachments(APK). This Python library can be installed using
pip install marrow.mailer
To use Marrow Mailer you instantiate a marrow.mailer.Mailer object with the configuration, then pass Message instances to the Mailer instance’s send() method.

You can refer to the following guides for more information about sending emails through command line:
https://github.com/marrow/mailer is the official repo of Marrow Mailer repository.
https://pypi.python.org/pypi/marrow.mailer
More detailled information on sending emails using Sendgrid can be found here https://www.daveperrett.com/articles/2013/03/19/setting-up-sendmail-with-sendgrid-on-ubuntu/