Better email notifications

Rework a bit on top of @bnjbvr's MR to add basic email notifications
support.

* Cleaned the code a bit.
* Add a `Date` and a `Message-ID` header to improve the spam score.

Thanks a lot @bnjbvr's for the MR!

flatisfy/email.py

@@ -1,17 +1,33 @@
 # coding: utf-8
-
 """
 Email notifications.
 """
 
 from __future__ import absolute_import, print_function, unicode_literals
+
+import logging
 import smtplib
 
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
+from email.utils import formatdate, make_msgid
 
-def send_email(server, port, subject, _from, to, txt, html):
+LOGGER = logging.getLogger(__name__)
-    if len(to) == 0:
+
+
+def send_email(server, port, subject, _from, _to, txt, html):
+    """
+    Send an email
+
+    :param server: SMTP server to use.
+    :param port: SMTP port to use.
+    :param subject: Subject of the email to send.
+    :param _from: Email address of the sender.
+    :param _to: List of email addresses of the receivers.
+    :param txt: Text version of the message.
+    :param html: HTML version of the message.
+    """
+    if not _to:
         LOGGER.warn("No recipients for the email notifications, aborting.")
         return
 
@@ -20,17 +36,26 @@ def send_email(server, port, subject, _from, to, txt, html):
     msg = MIMEMultipart('alternative')
     msg['Subject'] = subject
     msg['From'] = _from
-    msg['To'] = ', '.join(to)
+    msg['To'] = ', '.join(_to)
+    msg['Date'] = formatdate()
+    msg['Message-ID'] = make_msgid()
 
     msg.attach(MIMEText(txt, 'plain', 'utf-8'))
     msg.attach(MIMEText(html, 'html', 'utf-8'))
 
-    server.sendmail(_from, to, msg.as_string())
+    server.sendmail(_from, _to, msg.as_string())
     server.quit()
 
+
 def send_notification(config, flats):
+    """
+    Send an email notification about new available flats.
+
+    :param config: A config dict.
+    :param flats: List of flats to include in the notification.
+    """
     # Don't send an email if there are no new flats.
-    if len(flats) == 0:
+    if not flats:
         return
 
     txt = u'Hello dear user,\n\nThe following new flats have been found:\n\n'
@@ -53,8 +78,11 @@ def send_notification(config, flats):
         cost = unicode(flat.cost)
         currency = unicode(flat.currency)
 
-        txt += '- {}: {}#/flat/{} (area: {}, cost: {} {})\n' \
+        txt += (
-                .format(title, website_url, flat_id, area, cost, currency)
+            '- {}: {}#/flat/{} (area: {}, cost: {} {})\n'.format(
+                title, website_url, flat_id, area, cost, currency
+            )
+        )
 
         html += """
             <li>
@@ -65,9 +93,11 @@ def send_notification(config, flats):
 
     html += "</ul>"
 
-    SIGNATURE = u"\nHope you'll find what you were looking for.\n\nBye!\nFlatisfy"
+    signature = (
-    txt += SIGNATURE
+        u"\nHope you'll find what you were looking for.\n\nBye!\nFlatisfy"
-    html += SIGNATURE.replace('\n', '<br>')
+    )
+    txt += signature
+    html += signature.replace('\n', '<br>')
 
     html += """</p>
       </body>