Fix exception on DNS timeout

In 8243bd2, I re-organized the return value of validate_email to be an object rather than a dict. However I didn't properly handle the case when there's a DNS timeout.

I think my original intention was to hide timeout errors because it's unclear if a timeout is a local problem or a problem with the email's domain name. If it's the former, passing through the timeout exception to the caller would make sense. But if it's something that can be triggered by user input, then it shouldn't raise a timeout exception, it should raise a EmailUndeliverableError. Since we can't decide which is the case, we treat it as if the deliverability check was not performed at all.

Fixes #41

Author: JoshData

README.md

@@ -348,12 +348,15 @@ are:
 
 `mx`: A list of (priority, domain) tuples of MX records specified in the
     DNS for the domain (see [RFC 5321 section
-    5](https://tools.ietf.org/html/rfc5321#section-5)).
+    5](https://tools.ietf.org/html/rfc5321#section-5)). May be `None` if
+    the deliverability check could not be completed because of a temporary
+    issue like a timeout.
 
 `mx_fallback_type`: `None` if an `MX` record is found. If no MX records are actually
     specified in DNS and instead are inferred, through an obsolete
     mechanism, from A or AAAA records, the value is the type of DNS
-    record used instead (`A` or `AAAA`).
+    record used instead (`A` or `AAAA`). May be `None` if the deliverability check
+    could not be completed because of a temporary issue like a timeout.
 
 Assumptions
 -----------

email_validator/__init__.py

@@ -92,8 +92,8 @@ class ValidatedEmail(object):
     is False."""
     smtputf8 = None
 
-    """If a deliverability check is performed, a list of (priority, domain) tuples of MX
-    records specified in the DNS for the domain."""
+    """If a deliverability check is performed and if it succeeds, a list of (priority, domain)
+    tuples of MX records specified in the DNS for the domain."""
     mx = None
 
     """If no MX records are actually specified in DNS and instead are inferred, through an obsolete
@@ -263,8 +263,9 @@ def validate_email(
         # Validate the email address's deliverability and update the
         # return dict with metadata.
         deliverability_info = validate_email_deliverability(ret["domain"], ret["domain_i18n"], timeout)
-        ret.mx = deliverability_info["mx"]
-        ret.mx_fallback_type = deliverability_info["mx-fallback"]
+        if "mx" in deliverability_info:
+            ret.mx = deliverability_info["mx"]
+            ret.mx_fallback_type = deliverability_info["mx-fallback"]
 
     return ret
 
@@ -441,6 +442,12 @@ def validate_email_deliverability(domain, domain_i18n, timeout=DEFAULT_TIMEOUT):
     domain += '.'
 
     try:
+        # We need a way to check how timeouts are handled in the tests. So we
+        # have a secret variable that if set makes this method always test the
+        # handling of a timeout.
+        if getattr(validate_email_deliverability, 'TEST_CHECK_TIMEOUT', False):
+            raise dns.exception.Timeout()
+
         resolver = dns.resolver.get_default_resolver()
 
         if timeout:

tests/test_main.py

@@ -268,3 +268,11 @@ def test_deliverability_fails():
     domain = 'xkxufoekjvjfjeodlfmdfjcu.com'
     with pytest.raises(EmailUndeliverableError, match='The domain name {} does not exist'.format(domain)):
         validate_email_deliverability(domain, domain)
+
+def test_deliverability_dns_timeout():
+    validate_email_deliverability.TEST_CHECK_TIMEOUT = True
+    response = validate_email_deliverability('gmail.com', 'gmail.com')
+    assert "mx" not in response
+    assert response.get("unknown-deliverability") == "timeout"
+    email = validate_email('test@gmail.com')
+    del validate_email_deliverability.TEST_CHECK_TIMEOUT