dns: Add hostname helpers to Zone class

Add .hostname(), .fqdn(), and .prefix() helpers to the Zone class. Each
of these functions accepts a host prefix, complete name (unrooted
hostname), or FQDN (rooted hostname) associated with the Zone object and
returns the format implied by the function name.

Author: Aaron Sierra

libcloud/dns/base.py

@@ -102,6 +102,70 @@ def __repr__(self):
             self.driver.name,
         )
 
+    def prefix(self, subname):
+        """
+        Accept subordinate (or identity) names in multiple convenience formats.
+
+        In the following examples, "www" is returned:
+
+        |     Format      |       subname      |  self.domain  |
+        |-----------------|--------------------|---------------|
+        |  Bare host name | "www"              | "example.com" |
+        | FQDN (unrooted) | "www.example.com"  | "example.com" |
+        |   FQDN (rooted) | "www.example.com." | "example.com" |
+
+        :param subname: Hostname or FQDN.
+        :type subname: ``str``
+
+        :return: Bare record name, without domain part.
+        :rtype: ``str``
+        """
+
+        return subname.rstrip(".").removesuffix(self.domain).rstrip(".")
+
+    def hostname(self, subname):
+        """
+        Accept subordinate (or identity) names in multiple convenience formats.
+
+        In the following examples, "www.example.com" is returned:
+
+        |     Format      |       subname      |  self.domain  |
+        |-----------------|--------------------|---------------|
+        |  Bare host name | "www"              | "example.com" |
+        | FQDN (unrooted) | "www.example.com"  | "example.com" |
+        |   FQDN (rooted) | "www.example.com." | "example.com" |
+
+        :param subname: Hostname or FQDN.
+        :type subname: ``str``
+
+        :return: Complete hostname, including domain part.
+        :rtype: ``str``
+        """
+
+        prefix = self.prefix(subname)
+        return f"{prefix}.{self.domain}" if prefix else self.domain
+
+    def fqdn(self, subname):
+        """
+        Accept subordinate (or identity) names in multiple convenience formats.
+
+        In the following examples, "www.example.com." is returned:
+
+        |     Format      |       subname      |  self.domain  |
+        |-----------------|--------------------|---------------|
+        |  Bare host name | "www"              | "example.com" |
+        | FQDN (unrooted) | "www.example.com"  | "example.com" |
+        |   FQDN (rooted) | "www.example.com." | "example.com" |
+
+        :param subname: Hostname or FQDN.
+        :type subname: ``str``
+
+        :return: Complete hostname, including domain part.
+        :rtype: ``str``
+        """
+
+        return f"{self.hostname(subname)}."
+
 
 class Record:
     """

libcloud/test/dns/test_base.py

@@ -63,6 +63,26 @@ def setUp(self):
         self.driver = DNSDriver("none", "none")
         self.tmp_file = tempfile.mkstemp()
         self.tmp_path = self.tmp_file[1]
+        self.master_zone = Zone(
+            id=1, domain="example.com", type="master", ttl=900, driver=self.driver
+        )
+
+    def test_zone_helpers(self):
+        zone = self.master_zone
+
+        for func in (zone.prefix, zone.hostname, zone.fqdn):
+            with self.assertRaises(AttributeError):
+                self.assertEqual(func(None))
+
+        for apex in ("", "example.com", "example.com."):
+            self.assertEqual(zone.prefix(apex), "")
+            self.assertEqual(zone.hostname(apex), "example.com")
+            self.assertEqual(zone.fqdn(apex), "example.com.")
+
+        for sub in ("sub", "sub.example.com", "sub.example.com."):
+            self.assertEqual(zone.prefix(sub), "sub")
+            self.assertEqual(zone.hostname(sub), "sub.example.com")
+            self.assertEqual(zone.fqdn(sub), "sub.example.com.")
 
     def test_export_zone_to_bind_format_slave_should_throw(self):
         zone = Zone(id=1, domain="example.com", type="slave", ttl=900, driver=self.driver)