Accept empty string on construction

The empty string is already treated as invalid by the supplied
validators. It was incorrect to raise ValueError on construction since
this requires users to pre-validate strings before passing to a
validation library.

Fixes ypcrts#45

fqdn/__init__.py

@@ -40,7 +40,7 @@ def __init__(self, fqdn, *nothing, **kwargs):
         if unknown_kwargs:
             raise ValueError("got extra kwargs: {}".format(unknown_kwargs))
 
-        if not (fqdn and isinstance(fqdn, str)):
+        if not isinstance(fqdn, str):
             raise ValueError("fqdn must be str")
         self._fqdn = fqdn.lower()
         self._allow_underscores = kwargs.get("allow_underscores", False)

tests/test_fqdn.py

@@ -30,6 +30,11 @@ def test_str(self, a_u):
         f = FQDN(d, allow_underscores=a_u)
         assert f.absolute == str(f)
 
+    def test_empty(self, a_u):
+        d = ""
+        f = FQDN(d, allow_underscores=a_u)
+        assert not f.is_valid
+
     def test_rfc_1035_s_2_3_4__label_max_length(self, a_u):
         assert FQDN(
             "www.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.com",
@@ -159,8 +164,7 @@ def test_min_labels_defaults_to_require_2(self):
         assert not dn.is_valid
 
     def test_min_labels_valid_set_to_1(self):
-        with pytest.raises(ValueError):
-            FQDN("", min_labels=1).is_valid
+        assert not FQDN("", min_labels=1).is_valid
         assert FQDN("label", min_labels=1).is_valid
         assert not FQDN(".label", min_labels=1).is_valid
         assert FQDN("label.babel", min_labels=1).is_valid