validation: Reject iterators as core arguments

iter() always never re-wraps an iterator, so 'iter(i) is iter(i)' tests if we
wrapped a container or if we already had an iterator. I also tried
types.GeneratorType and inspect helpers but they did not work for this use
case.

mopidy/utils/validation.py

@@ -22,11 +22,13 @@ DISTINCT_FIELDS = {
 
 # TODO: _check_iterable(check, msg, **kwargs) + [check(a) for a in arg]?
 def _check_iterable(arg, msg, **kwargs):
-    """Ensure we have an iterable which is not a string."""
+    """Ensure we have an iterable which is not a string or an iterator"""
     if isinstance(arg, compat.string_types):
         raise exceptions.ValidationError(msg.format(arg=arg, **kwargs))
     elif not isinstance(arg, collections.Iterable):
         raise exceptions.ValidationError(msg.format(arg=arg, **kwargs))
+    elif iter(arg) is iter(arg):
+        raise exceptions.ValidationError(msg.format(arg=arg, **kwargs))
 
 
 def check_choice(arg, choices, msg='Expected one of {choices}, not {arg!r}'):

tests/utils/test_validation.py

@@ -72,6 +72,8 @@ def test_check_instances_with_invalid_values():
         validation.check_instances(None, compat.string_types)
     with raises(exceptions.ValidationError):
         validation.check_instances([None], compat.string_types)
+    with raises(exceptions.ValidationError):
+        validation.check_instances(iter(['abc']), compat.string_types)
 
 
 def test_check_instances_error_message():
@@ -110,7 +112,7 @@ def test_check_field_error_message():
 
 
 def test_check_query_invalid_values():
-    for value in '', None, 'foo', 123, [''], [None]:
+    for value in '', None, 'foo', 123, [''], [None], iter(['abc']):
         with raises(exceptions.ValidationError):
             validation.check_query({'any': value})
 
@@ -155,6 +157,8 @@ def test_check_uris_with_invalid_values():
         validation.check_uris([None])
     with raises(exceptions.ValidationError):
         validation.check_uris(['foobar:', 'foobar'])
+    with raises(exceptions.ValidationError):
+        validation.check_uris(iter(['http://example.com']))
 
 
 def test_check_uris_error_message():