feat(config): add a list Property for validating input lists

This property can validate the type and size (exact and maximum) of an
incoming list.

caput/config.py

@@ -328,6 +328,75 @@ def _prop(val):
     return prop
 
 
+def list_type(type_=None, length=None, maxlength=None, default=None):
+    """A property type that validates lists against required properties.
+
+    Parameters
+    ----------
+    type_ : type, optional
+        Type to apply. If `None` does not attempt to validate elements against type.
+    length : int, optional
+        Exact length of the list we expect. If `None` (default) accept any length.
+    maxlength : int, optional
+        Maximum length of the list. If `None` (default) there is no maximum length.
+    default : optional
+        The optional default value.
+
+    Returns
+    -------
+    prop : Property
+        A property instance setup to validate the type.
+
+    Examples
+    --------
+    Should be used like::
+
+        class Project(object):
+
+            mode = list_type(int, length=2, default=[3, 4])
+    """
+
+    def _prop(val):
+
+        if not isinstance(val, (list, tuple)):
+            raise ValueError("Expected to receive a list, but got '%s.'" % repr(val))
+
+        if type_:
+            for ii, item in enumerate(val):
+                if not isinstance(item, type_):
+                    raise ValueError(
+                        "Expected to receive a list with items of type %s, but got "
+                        "'%s.' at position %i" % (type_, val, ii)
+                    )
+
+        if length and len(val) != length:
+            raise ValueError(
+                "List expected to be of length %i, but was actually length %i"
+                % (length, len(val))
+            )
+
+        if maxlength and len(val) > maxlength:
+            raise ValueError(
+                "Maximum length of list is %i is, but list was actually length %i"
+                % (maxlength, len(val))
+            )
+
+        return val
+
+    if default:
+        try:
+            _prop(default)
+        except ValueError as e:
+            raise ValueError(
+                "Default value %s does not satisfy property requirements: %s"
+                % (default, repr(e))
+            )
+
+    prop = Property(proptype=_prop, default=default)
+
+    return prop
+
+
 if __name__ == "__main__":
     import doctest
 

caput/tests/test_config.py

@@ -22,6 +22,12 @@ class PersonWithPet(Person):
     petage = 36
 
 
+class ListTypeTests(Person):
+    list_max_length = config.list_type(maxlength=2)
+    list_exact_length = config.list_type(length=2)
+    list_type = config.list_type(type_=int)
+
+
 class TestConfig(unittest.TestCase):
 
     testdict = {"name": "Richard", "ageinyears": 40, "petname": "Sooty"}
@@ -67,3 +73,28 @@ def test_pickle(self):
         self.assertEqual(person2.name, "Richard")
         self.assertEqual(person2.age, 40.0)
         self.assertEqual(person2.petname, "Sooty")
+
+    def test_list_type(self):
+
+        lt = ListTypeTests()
+
+        with self.assertRaises(ValueError):
+            lt.read_config({"list_max_length": [1, 3, 4]})
+
+        # Should work fine
+        lt = ListTypeTests()
+        lt.read_config({"list_max_length": [1, 2]})
+
+        with self.assertRaises(ValueError):
+            lt.read_config({"list_exact_length": [3]})
+
+        # Work should fine
+        lt = ListTypeTests()
+        lt.read_config({"list_exact_length": [1, 2]})
+
+        with self.assertRaises(ValueError):
+            lt.read_config({"list_type": ["hello"]})
+
+        # Work should fine
+        lt = ListTypeTests()
+        lt.read_config({"list_type": [1, 2]})