Combine uniquness and length checks

tests/utilities/__init__.py

@@ -1,3 +1,26 @@
 """
-This directory contains utitlities that help with testing and debugging
+This sub-package contains utilities that help with testing and debugging.
+
+At the top level are some very basic testing functions, more specific testing
+is provides by modules inside the package.
 """
+
+from typing import Protocol, Iterable
+from collections.abc import Hashable
+
+
+class SizedIterableHashable(Iterable[Hashable], Protocol):
+    """A protocol for sized iterable of hashable objects"""
+
+    def __len__(self) -> int: ...
+
+
+def assert_unique_of_length(data: SizedIterableHashable, length: int) -> None:
+    """Assert that a list (or other iterable) has unique contents of a given length.
+
+    :param data: A list or other sized iterable of hashable objects. To be checked
+    for unique contents and length.
+    :param length: The expected length of data
+    """
+    assert len(data) == len(set(data))
+    assert len(data) == length