openpathsampling · dwhswenson · Oct 31, 2021 · Oct 27, 2021 · Oct 28, 2021 · Oct 31, 2021
diff --git a/paths_cli/tests/test_utils.py b/paths_cli/tests/test_utils.py
@@ -0,0 +1,40 @@
+from paths_cli.utils import *
+
+class TestOrderedSet:
+    def setup(self):
+        self.set = OrderedSet(['a', 'b', 'a', 'c', 'd', 'c', 'd'])
+
+    def test_len(self):
+        assert len(self.set) == 4
+
+    def test_empty(self):
+        ordered = OrderedSet()
+        assert len(ordered) == 0
+        for _ in ordered:
+            raise RuntimeError("This should not happen")
+
+    def test_order(self):
+        for truth, beauty in zip("abcd", self.set):
+            assert truth == beauty
+
+    def test_add_existing(self):
+        assert len(self.set) == 4
+        self.set.add('a')
+        assert len(self.set) == 4
+        assert list(self.set) == ['a', 'b', 'c', 'd']
+
+    def test_contains(self):
+        assert 'a' in self.set
+        assert not 'q' in self.set
+        assert 'q' not in self.set
+
+    def test_discard(self):
+        self.set.discard('a')
+        assert list(self.set) == ['b', 'c', 'd']
+
+    def test_discard_add_order(self):
+        assert list(self.set) == ['a', 'b', 'c', 'd']
+        self.set.discard('a')
+        self.set.add('a')
+        assert list(self.set) == ['b', 'c', 'd', 'a']
+
diff --git a/paths_cli/utils.py b/paths_cli/utils.py
@@ -1,8 +1,49 @@
 import importlib
 import pathlib
+from collections import abc
 import click
 from .plugin_management import FilePluginLoader, NamespacePluginLoader
 
+
+class OrderedSet(abc.MutableSet):
+    """Set-like object with ordered iterator (insertion order).
+
+    This is used to ensure that only one copy of each plugin is loaded
+    (set-like behavior) while retaining the insertion order.
+
+    Parameters
+    ----------
+    iterable : Iterable
+        iterable of objects to initialize with
+    """
+    def __init__(self, iterable=None):
+        self._set = set([])
+        self._list = []
+        if iterable is None:
+            iterable = []
+        for item in iterable:
+            self.add(item)
+
+    def __contains__(self, item):
+        return item in self._set
+
+    def __len__(self):
+        return len(self._set)
+
+    def __iter__(self):
+        return iter(self._list)
+
+    def add(self, item):
+        if item in self._set:
+            return
+        self._set.add(item)
+        self._list.append(item)
+
+    def discard(self, item):
+        self._list.remove(item)
+        self._set.discard(item)
+
+
 def tag_final_result(result, storage, tag='final_conditions'):
     """Save results to a tag in storage.
 
@@ -41,5 +82,5 @@ def get_installed_plugins(default_loader, plugin_types):
         NamespacePluginLoader('paths_cli_plugins', plugin_types)
 
     ]
-    plugins = set(sum([loader() for loader in loaders], []))
+    plugins = OrderedSet(sum([loader() for loader in loaders], []))
     return list(plugins)