From b577f00c3ef7c58bf31d5c0ad3f08ca8daf4b51e Mon Sep 17 00:00:00 2001
From: Itamar Shefi <itamarshefi@gmail.com>
Date: Wed, 21 Feb 2024 16:35:21 +0200
Subject: [PATCH] Add support to $stdDevPop operator

---
 mongomock/aggregate.py        | 10 ++++++++++
 tests/test__collection_api.py | 30 +++++++++++++++++++++++++-----
 tests/test__mongomock.py      |  4 +++-
 3 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/mongomock/aggregate.py b/mongomock/aggregate.py
index d262cd91d..a6c42e726 100644
--- a/mongomock/aggregate.py
+++ b/mongomock/aggregate.py
@@ -235,9 +235,19 @@ def _merge_objects_operation(values):
     return merged_doc
 
 
+def _std_dev_pop_operation(values):
+    values_list = list(v for v in values if isinstance(v, numbers.Number))
+    if not values_list:
+        return None
+    mean = sum(values_list) / float(len(values_list))
+    variance = sum((x - mean) ** 2 for x in values_list) / float(len(values_list))
+    return math.sqrt(variance)
+
+
 _GROUPING_OPERATOR_MAP = {
     '$sum': _sum_operation,
     '$avg': _avg_operation,
+    "$stdDevPop": _std_dev_pop_operation,
     '$mergeObjects': _merge_objects_operation,
     '$min': lambda values: _group_operation(values, min),
     '$max': lambda values: _group_operation(values, max),
diff --git a/tests/test__collection_api.py b/tests/test__collection_api.py
index 8eb90a970..46617c3db 100644
--- a/tests/test__collection_api.py
+++ b/tests/test__collection_api.py
@@ -4586,11 +4586,6 @@ def test__aggregate_unrecognized(self):
     def test__aggregate_not_implemented(self):
         self.db.collection.insert_one({})
 
-        with self.assertRaises(NotImplementedError):
-            self.db.collection.aggregate([
-                {'$project': {'a': {'$stdDevPop': 'scores'}}},
-            ])
-
         with self.assertRaises(NotImplementedError):
             self.db.collection.aggregate([
                 {'$project': {'a': {'$cmp': [1, 2]}}},
@@ -7350,6 +7345,31 @@ def test_aggregate_type(self):
         }])
 
         self.assertListEqual(expected, list(actual))
+    def test_std_dev_pop(self):
+        collection = self.db.collection
+        collection.insert_many([
+            {'_id': 1, 'grades': [80, 85, 90, 90, 90]},
+            {'_id': 2, 'grades': [96, 100, 100, 100, 100]},
+            {'_id': 3, 'grades': [90, 90, 90, 90, 90]},
+        ])
+
+        actual = collection.aggregate([
+            {
+                '$project': {
+                    'stdDevPop': {'$stdDevPop': '$grades'},
+                    '_id': 0
+                }
+            }
+        ])
+
+        expect = [
+            {'stdDevPop': 4.0},
+            {'stdDevPop': 1.6},
+            {'stdDevPop': 0.0},
+        ]
+
+        for i, item in enumerate(actual):
+            self.assertAlmostEqual(expect[i]['stdDevPop'], item['stdDevPop'], places=5)
 
     def test_aggregate_project_with_boolean(self):
         collection = self.db.collection
diff --git a/tests/test__mongomock.py b/tests/test__mongomock.py
index fc616a5ea..7407e315d 100644
--- a/tests/test__mongomock.py
+++ b/tests/test__mongomock.py
@@ -2654,7 +2654,8 @@ def test__aggregate20(self):
             'pow': {'$pow': [4, 2]},
             'sqrt': {'$sqrt': 100},
             'trunc': {'$trunc': 8.35},
-            'round': {'$round': 9.51}
+            'round': {'$round': 9.51},
+            'stdDevPop': {'$stdDevPop': '$a'},
         }}]
         self.cmp.compare.aggregate(pipeline)
 
@@ -3861,6 +3862,7 @@ def test__aggregate_exception(self):
         self.cmp.compare_exceptions.aggregate([{'$project': {'c': {'$sum': []}}}])
         self.cmp.compare_exceptions.aggregate([{'$project': {'c': {'$multiply': []}}}])
         self.cmp.compare_exceptions.aggregate([{'$project': {'c': {'$round': "12"}}}])
+        self.cmp.compare_exceptions.aggregate([{'$project': {'c': {'$stdDevPop': [12, 2, 3]}}}])
         self.cmp.compare_exceptions.aggregate([{'$project': {'n': {'$add': '$a'}}}])
         self.cmp.compare_exceptions.aggregate(
             [{'$project': {'q': {'$multiply': [1, '$non_existent_key']}}}])