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Std dev samp #1

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10 changes: 10 additions & 0 deletions mongomock/aggregate.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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),
Expand Down
30 changes: 25 additions & 5 deletions tests/test__collection_api.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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]}}},
Expand Down Expand Up @@ -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
Expand Down
4 changes: 3 additions & 1 deletion tests/test__mongomock.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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)

Expand Down Expand Up @@ -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']}}}])
Expand Down