PYTHON-3137 Handle falsey values for "let" parameter (mongodb#881)

Author: juliusgeo

pymongo/aggregation.py

@@ -55,7 +55,7 @@ def __init__(
             self._performs_write = True
 
         common.validate_is_mapping("options", options)
-        if let:
+        if let is not None:
             common.validate_is_mapping("let", let)
             options["let"] = let
         if comment is not None:

pymongo/collection.py

@@ -728,7 +728,7 @@ def _update(
                 hint = helpers._index_document(hint)
             update_doc["hint"] = hint
         command = SON([("update", self.name), ("ordered", ordered), ("updates", [update_doc])])
-        if let:
+        if let is not None:
             common.validate_is_mapping("let", let)
             command["let"] = let
         if not write_concern.is_server_default:
@@ -893,7 +893,7 @@ def replace_one(
         """
         common.validate_is_mapping("filter", filter)
         common.validate_ok_for_replace(replacement)
-        if let:
+        if let is not None:
             common.validate_is_mapping("let", let)
         write_concern = self._write_concern_for(session)
         return UpdateResult(
@@ -1189,7 +1189,7 @@ def _delete(
         if not write_concern.is_server_default:
             command["writeConcern"] = write_concern.document
 
-        if let:
+        if let is not None:
             common.validate_is_document_type("let", let)
             command["let"] = let
 
@@ -2728,7 +2728,7 @@ def __find_and_modify(
             )
         collation = validate_collation_or_none(kwargs.pop("collation", None))
         cmd = SON([("findAndModify", self.__name), ("query", filter), ("new", return_document)])
-        if let:
+        if let is not None:
             common.validate_is_mapping("let", let)
             cmd["let"] = let
         cmd.update(kwargs)

pymongo/cursor.py

@@ -248,7 +248,7 @@ def __init__(
         if projection is not None:
             projection = helpers._fields_list_to_dict(projection, "projection")
 
-        if let:
+        if let is not None:
             validate_is_document_type("let", let)
 
         self.__let = let

test/test_collection.py

@@ -2123,7 +2123,7 @@ def test_helpers_with_let(self):
             (c.find_one_and_replace, ({}, {})),
             (c.aggregate, ([], {})),
         ]
-        for let in [10, "str"]:
+        for let in [10, "str", [], False]:
             for helper, args in helpers:
                 with self.assertRaisesRegex(TypeError, "let must be an instance of dict"):
                     helper(*args, let=let)  # type: ignore