PYTHON-5508 - Add built-in DecimalEncoder and DecimalDecoder (#2499)

Author: NoahStapp

bson/decimal128.py

@@ -20,8 +20,11 @@
 
 import decimal
 import struct
+from decimal import Decimal
 from typing import Any, Sequence, Tuple, Type, Union
 
+from bson.codec_options import TypeDecoder, TypeEncoder
+
 _PACK_64 = struct.Struct("<Q").pack
 _UNPACK_64 = struct.Struct("<Q").unpack
 
@@ -58,6 +61,42 @@
 _VALUE_OPTIONS = Union[decimal.Decimal, float, str, Tuple[int, Sequence[int], int]]
 
 
+class DecimalEncoder(TypeEncoder):
+    """Converts Python :class:`decimal.Decimal` to BSON :class:`Decimal128`.
+
+    For example::
+        opts = CodecOptions(type_registry=TypeRegistry([DecimalEncoder()]))
+        bson.encode({"d": decimal.Decimal('1.0')}, codec_options=opts)
+
+    .. versionadded:: 4.15
+    """
+
+    @property
+    def python_type(self) -> Type[Decimal]:
+        return Decimal
+
+    def transform_python(self, value: Any) -> Decimal128:
+        return Decimal128(value)
+
+
+class DecimalDecoder(TypeDecoder):
+    """Converts BSON :class:`Decimal128` to Python :class:`decimal.Decimal`.
+
+    For example::
+        opts = CodecOptions(type_registry=TypeRegistry([DecimalDecoder()]))
+        bson.decode(data, codec_options=opts)
+
+    .. versionadded:: 4.15
+    """
+
+    @property
+    def bson_type(self) -> Type[Decimal128]:
+        return Decimal128
+
+    def transform_bson(self, value: Any) -> decimal.Decimal:
+        return value.to_decimal()
+
+
 def create_decimal128_context() -> decimal.Context:
     """Returns an instance of :class:`decimal.Context` appropriate
     for working with IEEE-754 128-bit decimal floating point values.

doc/changelog.rst

@@ -1,5 +1,12 @@
 Changelog
 =========
+Changes in Version 4.15.0 (XXXX/XX/XX)
+--------------------------------------
+PyMongo 4.15 brings a number of changes including:
+
+- Added :class:`bson.decimal128.DecimalEncoder` and :class:`bson.decimal128.DecimalDecoder`
+  to support encoding and decoding of BSON Decimal128 values to decimal.Decimal values using the TypeRegistry API.
+
 Changes in Version 4.14.1 (2025/08/19)
 --------------------------------------
 

test/asynchronous/test_custom_types.py

@@ -23,6 +23,7 @@
 from random import random
 from typing import Any, Tuple, Type, no_type_check
 
+from bson.decimal128 import DecimalDecoder, DecimalEncoder
 from gridfs.asynchronous.grid_file import AsyncGridIn, AsyncGridOut
 
 sys.path[0:0] = [""]
@@ -59,29 +60,7 @@
 _IS_SYNC = False
 
 
-class DecimalEncoder(TypeEncoder):
-    @property
-    def python_type(self):
-        return Decimal
-
-    def transform_python(self, value):
-        return Decimal128(value)
-
-
-class DecimalDecoder(TypeDecoder):
-    @property
-    def bson_type(self):
-        return Decimal128
-
-    def transform_bson(self, value):
-        return value.to_decimal()
-
-
-class DecimalCodec(DecimalDecoder, DecimalEncoder):
-    pass
-
-
-DECIMAL_CODECOPTS = CodecOptions(type_registry=TypeRegistry([DecimalCodec()]))
+DECIMAL_CODECOPTS = CodecOptions(type_registry=TypeRegistry([DecimalEncoder(), DecimalDecoder()]))
 
 
 class UndecipherableInt64Type:

test/test_custom_types.py

@@ -23,6 +23,7 @@
 from random import random
 from typing import Any, Tuple, Type, no_type_check
 
+from bson.decimal128 import DecimalDecoder, DecimalEncoder
 from gridfs.synchronous.grid_file import GridIn, GridOut
 
 sys.path[0:0] = [""]
@@ -59,29 +60,7 @@
 _IS_SYNC = True
 
 
-class DecimalEncoder(TypeEncoder):
-    @property
-    def python_type(self):
-        return Decimal
-
-    def transform_python(self, value):
-        return Decimal128(value)
-
-
-class DecimalDecoder(TypeDecoder):
-    @property
-    def bson_type(self):
-        return Decimal128
-
-    def transform_bson(self, value):
-        return value.to_decimal()
-
-
-class DecimalCodec(DecimalDecoder, DecimalEncoder):
-    pass
-
-
-DECIMAL_CODECOPTS = CodecOptions(type_registry=TypeRegistry([DecimalCodec()]))
+DECIMAL_CODECOPTS = CodecOptions(type_registry=TypeRegistry([DecimalEncoder(), DecimalDecoder()]))
 
 
 class UndecipherableInt64Type: