Fix float -> Decimal coercion precision loss (#6810)

pydantic · Jul 23, 2023 · 5320764 · 5320764
1 parent ebc6019
commit 5320764
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Showing 3 changed files with 22 additions and 11 deletions.
diff --git a/pydantic/_internal/_std_types_schema.py b/pydantic/_internal/_std_types_schema.py
@@ -180,17 +180,21 @@ def __get_pydantic_json_schema__(self, _schema: CoreSchema, handler: GetJsonSche
     def __get_pydantic_core_schema__(self, _source_type: Any, _handler: GetCoreSchemaHandler) -> CoreSchema:
         Decimal = decimal.Decimal
 
-        def to_decimal(v: Any) -> decimal.Decimal:
+        def to_decimal(v: str) -> decimal.Decimal:
             try:
                 return Decimal(v)
             except decimal.DecimalException as e:
                 raise PydanticCustomError('decimal_parsing', 'Input should be a valid decimal') from e
 
         primitive_schema = core_schema.union_schema(
             [
-                core_schema.float_schema(strict=True),
+                # if it's an int keep it like that and pass it straight to Decimal
+                # but if it's not make it a string
+                # we don't use JSON -> float because parsing to any float will cause
+                # loss of precision
                 core_schema.int_schema(strict=True),
                 core_schema.str_schema(strict=True, strip_whitespace=True),
+                core_schema.no_info_plain_validator_function(str),
             ],
         )
         json_schema = core_schema.no_info_after_validator_function(to_decimal, primitive_schema)

diff --git a/tests/test_edge_cases.py b/tests/test_edge_cases.py
@@ -446,7 +446,7 @@ class Model(BaseModel):
         },
         {
             'type': 'decimal_parsing',
-            'loc': ('v', 2, 'function-after[to_decimal(), union[float,int,constrained-str]]'),
+            'loc': ('v', 2, 'function-after[to_decimal(), union[int,constrained-str,function-plain[str()]]]'),
             'msg': 'Input should be a valid decimal',
             'input': 'x',
         },
@@ -1237,9 +1237,7 @@ class Model(BaseModel):
             'type': 'is_instance_of',
             'loc': (
                 'a',
-                'function-after[check_digits_validator(), json-or-python[json=function-after[to_decimal(), '
-                'union[float,int,constrained-str]],python=lax-or-strict[lax=union[is-instance[Decimal],'
-                'function-after[to_decimal(), union[float,int,constrained-str]]],strict=is-instance[Decimal]]]]',
+                'function-after[check_digits_validator(), json-or-python[json=function-after[to_decimal(), union[int,constrained-str,function-plain[str()]]],python=lax-or-strict[lax=union[is-instance[Decimal],function-after[to_decimal(), union[int,constrained-str,function-plain[str()]]]],strict=is-instance[Decimal]]]]',
                 'is-instance[Decimal]',
             ),
             'msg': 'Input should be an instance of Decimal',
@@ -1250,10 +1248,8 @@ class Model(BaseModel):
             'type': 'decimal_parsing',
             'loc': (
                 'a',
-                'function-after[check_digits_validator(), json-or-python[json=function-after[to_decimal(), '
-                'union[float,int,constrained-str]],python=lax-or-strict[lax=union[is-instance[Decimal],'
-                'function-after[to_decimal(), union[float,int,constrained-str]]],strict=is-instance[Decimal]]]]',
-                'function-after[to_decimal(), union[float,int,constrained-str]]',
+                'function-after[check_digits_validator(), json-or-python[json=function-after[to_decimal(), union[int,constrained-str,function-plain[str()]]],python=lax-or-strict[lax=union[is-instance[Decimal],function-after[to_decimal(), union[int,constrained-str,function-plain[str()]]]],strict=is-instance[Decimal]]]]',
+                'function-after[to_decimal(), union[int,constrained-str,function-plain[str()]]]',
             ),
             'msg': 'Input should be a valid decimal',
             'input': 'foobar',

diff --git a/tests/test_types.py b/tests/test_types.py
@@ -1292,7 +1292,7 @@ def __bool__(self) -> bool:
         ('uuid_check', b'\x12\x34\x56\x78' * 4, UUID('12345678-1234-5678-1234-567812345678')),
         ('uuid_check', 'ebcdab58-6eb8-46fb-a190-', ValidationError),
         ('uuid_check', 123, ValidationError),
-        ('decimal_check', 42.24, Decimal(42.24)),
+        ('decimal_check', 42.24, Decimal('42.24')),
         ('decimal_check', '42.24', Decimal('42.24')),
         ('decimal_check', b'42.24', ValidationError),
         ('decimal_check', '  42.24  ', Decimal('42.24')),
@@ -5615,3 +5615,14 @@ def test_string_constraints() -> None:
         Annotated[str, StringConstraints(strip_whitespace=True, to_lower=True), AfterValidator(lambda x: x * 2)]
     )
     assert ta.validate_python(' ABC ') == 'abcabc'
+
+
+def test_decimal_float_precision() -> None:
+    """https://github.com/pydantic/pydantic/issues/6807"""
+    ta = TypeAdapter(Decimal)
+    assert ta.validate_json('1.1') == Decimal('1.1')
+    assert ta.validate_python(1.1) == Decimal('1.1')
+    assert ta.validate_json('"1.1"') == Decimal('1.1')
+    assert ta.validate_python('1.1') == Decimal('1.1')
+    assert ta.validate_json('1') == Decimal('1')
+    assert ta.validate_python(1) == Decimal('1')