ENH: Format decimal.Decimal as full precision strings in `.to_json(…

…...)` (#60698)

* Format decimal.Decimal as full precision strings in .to_json(...)

* Fix failing tests

* Clean up Decimal to utf8 convertion and switch to using PyObject_Format() to suppress scientific notation

* Add whatsnew entry

doc/source/whatsnew/v3.0.0.rst

@@ -53,6 +53,7 @@ Other enhancements
 - :meth:`DataFrame.ewm` now allows ``adjust=False`` when ``times`` is provided (:issue:`54328`)
 - :meth:`DataFrame.fillna` and :meth:`Series.fillna` can now accept ``value=None``; for non-object dtype the corresponding NA value will be used (:issue:`57723`)
 - :meth:`DataFrame.pivot_table` and :func:`pivot_table` now allow the passing of keyword arguments to ``aggfunc`` through ``**kwargs`` (:issue:`57884`)
+- :meth:`DataFrame.to_json` now encodes ``Decimal`` as strings instead of floats (:issue:`60698`)
 - :meth:`Series.cummin` and :meth:`Series.cummax` now supports :class:`CategoricalDtype` (:issue:`52335`)
 - :meth:`Series.plot` now correctly handle the ``ylabel`` parameter for pie charts, allowing for explicit control over the y-axis label (:issue:`58239`)
 - :meth:`DataFrame.plot.scatter` argument ``c`` now accepts a column of strings, where rows with the same string are colored identically (:issue:`16827` and :issue:`16485`)

pandas/_libs/src/vendored/ujson/python/objToJSON.c

@@ -373,6 +373,27 @@ static char *PyTimeToJSON(JSOBJ _obj, JSONTypeContext *tc, size_t *outLen) {
   return outValue;
 }
 
+static char *PyDecimalToUTF8Callback(JSOBJ _obj, JSONTypeContext *tc,
+                                     size_t *len) {
+  PyObject *obj = (PyObject *)_obj;
+  PyObject *format_spec = PyUnicode_FromStringAndSize("f", 1);
+  PyObject *str = PyObject_Format(obj, format_spec);
+  Py_DECREF(format_spec);
+
+  if (str == NULL) {
+    ((JSONObjectEncoder *)tc->encoder)->errorMsg = "";
+    return NULL;
+  }
+
+  GET_TC(tc)->newObj = str;
+
+  Py_ssize_t s_len;
+  char *outValue = (char *)PyUnicode_AsUTF8AndSize(str, &s_len);
+  *len = s_len;
+
+  return outValue;
+}
+
 //=============================================================================
 // Numpy array iteration functions
 //=============================================================================
@@ -1467,8 +1488,18 @@ static void Object_beginTypeContext(JSOBJ _obj, JSONTypeContext *tc) {
     tc->type = JT_UTF8;
     return;
   } else if (object_is_decimal_type(obj)) {
-    pc->doubleValue = PyFloat_AsDouble(obj);
-    tc->type = JT_DOUBLE;
+    PyObject *is_nan_py = PyObject_RichCompare(obj, obj, Py_NE);
+    if (is_nan_py == NULL) {
+      goto INVALID;
+    }
+    int is_nan = (is_nan_py == Py_True);
+    Py_DECREF(is_nan_py);
+    if (is_nan) {
+      tc->type = JT_NULL;
+      return;
+    }
+    pc->PyTypeToUTF8 = PyDecimalToUTF8Callback;
+    tc->type = JT_UTF8;
     return;
   } else if (PyDateTime_Check(obj) || PyDate_Check(obj)) {
     if (object_is_nat_type(obj)) {

pandas/tests/io/json/test_json_table_schema_ext_dtype.py

@@ -159,7 +159,7 @@ def test_build_decimal_series(self, dc):
         expected = OrderedDict(
             [
                 ("schema", schema),
-                ("data", [OrderedDict([("id", 0), ("a", 10.0)])]),
+                ("data", [OrderedDict([("id", 0), ("a", "10")])]),
             ]
         )
 
@@ -245,7 +245,7 @@ def test_to_json(self, da, dc, sa, ia):
                 [
                     ("idx", 0),
                     ("A", "2021-10-10T00:00:00.000"),
-                    ("B", 10.0),
+                    ("B", "10"),
                     ("C", "pandas"),
                     ("D", 10),
                 ]

pandas/tests/io/json/test_pandas.py

@@ -1,6 +1,5 @@
 import datetime
 from datetime import timedelta
-from decimal import Decimal
 from io import StringIO
 import json
 import os
@@ -2025,12 +2024,8 @@ def test_to_s3(self, s3_public_bucket, s3so):
             timeout -= 0.1
             assert timeout > 0, "Timed out waiting for file to appear on moto"
 
-    def test_json_pandas_nulls(self, nulls_fixture, request):
+    def test_json_pandas_nulls(self, nulls_fixture):
         # GH 31615
-        if isinstance(nulls_fixture, Decimal):
-            mark = pytest.mark.xfail(reason="not implemented")
-            request.applymarker(mark)
-
         expected_warning = None
         msg = (
             "The default 'epoch' date format is deprecated and will be removed "

pandas/tests/io/json/test_ujson.py

@@ -57,56 +57,56 @@ def test_encode_decimal(self):
         sut = decimal.Decimal("1337.1337")
         encoded = ujson.ujson_dumps(sut, double_precision=15)
         decoded = ujson.ujson_loads(encoded)
-        assert decoded == 1337.1337
+        assert decoded == "1337.1337"
 
         sut = decimal.Decimal("0.95")
         encoded = ujson.ujson_dumps(sut, double_precision=1)
-        assert encoded == "1.0"
+        assert encoded == '"0.95"'
 
         decoded = ujson.ujson_loads(encoded)
-        assert decoded == 1.0
+        assert decoded == "0.95"
 
         sut = decimal.Decimal("0.94")
         encoded = ujson.ujson_dumps(sut, double_precision=1)
-        assert encoded == "0.9"
+        assert encoded == '"0.94"'
 
         decoded = ujson.ujson_loads(encoded)
-        assert decoded == 0.9
+        assert decoded == "0.94"
 
         sut = decimal.Decimal("1.95")
         encoded = ujson.ujson_dumps(sut, double_precision=1)
-        assert encoded == "2.0"
+        assert encoded == '"1.95"'
 
         decoded = ujson.ujson_loads(encoded)
-        assert decoded == 2.0
+        assert decoded == "1.95"
 
         sut = decimal.Decimal("-1.95")
         encoded = ujson.ujson_dumps(sut, double_precision=1)
-        assert encoded == "-2.0"
+        assert encoded == '"-1.95"'
 
         decoded = ujson.ujson_loads(encoded)
-        assert decoded == -2.0
+        assert decoded == "-1.95"
 
         sut = decimal.Decimal("0.995")
         encoded = ujson.ujson_dumps(sut, double_precision=2)
-        assert encoded == "1.0"
+        assert encoded == '"0.995"'
 
         decoded = ujson.ujson_loads(encoded)
-        assert decoded == 1.0
+        assert decoded == "0.995"
 
         sut = decimal.Decimal("0.9995")
         encoded = ujson.ujson_dumps(sut, double_precision=3)
-        assert encoded == "1.0"
+        assert encoded == '"0.9995"'
 
         decoded = ujson.ujson_loads(encoded)
-        assert decoded == 1.0
+        assert decoded == "0.9995"
 
         sut = decimal.Decimal("0.99999999999999944")
         encoded = ujson.ujson_dumps(sut, double_precision=15)
-        assert encoded == "1.0"
+        assert encoded == '"0.99999999999999944"'
 
         decoded = ujson.ujson_loads(encoded)
-        assert decoded == 1.0
+        assert decoded == "0.99999999999999944"
 
     @pytest.mark.parametrize("ensure_ascii", [True, False])
     def test_encode_string_conversion(self, ensure_ascii):