BUG: Preserve full float precision when double_precision=15 in to_json; avoid truncation before serialization (#62072)

Author: Tuhin-SnapD

pandas/io/json/_json.py

@@ -92,6 +92,44 @@
 FrameSeriesStrT = TypeVar("FrameSeriesStrT", bound=Literal["frame", "series"])
 
 
+# Helpers to stringify floats at full precision when requested
+def _format_float_full_precision(value: Any) -> Any:
+    """Return a string with full precision for finite floats; otherwise original.
+
+    Uses 17 significant digits to preserve IEEE-754 double precision.
+    """
+    try:
+        # Fast path for Python float and NumPy floating scalars
+        if isinstance(value, (float, np.floating)):
+            x = float(value)
+            if np.isfinite(x):
+                return format(x, ".17g")
+            return value
+    except Exception:
+        # On any unexpected error, fall back to original value
+        return value
+    return value
+
+
+def _stringify_floats_full_precision(obj: Any) -> Any:
+    """Recursively convert finite floats to full-precision strings.
+
+    Leaves non-finite floats (NaN, +/-Inf) and non-float types unchanged.
+    Works for pandas Series/DataFrame, Python dict/list, and NumPy arrays.
+    """
+    if isinstance(obj, Series):
+        return obj.map(_format_float_full_precision)
+    if isinstance(obj, DataFrame):
+        return obj.applymap(_format_float_full_precision)
+    if isinstance(obj, dict):
+        return {k: _stringify_floats_full_precision(v) for k, v in obj.items()}
+    if isinstance(obj, list):
+        return [_stringify_floats_full_precision(v) for v in obj]
+    if isinstance(obj, np.ndarray):
+        return [_stringify_floats_full_precision(v) for v in obj.tolist()]
+    # Scalars
+    return _format_float_full_precision(obj)
+
 # interface to/from
 @overload
 def to_json(
@@ -252,8 +290,14 @@ def _format_axes(self) -> None:
 
     def write(self) -> str:
         iso_dates = self.date_format == "iso"
+        payload = self.obj_to_write
+        # When maximum precision is requested, avoid truncation by stringifying
+        # finite float values at full IEEE-754 precision. Skip for orient='table'
+        # to preserve Table Schema type conformance.
+        if self.double_precision == 15 and not isinstance(self, JSONTableWriter):
+            payload = _stringify_floats_full_precision(payload)
         return ujson_dumps(
-            self.obj_to_write,
+            payload,
             orient=self.orient,
             double_precision=self.double_precision,
             ensure_ascii=self.ensure_ascii,