From 2edcecb7f9121dba1d7d680e6a038a0dc6acf7cc Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Mon, 19 Oct 2026 19:46:19 -0500
Subject: [PATCH] gh-159702: Add fast-path for float in _json encoder

Optimize json encoder for floats: up to 1.3x faster.

Avoid creating a temporary Unicode object when writing the JSON
representation of a float: decode directly the ASCII-encoded string
(PyOS_double_to_string()) into the writer.
---
 ...-10-19-19-55-47.gh-issue-159702.TL3O4R.rst |  2 +
 Modules/_json.c                               | 51 +++++++++++++++++--
 2 files changed, 49 insertions(+), 4 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-10-19-19-55-47.gh-issue-159702.TL3O4R.rst

diff --git a/Misc/NEWS.d/next/Library/2026-10-19-19-55-47.gh-issue-159702.TL3O4R.rst b/Misc/NEWS.d/next/Library/2026-10-19-19-55-47.gh-issue-159702.TL3O4R.rst
new file mode 100644
index 000000000000000..41a8fbd9266354f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-19-19-55-47.gh-issue-159702.TL3O4R.rst
@@ -0,0 +1,2 @@
+:mod:`json`: Optimize the JSON encoder for floats to make it up to 1.3x
+faster. Patch by Victor Stinner.
diff --git a/Modules/_json.c b/Modules/_json.c
index 3a724a3e72b185b..54299163e984e37 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -1532,6 +1532,52 @@ encoder_encode_float(PyEncoderObject *s, PyObject *obj)
     return PyFloat_Type.tp_repr(obj);
 }
 
+static int
+encoder_write_float(PyEncoderObject *s, PyUnicodeWriter *writer, PyObject *obj)
+{
+    /* Write the JSON representation of a PyFloat into writer. */
+    double i = PyFloat_AS_DOUBLE(obj);
+    if (!isfinite(i)) {
+        if (!s->allow_nan) {
+            PyErr_Format(
+                    PyExc_ValueError,
+                    "Out of range float values are not JSON compliant: %R",
+                    obj
+                    );
+            return -1;
+        }
+
+        const char *str;
+        size_t len;
+        if (i > 0) {
+            str = "Infinity";
+            len = strlen("Infinity");
+        }
+        else if (i < 0) {
+            str = "-Infinity";
+            len = strlen("-Infinity");
+        }
+        else {
+            str = "NaN";
+            len = strlen("NaN");
+        }
+        return PyUnicodeWriter_WriteASCII(writer, str, len);
+    }
+
+    // Similar to float_repr(), but writes directly into writer
+    char *buf = PyOS_double_to_string(PyFloat_AS_DOUBLE(obj),
+                                      'r', 0,
+                                      Py_DTSF_ADD_DOT_0,
+                                      NULL);
+    if (!buf) {
+        PyErr_NoMemory();
+        return -1;
+    }
+    int res = PyUnicodeWriter_WriteASCII(writer, buf, strlen(buf));
+    PyMem_Free(buf);
+    return res;
+}
+
 static int
 encoder_write_string(PyEncoderObject *s, PyUnicodeWriter *writer, PyObject *obj)
 {
@@ -1596,10 +1642,7 @@ encoder_listencode_obj(PyEncoderObject *s, PyUnicodeWriter *writer,
         return _steal_accumulate(writer, encoded);
     }
     else if (PyFloat_Check(obj)) {
-        PyObject *encoded = encoder_encode_float(s, obj);
-        if (encoded == NULL)
-            return -1;
-        return _steal_accumulate(writer, encoded);
+        return encoder_write_float(s, writer, obj);
     }
     else if (PyList_Check(obj) || PyTuple_Check(obj)) {
         if (_Py_EnterRecursiveCall(" while encoding a JSON object"))