Add a default keyword argument to dumps

dump and dumps functions in python json stdlib have a default keyword argument.
It's useful for serializing complex objects. Supporting this argument will improve compatibility and flexibility of ujson.

python/objToJSON.c

@@ -72,6 +72,12 @@ typedef struct __TypeContext
 
 #define GET_TC(__ptrtc) ((TypeContext *)((__ptrtc)->prv))
 
+// If newObj is set, we should use it rather than JSOBJ
+#define GET_OBJ(__jsobj, __ptrtc) (GET_TC(__ptrtc)->newObj ? GET_TC(__ptrtc)->newObj : __jsobj)
+
+// Avoid infinite loop caused by the default function
+#define DEFAULT_FN_MAX_DEPTH 3
+
 struct PyDictIterState
 {
   PyObject *keys;
@@ -432,7 +438,8 @@ static void SetupDictIter(PyObject *dictObj, TypeContext *pc, JSONObjectEncoder
 
 static void Object_beginTypeContext (JSOBJ _obj, JSONTypeContext *tc, JSONObjectEncoder *enc)
 {
-  PyObject *obj, *objRepr, *exc;
+  PyObject *obj, *objRepr, *exc, *defaultFn, *newObj;
+  int level = 0;
   TypeContext *pc;
   PRINTMARK();
   if (!_obj)
@@ -442,6 +449,7 @@ static void Object_beginTypeContext (JSOBJ _obj, JSONTypeContext *tc, JSONObject
   }
 
   obj = (PyObject*) _obj;
+  defaultFn = (PyObject*) enc->prv;
 
   tc->prv = PyObject_Malloc(sizeof(TypeContext));
   pc = (TypeContext *) tc->prv;
@@ -462,6 +470,7 @@ static void Object_beginTypeContext (JSOBJ _obj, JSONTypeContext *tc, JSONObject
   pc->longValue = 0;
   pc->rawJSONValue = NULL;
 
+BEGIN:
   if (PyIter_Check(obj))
   {
     PRINTMARK();
@@ -553,7 +562,6 @@ static void Object_beginTypeContext (JSOBJ _obj, JSONTypeContext *tc, JSONObject
     return;
   }
 
-
 ISITERABLE:
   if (PyDict_Check(obj))
   {
@@ -651,6 +659,31 @@ static void Object_beginTypeContext (JSOBJ _obj, JSONTypeContext *tc, JSONObject
     return;
   }
 
+DEFAULT:
+  if (defaultFn)
+  {
+    // Break infinite loop
+    if (level >= DEFAULT_FN_MAX_DEPTH)
+    {
+      PRINTMARK();
+      PyErr_Format(PyExc_TypeError, "maximum recursion depth exceeded");
+      goto INVALID;
+    }
+
+    newObj = PyObject_CallFunctionObjArgs(defaultFn, obj, NULL);
+    if (newObj)
+    {
+      PRINTMARK();
+      obj = pc->newObj = newObj;
+      level += 1;
+      goto BEGIN;
+    }
+    else
+    {
+      goto INVALID;
+    }
+  }
+
   PRINTMARK();
   PyErr_Clear();
 
@@ -682,33 +715,38 @@ static void Object_endTypeContext(JSOBJ obj, JSONTypeContext *tc)
 
 static const char *Object_getStringValue(JSOBJ obj, JSONTypeContext *tc, size_t *_outLen)
 {
+  obj = GET_OBJ(obj, tc);
   return GET_TC(tc)->PyTypeToJSON (obj, tc, NULL, _outLen);
 }
 
 static JSINT64 Object_getLongValue(JSOBJ obj, JSONTypeContext *tc)
 {
   JSINT64 ret;
+  obj = GET_OBJ(obj, tc);
   GET_TC(tc)->PyTypeToJSON (obj, tc, &ret, NULL);
   return ret;
 }
 
 static JSUINT64 Object_getUnsignedLongValue(JSOBJ obj, JSONTypeContext *tc)
 {
   JSUINT64 ret;
+  obj = GET_OBJ(obj, tc);
   GET_TC(tc)->PyTypeToJSON (obj, tc, &ret, NULL);
   return ret;
 }
 
 static JSINT32 Object_getIntValue(JSOBJ obj, JSONTypeContext *tc)
 {
   JSINT32 ret;
+  obj = GET_OBJ(obj, tc);
   GET_TC(tc)->PyTypeToJSON (obj, tc, &ret, NULL);
   return ret;
 }
 
 static double Object_getDoubleValue(JSOBJ obj, JSONTypeContext *tc)
 {
   double ret;
+  obj = GET_OBJ(obj, tc);
   GET_TC(tc)->PyTypeToJSON (obj, tc, &ret, NULL);
   return ret;
 }
@@ -720,27 +758,31 @@ static void Object_releaseObject(JSOBJ _obj)
 
 static int Object_iterNext(JSOBJ obj, JSONTypeContext *tc)
 {
+  obj = GET_OBJ(obj, tc);
   return GET_TC(tc)->iterNext(obj, tc);
 }
 
 static void Object_iterEnd(JSOBJ obj, JSONTypeContext *tc)
 {
+  obj = GET_OBJ(obj, tc);
   GET_TC(tc)->iterEnd(obj, tc);
 }
 
 static JSOBJ Object_iterGetValue(JSOBJ obj, JSONTypeContext *tc)
 {
+  obj = GET_OBJ(obj, tc);
   return GET_TC(tc)->iterGetValue(obj, tc);
 }
 
 static char *Object_iterGetName(JSOBJ obj, JSONTypeContext *tc, size_t *outLen)
 {
+  obj = GET_OBJ(obj, tc);
   return GET_TC(tc)->iterGetName(obj, tc, outLen);
 }
 
 PyObject* objToJSON(PyObject* self, PyObject *args, PyObject *kwargs)
 {
-  static char *kwlist[] = { "obj", "ensure_ascii", "encode_html_chars", "escape_forward_slashes", "sort_keys", "indent", "allow_nan", "reject_bytes", NULL };
+  static char *kwlist[] = { "obj", "ensure_ascii", "encode_html_chars", "escape_forward_slashes", "sort_keys", "indent", "allow_nan", "reject_bytes", "default", NULL };
 
   char buffer[65536];
   char *ret;
@@ -751,6 +793,7 @@ PyObject* objToJSON(PyObject* self, PyObject *args, PyObject *kwargs)
   PyObject *oencodeHTMLChars = NULL;
   PyObject *oescapeForwardSlashes = NULL;
   PyObject *osortKeys = NULL;
+  PyObject *odefaultFn = NULL;
   int allowNan = -1;
   int orejectBytes = -1;
 
@@ -785,7 +828,7 @@ PyObject* objToJSON(PyObject* self, PyObject *args, PyObject *kwargs)
 
   PRINTMARK();
 
-  if (!PyArg_ParseTupleAndKeywords(args, kwargs, "O|OOOOiii", kwlist, &oinput, &oensureAscii, &oencodeHTMLChars, &oescapeForwardSlashes, &osortKeys, &encoder.indent, &allowNan, &orejectBytes))
+  if (!PyArg_ParseTupleAndKeywords(args, kwargs, "O|OOOOiiiO", kwlist, &oinput, &oensureAscii, &oencodeHTMLChars, &oescapeForwardSlashes, &osortKeys, &encoder.indent, &allowNan, &orejectBytes, &odefaultFn))
   {
     return NULL;
   }
@@ -815,6 +858,12 @@ PyObject* objToJSON(PyObject* self, PyObject *args, PyObject *kwargs)
     encoder.allowNan = allowNan;
   }
 
+  if (odefaultFn != NULL && odefaultFn != Py_None)
+  {
+    // Here use prv to store default function
+    encoder.prv = odefaultFn;
+  }
+
   if (encoder.allowNan)
   {
     csInf = "Inf";

tests/test_ujson.py

@@ -1,9 +1,11 @@
+import datetime
 import decimal
 import io
 import json
 import math
 import re
 import sys
+import uuid
 from collections import OrderedDict
 
 import pytest
@@ -828,6 +830,39 @@ def test_encode_none_key():
     assert ujson.dumps(data) == '{"null":null}'
 
 
+def test_default_function():
+    iso8601_time_format = "%Y-%m-%dT%H:%M:%S.%f"
+
+    class CustomObject:
+        pass
+
+    class UnjsonableObject:
+        pass
+
+    def default(value):
+        if isinstance(value, datetime.datetime):
+            return value.strftime(iso8601_time_format)
+        elif isinstance(value, uuid.UUID):
+            return value.hex
+        elif isinstance(value, CustomObject):
+            raise ValueError("invalid value")
+        return value
+
+    dt = datetime.datetime.now()
+    assert ujson.dumps(dt, default=default) == '"%s"' % dt.strftime(iso8601_time_format)
+
+    uuid4 = uuid.uuid4()
+    assert ujson.dumps(uuid4, default=default) == '"%s"' % uuid4.hex
+
+    custom_obj = CustomObject()
+    with pytest.raises(ValueError, match="invalid value"):
+        ujson.dumps(custom_obj, default=default)
+
+    unjsonable_obj = UnjsonableObject()
+    with pytest.raises(TypeError, match="maximum recursion depth exceeded"):
+        ujson.dumps(unjsonable_obj, default=default)
+
+
 """
 def test_decode_numeric_int_frc_overflow():
 input = "X.Y"