Additional security hardening improvements:

* Remove unused namedtuple_as_object and tuple_as_array arguments from
  simplejson.load (SJ-PT-23-102)
* Remove vestigial _one_shot code from iterencode (SJ-PT-23-103)
* Change default of allow_nan from True to False and add allow_nan
  to decoder (SJ-PT-23-107)

8 files changed, 152 insertions, 121 deletions

diff --git a/CHANGES.txt b/CHANGES.txt
index 4b600ef..2ceab57 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -9,6 +9,11 @@ Version 3.19.0 released 2023-04-XX
 * Fix inconsistencies with error messages between the C and Python
   implementations (SJ-PT-23-100)
 * Remove unused unichr import from encoder (SJ-PT-23-101)
+* Remove unused namedtuple_as_object and tuple_as_array arguments from
+  simplejson.load (SJ-PT-23-102)
+* Remove vestigial _one_shot code from iterencode (SJ-PT-23-103)
+* Change default of allow_nan from True to False and add allow_nan
+  to decoder (SJ-PT-23-107)
 
 Version 3.18.4 released 2023-03-14
 
diff --git a/index.rst b/index.rst
index a10ccb7..f57d650 100644
--- a/index.rst
+++ b/index.rst
@@ -160,7 +160,7 @@ Basic Usage
 -----------
 
 .. function:: dump(obj, fp, skipkeys=False, ensure_ascii=True, \
-                   check_circular=True, allow_nan=True, cls=None, \
+                   check_circular=True, allow_nan=False, cls=None, \
                    indent=None, separators=None, encoding='utf-8', \
                    default=None, use_decimal=True, \
                    namedtuple_as_object=True, tuple_as_array=True, \
@@ -191,7 +191,7 @@ Basic Usage
         is highly optimized.
 
 .. function:: dumps(obj, skipkeys=False, ensure_ascii=True, \
-                    check_circular=True, allow_nan=True, cls=None, \
+                    check_circular=True, allow_nan=False, cls=None, \
                     indent=None, separators=None, encoding='utf-8', \
                     default=None, use_decimal=True, \
                     namedtuple_as_object=True, tuple_as_array=True, \
@@ -225,13 +225,17 @@ Basic Usage
     reference check for container types will be skipped and a circular
     reference will result in an :exc:`OverflowError` (or worse).
 
-    If *allow_nan* is false (default: ``True``), then it will be a
+    If *allow_nan* is false (default: ``False``), then it will be a
     :exc:`ValueError` to serialize out of range :class:`float` values
     (``nan``, ``inf``, ``-inf``) in strict compliance of the original
     JSON specification. If *allow_nan* is true, their JavaScript equivalents
     will be used (``NaN``, ``Infinity``, ``-Infinity``). See also *ignore_nan*
     for ECMA-262 compliant behavior.
 
+    .. versionchanged:: 3.19.0
+        The default for *allow_nan* was changed to False for better spec
+        compliance.
+
     If *indent* is a string, then JSON array elements and object members
     will be pretty-printed with a newline followed by that string repeated
     for each level of nesting. ``None`` (the default) selects the most compact
@@ -324,7 +328,7 @@ Basic Usage
 .. function:: load(fp, encoding='utf-8', cls=None, object_hook=None, \
                    parse_float=None, parse_int=None, \
                    parse_constant=None, object_pairs_hook=None, \
-                   use_decimal=None, **kw)
+                   use_decimal=None, allow_nan=False, **kw)
 
    Deserialize *fp* (a ``.read()``-supporting file-like object containing a JSON
    document) to a Python object using this
@@ -367,7 +371,7 @@ Basic Usage
 .. function:: loads(s, encoding='utf-8', cls=None, object_hook=None, \
                     parse_float=None, parse_int=None, \
                     parse_constant=None, object_pairs_hook=None, \
-                    use_decimal=None, **kw)
+                    use_decimal=None, allow_nan=False, **kw)
 
     Deserialize *s* (a :class:`str` or :class:`unicode` instance containing a JSON
     document) to a Python object. :exc:`JSONDecodeError` will be
@@ -419,10 +423,6 @@ Basic Usage
         suitable alternative parser is specified
         (e.g. :class:`decimal.Decimal`)
 
-    *parse_constant*, if specified, will be called with one of the following
-    strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``.  This can be used to
-    raise an exception if invalid JSON numbers are encountered.
-
     If *use_decimal* is true (default: ``False``) then *parse_float* is set to
     :class:`decimal.Decimal`. This is a convenience for parity with the
     :func:`dump` parameter.
@@ -443,12 +443,28 @@ Basic Usage
         Subclassing is not recommended. You should use *object_hook* or
         *object_pairs_hook*. This is faster and more portable than subclassing.
 
+
+    *allow_nan*, if True (default false), will allow the parser to
+    accept the non-standard floats
+    ``NaN``, ``Infinity``, and ``-Infinity``.
+
+    .. versionchanged:: 3.19.0
+
+        This argument was added to make it possible to use the legacy behavior
+        now that the parser is more strict about compliance to the standard.
+
+    *parse_constant*, if specified, will be
+    called with one of the following strings: ``'-Infinity'``,
+    ``'Infinity'``, ``'NaN'``. It is not recommended to use this feature,
+    as it is rare to parse non-compliant JSON containing these values.
+
+
 Encoders and decoders
 ---------------------
 
 .. class:: JSONDecoder(encoding='utf-8', object_hook=None, parse_float=None, \
                        parse_int=None, parse_constant=None, \
-                       object_pairs_hook=None, strict=True)
+                       object_pairs_hook=None, strict=True, allow_nan=False)
 
    Simple JSON decoder.
 
@@ -476,7 +492,8 @@ Encoders and decoders
    | null          | None      | None      |
    +---------------+-----------+-----------+
 
-   It also understands ``NaN``, ``Infinity``, and ``-Infinity`` as their
+   When *allow_nan* is True, it also understands
+   ``NaN``, ``Infinity``, and ``-Infinity`` as their
    corresponding ``float`` values, which is outside the JSON spec.
 
    *encoding* determines the encoding used to interpret any :class:`str` objects
@@ -516,15 +533,24 @@ Encoders and decoders
         suitable alternative parser is specified
         (e.g. :class:`decimal.Decimal`)
 
-   *parse_constant*, if specified, will be called with one of the following
-   strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``.  This can be used to
-   raise an exception if invalid JSON numbers are encountered.
+    *parse_constant*, if specified, will be
+    called with one of the following strings: ``'-Infinity'``,
+    ``'Infinity'``, ``'NaN'``. It is not recommended to use this feature,
+    as it is rare to parse non-compliant JSON containing these values.
 
    *strict* controls the parser's behavior when it encounters an invalid
    control character in a string. The default setting of ``True`` means that
    unescaped control characters are parse errors, if ``False`` then control
    characters will be allowed in strings.
 
+   *allow_nan* when True (not the default), the decoder will allow
+   ``NaN``, ``Infinity``, and ``-Infinity`` as their corresponding floats.
+
+    .. versionchanged:: 3.19.0
+        This argument was added to make it behave closer to the spec by
+        default. The previous behavior can be restored by setting this to
+        False.
+
    .. method:: decode(s)
 
       Return the Python representation of the JSON document *s*. See
@@ -546,7 +572,7 @@ Encoders and decoders
       document is not valid.
 
 .. class:: JSONEncoder(skipkeys=False, ensure_ascii=True, \
-                       check_circular=True, allow_nan=True, sort_keys=False, \
+                       check_circular=True, allow_nan=False, sort_keys=False, \
                        indent=None, separators=None, encoding='utf-8', \
                        default=None, use_decimal=True, \
                        namedtuple_as_object=True, tuple_as_array=True, \
@@ -587,7 +613,8 @@ Encoders and decoders
       wrapped in another type with an appropriate `for_json` method to
       transform the keys during encoding.
 
-   It also understands ``NaN``, ``Infinity``, and ``-Infinity`` as their
+   When *allow_nan* is True, it also understands
+   ``NaN``, ``Infinity``, and ``-Infinity`` as their
    corresponding ``float`` values, which is outside the JSON spec.
 
    To extend this to recognize other objects, subclass and implement a
@@ -613,12 +640,16 @@ Encoders and decoders
    prevent an infinite recursion (which would cause an :exc:`OverflowError`).
    Otherwise, no such check takes place.
 
-   If *allow_nan* is true (the default), then ``NaN``, ``Infinity``, and
+   If *allow_nan* is true (not the default), then ``NaN``, ``Infinity``, and
    ``-Infinity`` will be encoded as such. This behavior is not JSON
-   specification compliant, but is consistent with most JavaScript based
-   encoders and decoders.  Otherwise, it will be a :exc:`ValueError` to encode
+   specification compliant. Otherwise, it will be a :exc:`ValueError` to encode
    such floats. See also *ignore_nan* for ECMA-262 compliant behavior.
 
+    .. versionchanged:: 3.19.0
+        This default is now False to make it behave closer to the spec.
+        The previous behavior can be restored by setting this to
+        False.
+
    If *sort_keys* is true (not the default), then the output of dictionaries
    will be sorted by key; this is useful for regression tests to ensure that
    JSON serializations can be compared on a day-to-day basis.
@@ -730,7 +761,7 @@ Encoders and decoders
       :meth:`iterencode`.
 
 .. class:: JSONEncoderForHTML(skipkeys=False, ensure_ascii=True, \
-                              check_circular=True, allow_nan=True, \
+                              check_circular=True, allow_nan=False, \
                               sort_keys=False, indent=None, separators=None, \
                               encoding='utf-8', \
                               default=None, use_decimal=True, \
@@ -840,22 +871,28 @@ Infinite and NaN Number Values
 
 The RFC does not permit the representation of infinite or NaN number values.
 Despite that, by default, this module accepts and outputs ``Infinity``,
-``-Infinity``, and ``NaN`` as if they were valid JSON number literal values::
+``-Infinity``, and ``NaN`` as if they were valid JSON number literal values
+if the allow_nan flag is enabled::
 
    >>> # Neither of these calls raises an exception, but the results are not valid JSON
-   >>> json.dumps(float('-inf'))
+   >>> json.dumps(float('-inf'), allow_nan=True)
    '-Infinity'
-   >>> json.dumps(float('nan'))
+   >>> json.dumps(float('nan'), allow_nan=True)
    'NaN'
    >>> # Same when deserializing
-   >>> json.loads('-Infinity')
+   >>> json.loads('-Infinity', allow_nan=True)
    -inf
-   >>> json.loads('NaN')
+   >>> json.loads('NaN', allow_nan=True)
    nan
+   >>> # ignore_nan uses the ECMA-262 behavior to serialize these as null
+   >>> json.dumps(float('-inf'), ignore_nan=True)
+   'null'
+   >>> json.dumps(float('nan'), ignore_nan=True)
+   'null'
 
 In the serializer, the *allow_nan* parameter can be used to alter this
-behavior.  In the deserializer, the *parse_constant* parameter can be used to
-alter this behavior.
+behavior. In the deserializer, the *allow_nan* and
+*parse_constant* parameters can be used to alter this behavior.
 
 
 Repeated Names Within an Object
diff --git a/simplejson/__init__.py b/simplejson/__init__.py
index 92b6009..206e22d 100644
--- a/simplejson/__init__.py
+++ b/simplejson/__init__.py
@@ -149,28 +149,10 @@ def _import_c_make_encoder():
     except ImportError:
         return None
 
-_default_encoder = JSONEncoder(
-    skipkeys=False,
-    ensure_ascii=True,
-    check_circular=True,
-    allow_nan=True,
-    indent=None,
-    separators=None,
-    encoding='utf-8',
-    default=None,
-    use_decimal=True,
-    namedtuple_as_object=True,
-    tuple_as_array=True,
-    iterable_as_array=False,
-    bigint_as_string=False,
-    item_sort_key=None,
-    for_json=False,
-    ignore_nan=False,
-    int_as_string_bitcount=None,
-)
+_default_encoder = JSONEncoder()
 
 def dump(obj, fp, skipkeys=False, ensure_ascii=True, check_circular=True,
-         allow_nan=True, cls=None, indent=None, separators=None,
+         allow_nan=False, cls=None, indent=None, separators=None,
          encoding='utf-8', default=None, use_decimal=True,
          namedtuple_as_object=True, tuple_as_array=True,
          bigint_as_string=False, sort_keys=False, item_sort_key=None,
@@ -187,10 +169,10 @@ def dump(obj, fp, skipkeys=False, ensure_ascii=True, check_circular=True,
     contain non-ASCII characters, so long as they do not need to be escaped
     by JSON. When it is true, all non-ASCII characters are escaped.
 
-    If *allow_nan* is false, then it will be a ``ValueError`` to
-    serialize out of range ``float`` values (``nan``, ``inf``, ``-inf``)
-    in strict compliance of the original JSON specification, instead of using
-    the JavaScript equivalents (``NaN``, ``Infinity``, ``-Infinity``). See
+    If *allow_nan* is true (default: ``False``), then out of range ``float``
+    values (``nan``, ``inf``, ``-inf``) will be serialized to
+    their JavaScript equivalents (``NaN``, ``Infinity``, ``-Infinity``)
+    instead of raising a ValueError. See
     *ignore_nan* for ECMA-262 compliant behavior.
 
     If *indent* is a string, then JSON array elements and object members
@@ -258,7 +240,7 @@ def dump(obj, fp, skipkeys=False, ensure_ascii=True, check_circular=True,
     """
     # cached encoder
     if (not skipkeys and ensure_ascii and
-        check_circular and allow_nan and
+        check_circular and not allow_nan and
         cls is None and indent is None and separators is None and
         encoding == 'utf-8' and default is None and use_decimal
         and namedtuple_as_object and tuple_as_array and not iterable_as_array
@@ -292,7 +274,7 @@ def dump(obj, fp, skipkeys=False, ensure_ascii=True, check_circular=True,
 
 
 def dumps(obj, skipkeys=False, ensure_ascii=True, check_circular=True,
-          allow_nan=True, cls=None, indent=None, separators=None,
+          allow_nan=False, cls=None, indent=None, separators=None,
           encoding='utf-8', default=None, use_decimal=True,
           namedtuple_as_object=True, tuple_as_array=True,
           bigint_as_string=False, sort_keys=False, item_sort_key=None,
@@ -312,10 +294,11 @@ def dumps(obj, skipkeys=False, ensure_ascii=True, check_circular=True,
     for container types will be skipped and a circular reference will
     result in an ``OverflowError`` (or worse).
 
-    If ``allow_nan`` is false, then it will be a ``ValueError`` to
-    serialize out of range ``float`` values (``nan``, ``inf``, ``-inf``) in
-    strict compliance of the JSON specification, instead of using the
-    JavaScript equivalents (``NaN``, ``Infinity``, ``-Infinity``).
+    If *allow_nan* is true (default: ``False``), then out of range ``float``
+    values (``nan``, ``inf``, ``-inf``) will be serialized to
+    their JavaScript equivalents (``NaN``, ``Infinity``, ``-Infinity``)
+    instead of raising a ValueError. See
+    *ignore_nan* for ECMA-262 compliant behavior.
 
     If ``indent`` is a string, then JSON array elements and object members
     will be pretty-printed with a newline followed by that string repeated
@@ -383,7 +366,7 @@ def dumps(obj, skipkeys=False, ensure_ascii=True, check_circular=True,
     """
     # cached encoder
     if (not skipkeys and ensure_ascii and
-        check_circular and allow_nan and
+        check_circular and not allow_nan and
         cls is None and indent is None and separators is None and
         encoding == 'utf-8' and default is None and use_decimal
         and namedtuple_as_object and tuple_as_array and not iterable_as_array
@@ -412,14 +395,12 @@ def dumps(obj, skipkeys=False, ensure_ascii=True, check_circular=True,
         **kw).encode(obj)
 
 
-_default_decoder = JSONDecoder(encoding=None, object_hook=None,
-                               object_pairs_hook=None)
+_default_decoder = JSONDecoder()
 
 
 def load(fp, encoding=None, cls=None, object_hook=None, parse_float=None,
         parse_int=None, parse_constant=None, object_pairs_hook=None,
-        use_decimal=False, namedtuple_as_object=True, tuple_as_array=True,
-        **kw):
+        use_decimal=False, allow_nan=False, **kw):
     """Deserialize ``fp`` (a ``.read()``-supporting file-like object containing
     a JSON document as `str` or `bytes`) to a Python object.
 
@@ -451,14 +432,18 @@ def load(fp, encoding=None, cls=None, object_hook=None, parse_float=None,
     ``int(num_str)``.  This can be used to use another datatype or parser
     for JSON integers (e.g. :class:`float`).
 
-    *parse_constant*, if specified, will be called with one of the
-    following strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``.  This
-    can be used to raise an exception if invalid JSON numbers are
-    encountered.
+    *allow_nan*, if True (default false), will allow the parser to
+    accept the non-standard floats ``NaN``, ``Infinity``, and ``-Infinity``
+    and enable the use of the deprecated *parse_constant*.
 
     If *use_decimal* is true (default: ``False``) then it implies
     parse_float=decimal.Decimal for parity with ``dump``.
 
+    *parse_constant*, if specified, will be
+    called with one of the following strings: ``'-Infinity'``,
+    ``'Infinity'``, ``'NaN'``. It is not recommended to use this feature,
+    as it is rare to parse non-compliant JSON containing these values.
+
     To use a custom ``JSONDecoder`` subclass, specify it with the ``cls``
     kwarg. NOTE: You should use *object_hook* or *object_pairs_hook* instead
     of subclassing whenever possible.
@@ -468,12 +453,12 @@ def load(fp, encoding=None, cls=None, object_hook=None, parse_float=None,
         encoding=encoding, cls=cls, object_hook=object_hook,
         parse_float=parse_float, parse_int=parse_int,
         parse_constant=parse_constant, object_pairs_hook=object_pairs_hook,
-        use_decimal=use_decimal, **kw)
+        use_decimal=use_decimal, allow_nan=allow_nan, **kw)
 
 
 def loads(s, encoding=None, cls=None, object_hook=None, parse_float=None,
         parse_int=None, parse_constant=None, object_pairs_hook=None,
-        use_decimal=False, **kw):
+        use_decimal=False, allow_nan=False, **kw):
     """Deserialize ``s`` (a ``str`` or ``unicode`` instance containing a JSON
     document) to a Python object.
 
@@ -505,14 +490,18 @@ def loads(s, encoding=None, cls=None, object_hook=None, parse_float=None,
     ``int(num_str)``.  This can be used to use another datatype or parser
     for JSON integers (e.g. :class:`float`).
 
-    *parse_constant*, if specified, will be called with one of the
-    following strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``.  This
-    can be used to raise an exception if invalid JSON numbers are
-    encountered.
+    *allow_nan*, if True (default false), will allow the parser to
+    accept the non-standard floats ``NaN``, ``Infinity``, and ``-Infinity``
+    and enable the use of the deprecated *parse_constant*.
 
     If *use_decimal* is true (default: ``False``) then it implies
     parse_float=decimal.Decimal for parity with ``dump``.
 
+    *parse_constant*, if specified, will be
+    called with one of the following strings: ``'-Infinity'``,
+    ``'Infinity'``, ``'NaN'``. It is not recommended to use this feature,
+    as it is rare to parse non-compliant JSON containing these values.
+
     To use a custom ``JSONDecoder`` subclass, specify it with the ``cls``
     kwarg. NOTE: You should use *object_hook* or *object_pairs_hook* instead
     of subclassing whenever possible.
@@ -521,7 +510,7 @@ def loads(s, encoding=None, cls=None, object_hook=None, parse_float=None,
     if (cls is None and encoding is None and object_hook is None and
             parse_int is None and parse_float is None and
             parse_constant is None and object_pairs_hook is None
-            and not use_decimal and not kw):
+            and not use_decimal and not allow_nan and not kw):
         return _default_decoder.decode(s)
     if cls is None:
         cls = JSONDecoder
@@ -539,6 +528,8 @@ def loads(s, encoding=None, cls=None, object_hook=None, parse_float=None,
         if parse_float is not None:
             raise TypeError("use_decimal=True implies parse_float=Decimal")
         kw['parse_float'] = Decimal
+    if allow_nan:
+        kw['allow_nan'] = True
     return cls(encoding=encoding, **kw).decode(s)
 
 
@@ -560,22 +551,9 @@ def _toggle_speedups(enabled):
         scan.make_scanner = scan.py_make_scanner
     dec.make_scanner = scan.make_scanner
     global _default_decoder
-    _default_decoder = JSONDecoder(
-        encoding=None,
-        object_hook=None,
-        object_pairs_hook=None,
-    )
+    _default_decoder = JSONDecoder()
     global _default_encoder
-    _default_encoder = JSONEncoder(
-       skipkeys=False,
-       ensure_ascii=True,
-       check_circular=True,
-       allow_nan=True,
-       indent=None,
-       separators=None,
-       encoding='utf-8',
-       default=None,
-   )
+    _default_encoder = JSONEncoder()
 
 def simple_first(kv):
     """Helper function to pass to item_sort_key to sort simple
diff --git a/simplejson/_speedups.c b/simplejson/_speedups.c
index e08f4c4..bd56b4d 100644
--- a/simplejson/_speedups.c
+++ b/simplejson/_speedups.c
@@ -1843,7 +1843,7 @@ bail:
 }
 
 static PyObject *
-_parse_constant(PyScannerObject *s, PyObject *constant, Py_ssize_t idx, Py_ssize_t *next_idx_ptr)
+_parse_constant(PyScannerObject *s, PyObject *pystr, PyObject *constant, Py_ssize_t idx, Py_ssize_t *next_idx_ptr)
 {
     /* Read a JSON constant from PyString pystr.
     constant is the Python string that was found
@@ -1855,6 +1855,10 @@ _parse_constant(PyScannerObject *s, PyObject *constant, Py_ssize_t idx, Py_ssize
     Returns the result of parse_constant
     */
     PyObject *rval;
+    if (s->parse_constant == Py_None) {
+        raise_errmsg(ERR_EXPECTING_VALUE, pystr, idx);
+        return NULL;
+    }
 
     /* rval = parse_constant(constant) */
     rval = PyObject_CallOneArg(s->parse_constant, constant);
@@ -2158,7 +2162,7 @@ scan_once_str(PyScannerObject *s, PyObject *pystr, Py_ssize_t idx, Py_ssize_t *n
         case 'N':
             /* NaN */
             if ((idx + 2 < length) && str[idx + 1] == 'a' && str[idx + 2] == 'N') {
-                rval = _parse_constant(s, JSON_NaN, idx, next_idx_ptr);
+                rval = _parse_constant(s, pystr, JSON_NaN, idx, next_idx_ptr);
             }
             else
                 fallthrough = 1;
@@ -2166,7 +2170,7 @@ scan_once_str(PyScannerObject *s, PyObject *pystr, Py_ssize_t idx, Py_ssize_t *n
         case 'I':
             /* Infinity */
             if ((idx + 7 < length) && str[idx + 1] == 'n' && str[idx + 2] == 'f' && str[idx + 3] == 'i' && str[idx + 4] == 'n' && str[idx + 5] == 'i' && str[idx + 6] == 't' && str[idx + 7] == 'y') {
-                rval = _parse_constant(s, JSON_Infinity, idx, next_idx_ptr);
+                rval = _parse_constant(s, pystr, JSON_Infinity, idx, next_idx_ptr);
             }
             else
                 fallthrough = 1;
@@ -2174,7 +2178,7 @@ scan_once_str(PyScannerObject *s, PyObject *pystr, Py_ssize_t idx, Py_ssize_t *n
         case '-':
             /* -Infinity */
             if ((idx + 8 < length) && str[idx + 1] == 'I' && str[idx + 2] == 'n' && str[idx + 3] == 'f' && str[idx + 4] == 'i' && str[idx + 5] == 'n' && str[idx + 6] == 'i' && str[idx + 7] == 't' && str[idx + 8] == 'y') {
-                rval = _parse_constant(s, JSON_NegInfinity, idx, next_idx_ptr);
+                rval = _parse_constant(s, pystr, JSON_NegInfinity, idx, next_idx_ptr);
             }
             else
                 fallthrough = 1;
@@ -2277,7 +2281,7 @@ scan_once_unicode(PyScannerObject *s, PyObject *pystr, Py_ssize_t idx, Py_ssize_
             if ((idx + 2 < length) &&
                 PyUnicode_READ(kind, str, idx + 1) == 'a' &&
                 PyUnicode_READ(kind, str, idx + 2) == 'N') {
-                rval = _parse_constant(s, JSON_NaN, idx, next_idx_ptr);
+                rval = _parse_constant(s, pystr, JSON_NaN, idx, next_idx_ptr);
             }
             else
                 fallthrough = 1;
@@ -2292,7 +2296,7 @@ scan_once_unicode(PyScannerObject *s, PyObject *pystr, Py_ssize_t idx, Py_ssize_
                 PyUnicode_READ(kind, str, idx + 5) == 'i' &&
                 PyUnicode_READ(kind, str, idx + 6) == 't' &&
                 PyUnicode_READ(kind, str, idx + 7) == 'y') {
-                rval = _parse_constant(s, JSON_Infinity, idx, next_idx_ptr);
+                rval = _parse_constant(s, pystr, JSON_Infinity, idx, next_idx_ptr);
             }
             else
                 fallthrough = 1;
@@ -2308,7 +2312,7 @@ scan_once_unicode(PyScannerObject *s, PyObject *pystr, Py_ssize_t idx, Py_ssize_
                 PyUnicode_READ(kind, str, idx + 6) == 'i' &&
                 PyUnicode_READ(kind, str, idx + 7) == 't' &&
                 PyUnicode_READ(kind, str, idx + 8) == 'y') {
-                rval = _parse_constant(s, JSON_NegInfinity, idx, next_idx_ptr);
+                rval = _parse_constant(s, pystr, JSON_NegInfinity, idx, next_idx_ptr);
             }
             else
                 fallthrough = 1;
diff --git a/simplejson/decoder.py b/simplejson/decoder.py
index bbde82c..c99a976 100644
--- a/simplejson/decoder.py
+++ b/simplejson/decoder.py
@@ -306,14 +306,15 @@ class JSONDecoder(object):
     | null          | None              |
     +---------------+-------------------+
 
-    It also understands ``NaN``, ``Infinity``, and ``-Infinity`` as
+    When allow_nan=True, it also understands
+    ``NaN``, ``Infinity``, and ``-Infinity`` as
     their corresponding ``float`` values, which is outside the JSON spec.
 
     """
 
     def __init__(self, encoding=None, object_hook=None, parse_float=None,
             parse_int=None, parse_constant=None, strict=True,
-            object_pairs_hook=None):
+            object_pairs_hook=None, allow_nan=False):
         """
         *encoding* determines the encoding used to interpret any
         :class:`str` objects decoded by this instance (``'utf-8'`` by
@@ -346,10 +347,13 @@ class JSONDecoder(object):
         ``int(num_str)``.  This can be used to use another datatype or parser
         for JSON integers (e.g. :class:`float`).
 
-        *parse_constant*, if specified, will be called with one of the
-        following strings: ``'-Infinity'``, ``'Infinity'``, ``'NaN'``.  This
-        can be used to raise an exception if invalid JSON numbers are
-        encountered.
+        *allow_nan*, if True (default false), will allow the parser to
+        accept the non-standard floats ``NaN``, ``Infinity``, and ``-Infinity``.
+
+        *parse_constant*, if specified, will be
+        called with one of the following strings: ``'-Infinity'``,
+        ``'Infinity'``, ``'NaN'``. It is not recommended to use this feature,
+        as it is rare to parse non-compliant JSON containing these values.
 
         *strict* controls the parser's behavior when it encounters an
         invalid control character in a string. The default setting of
@@ -364,7 +368,7 @@ class JSONDecoder(object):
         self.object_pairs_hook = object_pairs_hook
         self.parse_float = parse_float or float
         self.parse_int = parse_int or bounded_int
-        self.parse_constant = parse_constant or _CONSTANTS.__getitem__
+        self.parse_constant = parse_constant or (allow_nan and _CONSTANTS.__getitem__ or None)
         self.strict = strict
         self.parse_object = JSONObject
         self.parse_array = JSONArray
diff --git a/simplejson/encoder.py b/simplejson/encoder.py
index 3f94f3a..661ff36 100644
--- a/simplejson/encoder.py
+++ b/simplejson/encoder.py
@@ -140,7 +140,7 @@ class JSONEncoder(object):
     key_separator = ': '
 
     def __init__(self, skipkeys=False, ensure_ascii=True,
-                 check_circular=True, allow_nan=True, sort_keys=False,
+                 check_circular=True, allow_nan=False, sort_keys=False,
                  indent=None, separators=None, encoding='utf-8', default=None,
                  use_decimal=True, namedtuple_as_object=True,
                  tuple_as_array=True, bigint_as_string=False,
@@ -161,10 +161,11 @@ class JSONEncoder(object):
         prevent an infinite recursion (which would cause an OverflowError).
         Otherwise, no such check takes place.
 
-        If allow_nan is true, then NaN, Infinity, and -Infinity will be
-        encoded as such.  This behavior is not JSON specification compliant,
-        but is consistent with most JavaScript based encoders and decoders.
-        Otherwise, it will be a ValueError to encode such floats.
+        If allow_nan is true (default: False), then out of range float
+        values (nan, inf, -inf) will be serialized to
+        their JavaScript equivalents (NaN, Infinity, -Infinity)
+        instead of raising a ValueError. See
+        ignore_nan for ECMA-262 compliant behavior.
 
         If sort_keys is true, then the output of dictionaries will be
         sorted by key; this is useful for regression tests to ensure
@@ -294,7 +295,7 @@ class JSONEncoder(object):
         # This doesn't pass the iterator directly to ''.join() because the
         # exceptions aren't as detailed.  The list call should be roughly
         # equivalent to the PySequence_Fast that ''.join() would do.
-        chunks = self.iterencode(o, _one_shot=True)
+        chunks = self.iterencode(o)
         if not isinstance(chunks, (list, tuple)):
             chunks = list(chunks)
         if self.ensure_ascii:
@@ -302,7 +303,7 @@ class JSONEncoder(object):
         else:
             return u''.join(chunks)
 
-    def iterencode(self, o, _one_shot=False):
+    def iterencode(self, o):
         """Encode the given object and yield each string
         representation as available.
 
@@ -356,8 +357,7 @@ class JSONEncoder(object):
         key_memo = {}
         int_as_string_bitcount = (
             53 if self.bigint_as_string else self.int_as_string_bitcount)
-        if (_one_shot and c_make_encoder is not None
-                and self.indent is None):
+        if (c_make_encoder is not None and self.indent is None):
             _iterencode = c_make_encoder(
                 markers, self.default, _encoder, self.indent,
                 self.key_separator, self.item_separator, self.sort_keys,
@@ -370,7 +370,7 @@ class JSONEncoder(object):
             _iterencode = _make_iterencode(
                 markers, self.default, _encoder, self.indent, floatstr,
                 self.key_separator, self.item_separator, self.sort_keys,
-                self.skipkeys, _one_shot, self.use_decimal,
+                self.skipkeys, self.use_decimal,
                 self.namedtuple_as_object, self.tuple_as_array,
                 int_as_string_bitcount,
                 self.item_sort_key, self.encoding, self.for_json,
@@ -398,14 +398,14 @@ class JSONEncoderForHTML(JSONEncoder):
     def encode(self, o):
         # Override JSONEncoder.encode because it has hacks for
         # performance that make things more complicated.
-        chunks = self.iterencode(o, True)
+        chunks = self.iterencode(o)
         if self.ensure_ascii:
             return ''.join(chunks)
         else:
             return u''.join(chunks)
 
-    def iterencode(self, o, _one_shot=False):
-        chunks = super(JSONEncoderForHTML, self).iterencode(o, _one_shot)
+    def iterencode(self, o):
+        chunks = super(JSONEncoderForHTML, self).iterencode(o)
         for chunk in chunks:
             chunk = chunk.replace('&', '\\u0026')
             chunk = chunk.replace('<', '\\u003c')
@@ -419,7 +419,7 @@ class JSONEncoderForHTML(JSONEncoder):
 
 
 def _make_iterencode(markers, _default, _encoder, _indent, _floatstr,
-        _key_separator, _item_separator, _sort_keys, _skipkeys, _one_shot,
+        _key_separator, _item_separator, _sort_keys, _skipkeys,
         _use_decimal, _namedtuple_as_object, _tuple_as_array,
         _int_as_string_bitcount, _item_sort_key,
         _encoding,_for_json,
diff --git a/simplejson/scanner.py b/simplejson/scanner.py
index 85e385e..34710d6 100644
--- a/simplejson/scanner.py
+++ b/simplejson/scanner.py
@@ -60,11 +60,11 @@ def py_make_scanner(context):
             else:
                 res = parse_int(integer)
             return res, m.end()
-        elif nextchar == 'N' and string[idx:idx + 3] == 'NaN':
+        elif parse_constant and nextchar == 'N' and string[idx:idx + 3] == 'NaN':
             return parse_constant('NaN'), idx + 3
-        elif nextchar == 'I' and string[idx:idx + 8] == 'Infinity':
+        elif parse_constant and nextchar == 'I' and string[idx:idx + 8] == 'Infinity':
             return parse_constant('Infinity'), idx + 8
-        elif nextchar == '-' and string[idx:idx + 9] == '-Infinity':
+        elif parse_constant and nextchar == '-' and string[idx:idx + 9] == '-Infinity':
             return parse_constant('-Infinity'), idx + 9
         else:
             raise JSONDecodeError(errmsg, string, idx)
diff --git a/simplejson/tests/test_float.py b/simplejson/tests/test_float.py
index e382ec2..a977969 100644
--- a/simplejson/tests/test_float.py
+++ b/simplejson/tests/test_float.py
@@ -7,9 +7,9 @@ from simplejson.decoder import NaN, PosInf, NegInf
 class TestFloat(TestCase):
     def test_degenerates_allow(self):
         for inf in (PosInf, NegInf):
-            self.assertEqual(json.loads(json.dumps(inf)), inf)
+            self.assertEqual(json.loads(json.dumps(inf, allow_nan=True), allow_nan=True), inf)
         # Python 2.5 doesn't have math.isnan
-        nan = json.loads(json.dumps(NaN))
+        nan = json.loads(json.dumps(NaN, allow_nan=True), allow_nan=True)
         self.assertTrue((0 + nan) != nan)
 
     def test_degenerates_ignore(self):
@@ -19,6 +19,9 @@ class TestFloat(TestCase):
     def test_degenerates_deny(self):
         for f in (PosInf, NegInf, NaN):
             self.assertRaises(ValueError, json.dumps, f, allow_nan=False)
+        for s in ('Infinity', '-Infinity', 'NaN'):
+            self.assertRaises(ValueError, json.loads, s, allow_nan=False)
+            self.assertRaises(ValueError, json.loads, s)
 
     def test_floats(self):
         for num in [1617161771.7650001, math.pi, math.pi**100,