bpo-20774: Add a JSON serializer to collections.deque

Doc/library/json.rst

@@ -390,7 +390,7 @@ Encoders and Decoders
    +========================================+===============+
    | dict                                   | object        |
    +----------------------------------------+---------------+
-   | list, tuple                            | array         |
+   | list, tuple, deque                     | array         |
    +----------------------------------------+---------------+
    | str                                    | string        |
    +----------------------------------------+---------------+
@@ -406,6 +406,9 @@ Encoders and Decoders
    .. versionchanged:: 3.4
       Added support for int- and float-derived Enum classes.
 
+  .. versionchanged:: 3.8
+     Added support for deque.
+
    To extend this to recognize other objects, subclass and implement a
    :meth:`default` method with another method that returns a serializable object
    for ``o`` if possible, otherwise it should call the superclass implementation

Lib/json/encoder.py

@@ -1,7 +1,7 @@
 """Implementation of JSONEncoder
 """
 import re
-
+from collections import deque
 try:
     from _json import encode_basestring_ascii as c_encode_basestring_ascii
 except ImportError:
@@ -316,7 +316,7 @@ def _iterencode_list(lst, _current_indent_level):
                 yield buf + _floatstr(value)
             else:
                 yield buf
-                if isinstance(value, (list, tuple)):
+                if isinstance(value, (list, tuple, deque)):
                     chunks = _iterencode_list(value, _current_indent_level)
                 elif isinstance(value, dict):
                     chunks = _iterencode_dict(value, _current_indent_level)
@@ -395,7 +395,7 @@ def _iterencode_dict(dct, _current_indent_level):
                 # see comment for int/float in _make_iterencode
                 yield _floatstr(value)
             else:
-                if isinstance(value, (list, tuple)):
+                if isinstance(value, (list, tuple, deque)):
                     chunks = _iterencode_list(value, _current_indent_level)
                 elif isinstance(value, dict):
                     chunks = _iterencode_dict(value, _current_indent_level)
@@ -424,7 +424,7 @@ def _iterencode(o, _current_indent_level):
         elif isinstance(o, float):
             # see comment for int/float in _make_iterencode
             yield _floatstr(o)
-        elif isinstance(o, (list, tuple)):
+        elif isinstance(o, (list, tuple, deque)):
             yield from _iterencode_list(o, _current_indent_level)
         elif isinstance(o, dict):
             yield from _iterencode_dict(o, _current_indent_level)

Lib/test/test_json/test_deque.py

@@ -0,0 +1,16 @@
+from collections import deque
+from test.test_json import PyTest, CTest
+
+
+class TestDeque:
+    def test_deque_ints(self):
+        d = deque(range(0, 10))
+        self.assertEqual(self.dumps(d), '[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]')
+
+    def test_deque_strings(self):
+        d = deque('hello')
+        self.assertEqual(self.dumps(d), '["h", "e", "l", "l", "o"]')
+
+
+class TestPyDeque(TestDeque, PyTest): pass
+class TestCDeque(TestDeque, CTest): pass

Modules/_json.c

@@ -1493,7 +1493,10 @@ encoder_listencode_obj(PyEncoderObject *s, _PyAccu *acc,
     /* Encode Python object obj to a JSON term */
     PyObject *newobj;
     int rv;
+    PyObject *mo = PyImport_ImportModule("collections");
+    PyObject *deque_type = PyObject_GetAttrString(mo, "deque");
 
+    Py_DECREF(mo);
     if (obj == Py_None || obj == Py_True || obj == Py_False) {
         PyObject *cstr = _encoded_const(obj);
         if (cstr == NULL)
@@ -1519,7 +1522,8 @@ encoder_listencode_obj(PyEncoderObject *s, _PyAccu *acc,
             return -1;
         return _steal_accumulate(acc, encoded);
     }
-    else if (PyList_Check(obj) || PyTuple_Check(obj)) {
+    else if (PyList_Check(obj) || PyTuple_Check(obj) || PyObject_IsInstance(obj, deque_type)) {
+        Py_DECREF(deque_type);
         if (Py_EnterRecursiveCall(" while encoding a JSON object"))
             return -1;
         rv = encoder_listencode_list(s, acc, obj, indent_level);