Add len to mapping protocol

Author: atpalmer

src/cstring.c

@@ -141,6 +141,10 @@ static PySequenceMethods cstring_as_sequence = {
     .sq_contains = cstring_contains,
 };
 
+static PyMappingMethods cstring_as_mapping = {
+    .mp_length = cstring_len,
+};
+
 static PyTypeObject cstring_type = {
     PyVarObject_HEAD_INIT(NULL, 0)
     .tp_name = "cstring",
@@ -154,6 +158,7 @@ static PyTypeObject cstring_type = {
     .tp_str = cstring_str,
     .tp_hash = cstring_hash,
     .tp_as_sequence = &cstring_as_sequence,
+    .tp_as_mapping = &cstring_as_mapping,
 };
 
 static struct PyModuleDef module = {

test/test_mapping.py

@@ -0,0 +1,7 @@
+from cstring import cstring
+
+
+def test_len():
+    result = cstring('hello, world')
+    assert len(result) == 12
+