Extend attribute and item accessor interface using object_api

docs/changelog.rst

@@ -53,6 +53,8 @@ Breaking changes queued for v2.0.0 (Not yet released)
 * Added ``py::dict`` keyword constructor:``auto d = dict("number"_a=42, "name"_a="World");``
 * Added ``py::str::format()`` method and ``_s`` literal:
   ``py::str s = "1 + 2 = {}"_s.format(3);``
+* Attribute and item accessors now have a more complete interface which makes it possible
+  to chain attributes ``obj.attr("a")[key].attr("b").attr("method")(1, 2, 3)```.
 * Various minor improvements of library internals (no user-visible changes)
 
 1.8.1 (July 12, 2016)

include/pybind11/cast.h

@@ -1219,7 +1219,7 @@ private:
 
     void process(list &args_list, detail::args_proxy ap) {
         for (const auto &a : ap) {
-            args_list.append(a.cast<object>());
+            args_list.append(a);
         }
     }
 

include/pybind11/numpy.h

@@ -125,7 +125,7 @@ private:
 
     static npy_api lookup() {
         module m = module::import("numpy.core.multiarray");
-        auto c = m.attr("_ARRAY_API").cast<object>();
+        auto c = m.attr("_ARRAY_API");
 #if PY_MAJOR_VERSION >= 3
         void **api_ptr = (void **) PyCapsule_GetPointer(c.ptr(), NULL);
 #else
@@ -220,9 +220,7 @@ private:
         struct field_descr { PYBIND11_STR_TYPE name; object format; pybind11::int_ offset; };
         std::vector<field_descr> field_descriptors;
 
-        auto fields = attr("fields").cast<object>();
-        auto items = fields.attr("items").cast<object>();
-        for (auto field : items()) {
+        for (auto field : attr("fields").attr("items")()) {
             auto spec = object(field, true).cast<tuple>();
             auto name = spec[0].cast<pybind11::str>();
             auto format = spec[1].cast<tuple>()[0].cast<dtype>();

include/pybind11/pybind11.h

@@ -176,7 +176,7 @@ protected:
             if (a.descr)
                 a.descr = strdup(a.descr);
             else if (a.value)
-                a.descr = strdup(((std::string) ((object) handle(a.value).attr("__repr__"))().str()).c_str());
+                a.descr = strdup(a.value.attr("__repr__")().cast<std::string>().c_str());
         }
 
         auto const &registered_types = detail::get_internals().registered_types_cpp;
@@ -723,8 +723,7 @@ protected:
         if (ob_type == &PyType_Type) {
             std::string name_ = std::string(ht_type.tp_name) + "__Meta";
 #if PY_MAJOR_VERSION >= 3 && PY_MINOR_VERSION >= 3
-            object ht_qualname(PyUnicode_FromFormat(
-                "%U__Meta", ((object) attr("__qualname__")).ptr()), false);
+            object ht_qualname(PyUnicode_FromFormat("%U__Meta", attr("__qualname__").ptr()), false);
 #endif
             object name(PYBIND11_FROM_STRING(name_.c_str()), false);
             object type_holder(PyType_Type.tp_alloc(&PyType_Type, 0), false);
@@ -1342,16 +1341,16 @@ PYBIND11_NOINLINE inline void print(tuple args, dict kwargs) {
         strings[i] = args[i].cast<object>().str();
     }
     auto sep = kwargs.contains("sep") ? kwargs["sep"] : cast(" ");
-    auto line = sep.attr("join").cast<object>()(strings);
+    auto line = sep.attr("join")(strings);
 
     auto file = kwargs.contains("file") ? kwargs["file"].cast<object>()
                                         : module::import("sys").attr("stdout");
-    auto write = file.attr("write").cast<object>();
+    auto write = file.attr("write");
     write(line);
     write(kwargs.contains("end") ? kwargs["end"] : cast("\n"));
 
     if (kwargs.contains("flush") && kwargs["flush"].cast<bool>()) {
-        file.attr("flush").cast<object>()();
+        file.attr("flush")();
     }
 }
 NAMESPACE_END(detail)

include/pybind11/pytypes.h

@@ -21,7 +21,18 @@ class str; class iterator;
 struct arg; struct arg_v;
 
 NAMESPACE_BEGIN(detail)
-class accessor; class args_proxy;
+class args_proxy;
+
+// Accessor forward declarations
+template <typename Policy> class accessor;
+namespace accessor_policies {
+    struct obj_attr;
+    struct str_attr;
+    struct generic_item;
+}
+using obj_attr_accessor = accessor<accessor_policies::obj_attr>;
+using str_attr_accessor = accessor<accessor_policies::str_attr>;
+using item_accessor = accessor<accessor_policies::generic_item>;
 
 /// Tag and check to identify a class which implements the Python object API
 class pyobject_tag { };
@@ -36,10 +47,10 @@ class object_api : public pyobject_tag {
 public:
     iterator begin() const;
     iterator end() const;
-    accessor operator[](handle key) const;
-    accessor operator[](const char *key) const;
-    accessor attr(handle key) const;
-    accessor attr(const char *key) const;
+    item_accessor operator[](handle key) const;
+    item_accessor operator[](const char *key) const;
+    obj_attr_accessor attr(handle key) const;
+    str_attr_accessor attr(const char *key) const;
     args_proxy operator*() const;
     template <typename T> bool contains(T &&key) const;
 
@@ -177,40 +188,60 @@ inline handle get_function(handle value) {
     return value;
 }
 
-class accessor {
+template <typename Policy>
+class accessor : public object_api<accessor<Policy>> {
+    using key_type = typename Policy::key_type;
+
 public:
-    accessor(handle obj, handle key, bool attr)
-        : obj(obj), key(key, true), attr(attr) { }
-    accessor(handle obj, const char *key, bool attr)
-        : obj(obj), key(PyUnicode_FromString(key), false), attr(attr) { }
-    accessor(const accessor &a) : obj(a.obj), key(a.key), attr(a.attr) { }
-
-    void operator=(accessor o) { operator=(object(o)); }
-
-    void operator=(const handle &value) {
-        if (attr) {
-            if (PyObject_SetAttr(obj.ptr(), key.ptr(), value.ptr()) == -1)
-                throw error_already_set();
-        } else {
-            if (PyObject_SetItem(obj.ptr(), key.ptr(), value.ptr()) == -1)
-                throw error_already_set();
-        }
+    accessor(handle obj, key_type key) : obj(obj), key(std::move(key)) { }
+
+    void operator=(const accessor &a) { operator=(handle(a)); }
+    void operator=(const object &o) { operator=(handle(o)); }
+    void operator=(handle value) { Policy::set(obj, key, value); }
+
+    operator object() const { return get_cache(); }
+    PyObject *ptr() const { return get_cache().ptr(); }
+    template <typename T> T cast() const { return get_cache().template cast<T>(); }
+
+private:
+    const object &get_cache() const {
+        if (!cache) { cache = Policy::get(obj, key); }
+        return cache;
     }
 
-    operator object() const {
-        PyObject *result = attr ? PyObject_GetAttr(obj.ptr(), key.ptr())
-                                : PyObject_GetItem(obj.ptr(), key.ptr());
+private:
+    handle obj;
+    key_type key;
+    mutable object cache;
+};
+
+NAMESPACE_BEGIN(accessor_policies)
+struct obj_attr {
+    using key_type = object;
+    static object get(handle obj, handle key) { return getattr(obj, key); }
+    static void set(handle obj, handle key, handle val) { setattr(obj, key, val); }
+};
+
+struct str_attr {
+    using key_type = const char *;
+    static object get(handle obj, const char *key) { return getattr(obj, key); }
+    static void set(handle obj, const char *key, handle val) { setattr(obj, key, val); }
+};
+
+struct generic_item {
+    using key_type = object;
+
+    static object get(handle obj, handle key) {
+        PyObject *result = PyObject_GetItem(obj.ptr(), key.ptr());
         if (!result) { throw error_already_set(); }
         return {result, false};
     }
 
-    template <typename T> T cast() const { return operator object().cast<T>(); }
-
-private:
-    handle obj;
-    object key;
-    bool attr;
+    static void set(handle obj, handle key, handle val) {
+        if (PyObject_SetItem(obj.ptr(), key.ptr(), val.ptr()) != 0) { throw error_already_set(); }
+    }
 };
+NAMESPACE_END(accessor_policies)
 
 struct list_accessor {
 public:
@@ -442,7 +473,7 @@ public:
 
     template <typename... Args>
     str format(Args &&...args) const {
-        return attr("format").cast<object>()(std::forward<Args>(args)...);
+        return attr("format")(std::forward<Args>(args)...);
     }
 };
 
@@ -729,13 +760,13 @@ inline size_t len(handle h) {
 NAMESPACE_BEGIN(detail)
 template <typename D> iterator object_api<D>::begin() const { return {PyObject_GetIter(derived().ptr()), false}; }
 template <typename D> iterator object_api<D>::end() const { return {nullptr, false}; }
-template <typename D> accessor object_api<D>::operator[](handle key) const { return {derived(), key, false}; }
-template <typename D> accessor object_api<D>::operator[](const char *key) const { return {derived(), key, false}; }
-template <typename D> accessor object_api<D>::attr(handle key) const { return {derived(), key, true}; }
-template <typename D> accessor object_api<D>::attr(const char *key) const { return {derived(), key, true}; }
+template <typename D> item_accessor object_api<D>::operator[](handle key) const { return {derived(), object(key, true)}; }
+template <typename D> item_accessor object_api<D>::operator[](const char *key) const { return {derived(), pybind11::str(key)}; }
+template <typename D> obj_attr_accessor object_api<D>::attr(handle key) const { return {derived(), object(key, true)}; }
+template <typename D> str_attr_accessor object_api<D>::attr(const char *key) const { return {derived(), key}; }
 template <typename D> args_proxy object_api<D>::operator*() const { return {derived().ptr()}; }
 template <typename D> template <typename T> bool object_api<D>::contains(T &&key) const {
-    return attr("__contains__").template cast<object>()(std::forward<T>(key)).template cast<bool>();
+    return attr("__contains__")(std::forward<T>(key)).template cast<bool>();
 }
 
 template <typename D>

tests/constructor_stats.h

@@ -103,7 +103,7 @@ public:
 
     int alive() {
         // Force garbage collection to ensure any pending destructors are invoked:
-        py::module::import("gc").attr("collect").operator py::object()();
+        py::module::import("gc").attr("collect")();
         int total = 0;
         for (const auto &p : _instances) if (p.second > 0) total += p.second;
         return total;

tests/test_python_types.cpp

@@ -203,7 +203,7 @@ test_initializer python_types([](py::module &m) {
         py::print("no new line here", "end"_a=" -- ");
         py::print("next print");
 
-        auto py_stderr = py::module::import("sys").attr("stderr").cast<py::object>();
+        auto py_stderr = py::module::import("sys").attr("stderr");
         py::print("this goes to stderr", "file"_a=py_stderr);
 
         py::print("flush", "flush"_a=true);
@@ -222,4 +222,39 @@ test_initializer python_types([](py::module &m) {
         auto d2 = py::dict("z"_a=3, **d1);
         return d2;
     });
+
+    m.def("test_accessor_api", [](py::object o) {
+        auto d = py::dict();
+
+        d["basic_attr"] = o.attr("basic_attr");
+
+        auto l = py::list();
+        for (const auto &item : o.attr("begin_end")) {
+            l.append(item);
+        }
+        d["begin_end"] = l;
+
+        d["operator[object]"] = o.attr("d")["operator[object]"_s];
+        d["operator[char *]"] = o.attr("d")["operator[char *]"];
+
+        d["attr(object)"] = o.attr("sub").attr("attr_obj");
+        d["attr(char *)"] = o.attr("sub").attr("attr_char");
+        try {
+            o.attr("sub").attr("missing").ptr();
+        } catch (const py::error_already_set &) {
+            d["missing_attr_ptr"] = "raised"_s;
+        }
+        try {
+            o.attr("missing").attr("doesn't matter");
+        } catch (const py::error_already_set &) {
+            d["missing_attr_chain"] = "raised"_s;
+        }
+
+        d["is_none"] = py::cast(o.attr("basic_attr").is_none());
+
+        d["operator()"] = o.attr("func")(1);
+        d["operator*"] = o.attr("func")(*o.attr("begin_end"));
+
+        return d;
+    });
 });

tests/test_python_types.py

@@ -248,3 +248,33 @@ def test_dict_api():
     from pybind11_tests import test_dict_keyword_constructor
 
     assert test_dict_keyword_constructor() == {"x": 1, "y": 2, "z": 3}
+
+
+def test_accessors():
+    from pybind11_tests import test_accessor_api
+
+    class SubTestObject:
+        attr_obj = 1
+        attr_char = 2
+
+    class TestObject:
+        basic_attr = 1
+        begin_end = [1, 2, 3]
+        d = {"operator[object]": 1, "operator[char *]": 2}
+        sub = SubTestObject()
+
+        def func(self, x, *args):
+            return self.basic_attr + x + sum(args)
+
+    d = test_accessor_api(TestObject())
+    assert d["basic_attr"] == 1
+    assert d["begin_end"] == [1, 2, 3]
+    assert d["operator[object]"] == 1
+    assert d["operator[char *]"] == 2
+    assert d["attr(object)"] == 1
+    assert d["attr(char *)"] == 2
+    assert d["missing_attr_ptr"] == "raised"
+    assert d["missing_attr_chain"] == "raised"
+    assert d["is_none"] is False
+    assert d["operator()"] == 2
+    assert d["operator*"] == 7