DOC, TST: test and document external extension type attribute aliasing

docs/src/userguide/extension_types.rst

@@ -771,6 +771,72 @@ For public extension types, the object and type clauses are both required,
 because Cython must be able to generate code that is compatible with external C
 code.
 
+Attribute name matching and aliasing
+------------------------------------
+
+Sometimes the type's C struct as specified in ``object_struct_name`` may use
+different labels for the fields. For example we may have an extension module
+``foo_extension``::
+
+    cdef class Foo:
+        cdef public int field0, field1, field2;
+
+        def __init__(self, f0, f1, f2):
+            self.field0 = f0
+            self.field1 = f1
+            self.field2 = f2
+
+but a C struct in a file ``foo_nominal.h``::
+
+   typedef struct {
+        PyObject_HEAD
+        int f0;
+        int f1;
+        int f2;
+    } FooStructNominal;
+
+Note that the struct uses ``f0``, ``f1``, ``f2`` but they are ``field0``,
+``field1``, and ``field2`` in ``Foo``. We are given this situation, including
+a header file with that struct, and we wish to write a function to sum the
+values. If we write an extension module ``wrapper``::
+
+    cdef extern from "foo_nominal.h":
+
+        ctypedef class foo_extension.Foo [object FooStructNominal]:
+            cdef:
+                int field0
+                int field1
+                int feild2
+
+    def sum(Foo f):
+        return f.field0 + f.field1 + f.field2
+
+then ``wrapper.sum(f)`` (where ``f = foo_extension.Foo(1, 2, 3)``) will still
+use the C-API equivalent of::
+
+    return f.__getattr__('field0') +
+           f.__getattr__('field1') +
+           f.__getattr__('field1')
+
+instead of the desired C equivalent of ``return f->f0 + f->f1 + f->f2``. We can
+alias the fields by using::
+    cdef extern from "foo_nominal.h":
+
+        ctypedef class foo_extension.Foo [object FooStructNominal]:
+            cdef:
+                int field0 "f0"
+                int field1 "f1"
+                int field2 "f2"
+
+    def sum(Foo f) except -1:
+        return f.field0 + f.field1 + f.field2
+
+and now Cython will replace the slow ``__getattr__`` with direct C access to
+the FooStructNominal fields. This is useful when directly processing Python
+code. No changes to Python need be made to achieve significant speedups, even
+though the field names in Python and C are different. Of course, one should
+make sure the fields are equivalent.
+
 Implicit importing
 ------------------
 

docs/src/userguide/external_C_code.rst

@@ -219,6 +219,7 @@ same applies equally to union and enum declarations.
 |   } Foo;                |                                             |                                                                       |
 +-------------------------+---------------------------------------------+-----------------------------------------------------------------------+
 
+See also use of :ref:`external_extension_types`.
 Note that in all the cases below, you refer to the type in Cython code simply
 as :c:type:`Foo`, not ``struct Foo``.
 

tests/run/ext_attr_getter.srctree

+PYTHON setup.py build_ext --inplace
+PYTHON -c "import runner"
+
+######## setup.py ########
+
+from Cython.Build.Dependencies import cythonize
+from distutils.core import setup
+
+# force the build order
+setup(ext_modules= cythonize("foo_extension.pyx"))
+
+setup(ext_modules = cythonize("getter*.pyx"))
+
+######## foo_nominal.h ########
+
+#include <Python.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct {
+    PyObject_HEAD
+    int f0;
+    int f1;
+    int f2;
+} FooStructNominal;
+
+#ifdef __cplusplus
+}
+#endif
+
+######## foo_extension.pyx ########
+
+cdef class Foo:
+    cdef public int field0, field1, field2;
+
+    def __init__(self, f0, f1, f2):
+        self.field0 = f0
+        self.field1 = f1
+        self.field2 = f2
+
+cdef get_field0(Foo f):
+    return f.field0
+
+cdef get_field1(Foo f):
+    return f.field1
+
+cdef get_field2(Foo f):
+    return f.field2
+
+# A pure-python class that disallows direct access to fields
+class OpaqueFoo(Foo):
+
+    @property
+    def field0(self):
+        raise AttributeError('no direct access to field0')
+
+    @property
+    def field1(self):
+        raise AttributeError('no direct access to field1')
+
+    @property
+    def field2(self):
+        raise AttributeError('no direct access to field2')
+
+
+######## getter0.pyx ########
+
+# Access base Foo fields from C via aliased field names
+
+cdef extern from "foo_nominal.h":
+
+    ctypedef class foo_extension.Foo [object FooStructNominal]:
+        cdef:
+            int field0 "f0"
+            int field1 "f1"
+            int field2 "f2"
+
+def sum(Foo f):
+    # the f.__getattr__('field0') is replaced in c by f->f0
+    return f.field0 + f.field1 + f.field2
+
+######## runner.py ########
+
+import foo_extension, getter0
+
+foo = foo_extension.Foo(23, 123, 1023)
+
+assert foo.field0 == 23
+assert foo.field1 == 123
+assert foo.field2 == 1023
+
+ret =  getter0.sum(foo)
+assert ret == foo.field0 + foo.field1 + foo.field2
+
+opaque_foo = foo_extension.OpaqueFoo(23, 123, 1023)
+
+# C can access the fields through the aliases
+opaque_ret = getter0.sum(opaque_foo)
+assert opaque_ret == ret
+try:
+    # Python cannot access the fields
+    f0 = opaque_ret.field0
+    assert False
+except AttributeError as e:
+    pass
+
+