Support more natural syntax for vector extend

docs/changelog.rst

@@ -94,6 +94,11 @@ v2.2.4 (September 11, 2018)
 * A few minor typo fixes and improvements to the test suite, and
   patches that silence compiler warnings.
 
+* Vectors now support construction from generators, as well as ``extend()`` from a
+  list or generator.
+  `#1496 <https://github.com/pybind/pybind11/pull/1496>`_.
+
+
 v2.2.3 (April 29, 2018)
 -----------------------------------------------------
 

include/pybind11/pytypes.h

@@ -1346,6 +1346,21 @@ inline size_t len(handle h) {
     return (size_t) result;
 }
 
+inline size_t len_hint(handle h) {
+#if PY_VERSION_HEX >= 0x03040000
+    ssize_t result = PyObject_LengthHint(h.ptr(), 0);
+#else
+    ssize_t result = PyObject_Length(h.ptr());
+#endif
+    if (result < 0) {
+        // Sometimes a length can't be determined at all (eg generators)
+        // In which case simply return 0
+        PyErr_Clear();
+        return 0;
+    }
+    return (size_t) result;
+}
+
 inline str repr(handle h) {
     PyObject *str_value = PyObject_Repr(h.ptr());
     if (!str_value) throw error_already_set();

include/pybind11/stl_bind.h

@@ -122,7 +122,7 @@ void vector_modifiers(enable_if_t<is_copy_constructible<typename Vector::value_t
 
     cl.def(init([](iterable it) {
         auto v = std::unique_ptr<Vector>(new Vector());
-        v->reserve(len(it));
+        v->reserve(len_hint(it));
         for (handle h : it)
            v->push_back(h.cast<T>());
         return v.release();
@@ -136,6 +136,28 @@ void vector_modifiers(enable_if_t<is_copy_constructible<typename Vector::value_t
        "Extend the list by appending all the items in the given list"
     );
 
+    cl.def("extend",
+       [](Vector &v, iterable it) {
+           const size_t old_size = v.size();
+           v.reserve(old_size + len_hint(it));
+           try {
+               for (handle h : it) {
+                   v.push_back(h.cast<T>());
+               }
+           } catch (const cast_error &) {
+               v.erase(v.begin() + static_cast<typename Vector::difference_type>(old_size), v.end());
+               try {
+                   v.shrink_to_fit();
+               } catch (const std::exception &) {
+                   // Do nothing
+               }
+               throw;
+           }
+       },
+       arg("L"),
+       "Extend the list by appending all the items in the given list"
+    );
+
     cl.def("insert",
         [](Vector &v, SizeType i, const T &x) {
             if (i > v.size())

tests/test_stl_binders.py

@@ -11,6 +11,10 @@ def test_vector_int():
     assert len(v_int) == 2
     assert bool(v_int) is True
 
+    # test construction from a generator
+    v_int1 = m.VectorInt(x for x in range(5))
+    assert v_int1 == m.VectorInt([0, 1, 2, 3, 4])
+
     v_int2 = m.VectorInt([0, 0])
     assert v_int == v_int2
     v_int2[1] = 1
@@ -33,6 +37,22 @@ def test_vector_int():
     del v_int2[0]
     assert v_int2 == m.VectorInt([0, 99, 2, 3])
 
+    v_int2.extend(m.VectorInt([4, 5]))
+    assert v_int2 == m.VectorInt([0, 99, 2, 3, 4, 5])
+
+    v_int2.extend([6, 7])
+    assert v_int2 == m.VectorInt([0, 99, 2, 3, 4, 5, 6, 7])
+
+    # test error handling, and that the vector is unchanged
+    with pytest.raises(RuntimeError):
+        v_int2.extend([8, 'a'])
+
+    assert v_int2 == m.VectorInt([0, 99, 2, 3, 4, 5, 6, 7])
+
+    # test extending from a generator
+    v_int2.extend(x for x in range(5))
+    assert v_int2 == m.VectorInt([0, 99, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 4])
+
 
 # related to the PyPy's buffer protocol.
 @pytest.unsupported_on_pypy