refactor LGBM_DatasetGetFeatureNames (#3022)

R-package/src/lightgbm_R.cpp

@@ -158,16 +158,23 @@ LGBM_SE LGBM_DatasetGetFeatureNames_R(LGBM_SE handle,
   R_API_BEGIN();
   int len = 0;
   CHECK_CALL(LGBM_DatasetGetNumFeature(R_GET_PTR(handle), &len));
+  const size_t reserved_string_size = 256;
   std::vector<std::vector<char>> names(len);
   std::vector<char*> ptr_names(len);
   for (int i = 0; i < len; ++i) {
-    names[i].resize(256);
+    names[i].resize(reserved_string_size);
     ptr_names[i] = names[i].data();
   }
   int out_len;
-  CHECK_CALL(LGBM_DatasetGetFeatureNames(R_GET_PTR(handle),
-    ptr_names.data(), &out_len));
+  size_t required_string_size;
+  CHECK_CALL(
+    LGBM_DatasetGetFeatureNames(
+      R_GET_PTR(handle),
+      len, &out_len,
+      reserved_string_size, &required_string_size,
+      ptr_names.data()));
   CHECK_EQ(len, out_len);
+  CHECK_GE(reserved_string_size, required_string_size);
   auto merge_str = Join<char*>(ptr_names, "\t");
   EncodeChar(feature_names, merge_str.c_str(), buf_len, actual_len, merge_str.size() + 1);
   R_API_END();

include/LightGBM/c_api.h

@@ -280,13 +280,21 @@ LIGHTGBM_C_EXPORT int LGBM_DatasetSetFeatureNames(DatasetHandle handle,
 /*!
  * \brief Get feature names of dataset.
  * \param handle Handle of dataset
- * \param[out] feature_names Feature names, should pre-allocate memory
+ * \param len Number of ``char*`` pointers stored at ``out_strs``.
+ *            If smaller than the max size, only this many strings are copied
  * \param[out] num_feature_names Number of feature names
+ * \param buffer_len Size of pre-allocated strings.
+ *                   Content is copied up to ``buffer_len - 1`` and null-terminated
+ * \param[out] out_buffer_len String sizes required to do the full string copies
+ * \param[out] feature_names Feature names, should pre-allocate memory
  * \return 0 when succeed, -1 when failure happens
  */
 LIGHTGBM_C_EXPORT int LGBM_DatasetGetFeatureNames(DatasetHandle handle,
-                                                  char** feature_names,
-                                                  int* num_feature_names);
+                                                  const int len,
+                                                  int* num_feature_names,
+                                                  const size_t buffer_len,
+                                                  size_t* out_buffer_len,
+                                                  char** feature_names);
 
 /*!
  * \brief Free space for dataset.

python-package/lightgbm/basic.py

@@ -1553,6 +1553,38 @@ def set_group(self, group):
             self.set_field('group', group)
         return self
 
+    def get_feature_name(self):
+        """Get the names of columns (features) in the Dataset.
+
+        Returns
+        -------
+        feature_names : list
+            The names of columns (features) in the Dataset.
+        """
+        if self.handle is None:
+            raise LightGBMError("Cannot get feature_name before construct dataset")
+        num_feature = self.num_feature()
+        tmp_out_len = ctypes.c_int(0)
+        reserved_string_buffer_size = 255
+        required_string_buffer_size = ctypes.c_size_t(0)
+        string_buffers = [ctypes.create_string_buffer(reserved_string_buffer_size) for i in range_(num_feature)]
+        ptr_string_buffers = (ctypes.c_char_p * num_feature)(*map(ctypes.addressof, string_buffers))
+        _safe_call(_LIB.LGBM_DatasetGetFeatureNames(
+            self.handle,
+            num_feature,
+            ctypes.byref(tmp_out_len),
+            reserved_string_buffer_size,
+            ctypes.byref(required_string_buffer_size),
+            ptr_string_buffers))
+        if num_feature != tmp_out_len.value:
+            raise ValueError("Length of feature names doesn't equal with num_feature")
+        if reserved_string_buffer_size < required_string_buffer_size.value:
+            raise BufferError(
+                "Allocated feature name buffer size ({}) was inferior to the needed size ({})."
+                .format(reserved_string_buffer_size, required_string_buffer_size.value)
+            )
+        return [string_buffers[i].value.decode('utf-8') for i in range_(num_feature)]
+
     def get_label(self):
         """Get the label of the Dataset.
 

src/c_api.cpp

@@ -1110,15 +1110,23 @@ int LGBM_DatasetSetFeatureNames(
 }
 
 int LGBM_DatasetGetFeatureNames(
-  DatasetHandle handle,
-  char** feature_names,
-  int* num_feature_names) {
-  API_BEGIN();
+    DatasetHandle handle,
+    const int len,
+    int* num_feature_names,
+    const size_t buffer_len,
+    size_t* out_buffer_len,
+    char** feature_names) {
+  API_BEGIN();
+  *out_buffer_len = 0;
   auto dataset = reinterpret_cast<Dataset*>(handle);
   auto inside_feature_name = dataset->feature_names();
   *num_feature_names = static_cast<int>(inside_feature_name.size());
   for (int i = 0; i < *num_feature_names; ++i) {
-    std::memcpy(feature_names[i], inside_feature_name[i].c_str(), inside_feature_name[i].size() + 1);
+    if (i < len) {
+      std::memcpy(feature_names[i], inside_feature_name[i].c_str(), std::min(inside_feature_name[i].size() + 1, buffer_len));
+      feature_names[i][buffer_len - 1] = '\0';
+    }
+    *out_buffer_len = std::max(inside_feature_name[i].size() + 1, *out_buffer_len);
   }
   API_END();
 }

tests/python_package_test/test_basic.py

@@ -271,15 +271,20 @@ def check_asserts(data):
             self.assertTrue(np.all(np.isclose([data.label[0], data.weight[0], data.init_score[0]],
                                               data.label[0])))
             self.assertAlmostEqual(data.label[1], data.weight[1])
+            self.assertListEqual(data.feature_name, data.get_feature_name())
 
         X, y = load_breast_cancer(True)
         sequence = np.ones(y.shape[0])
         sequence[0] = np.nan
         sequence[1] = np.inf
-        lgb_data = lgb.Dataset(X, sequence, weight=sequence, init_score=sequence).construct()
+        feature_names = ['f{0}'.format(i) for i in range(X.shape[1])]
+        lgb_data = lgb.Dataset(X, sequence,
+                               weight=sequence, init_score=sequence,
+                               feature_name=feature_names).construct()
         check_asserts(lgb_data)
         lgb_data = lgb.Dataset(X, y).construct()
         lgb_data.set_label(sequence)
         lgb_data.set_weight(sequence)
         lgb_data.set_init_score(sequence)
+        lgb_data.set_feature_name(feature_names)
         check_asserts(lgb_data)