Fix config.get for iterable sections (#2020)

* Fix config.get for iterable segments

* Add unit tests for coverage

* Lint fixes

src/sqlfluff/core/config.py

@@ -615,7 +615,11 @@ def get(
         self, val: str, section: Union[str, Iterable[str]] = "core", default: Any = None
     ):
         """Get a particular value from the config."""
-        return self._configs[section].get(val, default)
+        section_dict = self.get_section(section)
+        if section_dict is None:
+            return default
+
+        return section_dict.get(val, default)
 
     def get_section(self, section: Union[str, Iterable[str]]) -> Union[dict, None]:
         """Return a whole section of config as a dict.

test/core/config_test.py

@@ -23,6 +23,11 @@
     "bar": {"foo": "barbar"},
 }
 
+config_b = {
+    "core": {"rules": "L007"},
+    "rules": {"L007": {"operator_new_lines": "before"}},
+}
+
 
 @pytest.fixture
 def mock_xdg_home(monkeypatch):
@@ -248,3 +253,29 @@ def test__config__glob_include_config_tests():
         assert ("L052", 12, 9) in violations[k]
         assert ("L027", 10, 8) in violations[k]
         assert "L044" not in [c[0] for c in violations[k]]
+
+
+def test__config__get_section():
+    """Test FluffConfig.get_section method."""
+    cfg = FluffConfig(config_b)
+
+    assert cfg.get_section("core").get("rules", None) == "L007"
+    assert cfg.get_section(["rules", "L007"]) == {"operator_new_lines": "before"}
+    assert cfg.get_section("non_existent") is None
+
+
+def test__config__get():
+    """Test FluffConfig.get method."""
+    cfg = FluffConfig(config_b)
+
+    assert cfg.get("rules") == "L007"
+    assert cfg.get("rulez") is None
+    assert cfg.get("rulez", section="core", default=123) == 123
+    assert (
+        cfg.get("operator_new_lines", section=["rules", "L007"], default=None)
+        == "before"
+    )
+    assert (
+        cfg.get("operator_new_lines", section=["rules", "ASDFSDG007"], default=None)
+        is None
+    )