feat: Add flatten array function (#562)

* Updated functions.rs, test_functions.py - Flatten

* Updated functions.rs - Formatting

* Updated test_functions.py - Converted test array to a literal for testing

* Updated test_functions.py - Linting

datafusion/tests/test_functions.py

@@ -238,6 +238,15 @@ def py_arr_replace(arr, from_, to, n=None):
 
         return new_arr
 
+    def py_flatten(arr):
+        result = []
+        for elem in arr:
+            if isinstance(elem, list):
+                result.extend(py_flatten(elem))
+            else:
+                result.append(elem)
+        return result
+
     col = column("arr")
     test_items = [
         [
@@ -432,6 +441,7 @@ def py_arr_replace(arr, from_, to, n=None):
             f.list_slice(col, literal(-1), literal(2)),
             lambda: [arr[-1:2] for arr in data],
         ],
+        [f.flatten(literal(data)), lambda: [py_flatten(data)]],
     ]
 
     for stmt, py_expr in test_items:

src/functions.rs

@@ -409,6 +409,7 @@ scalar_function!(array_replace_all, ArrayReplaceAll);
 scalar_function!(list_replace_all, ArrayReplaceAll);
 scalar_function!(array_slice, ArraySlice);
 scalar_function!(list_slice, ArraySlice);
+scalar_function!(flatten, Flatten);
 
 aggregate_function!(approx_distinct, ApproxDistinct);
 aggregate_function!(approx_median, ApproxMedian);
@@ -651,6 +652,7 @@ pub(crate) fn init_module(m: &PyModule) -> PyResult<()> {
     m.add_wrapped(wrap_pyfunction!(list_replace_all))?;
     m.add_wrapped(wrap_pyfunction!(array_slice))?;
     m.add_wrapped(wrap_pyfunction!(list_slice))?;
+    m.add_wrapped(wrap_pyfunction!(flatten))?;
 
     Ok(())
 }