Add tests for merge_scan_task

tests/conftest.py

@@ -10,6 +10,16 @@
 from daft.table import MicroPartition
 
 
+@pytest.fixture(scope="session", autouse=True)
+def set_configs():
+    """Sets global Daft config for testing"""
+    daft.context.set_config(
+        # Disables merging of ScanTasks
+        merge_scan_tasks_min_size_bytes=0,
+        merge_scan_tasks_max_size_bytes=0,
+    )
+
+
 def pytest_configure(config):
     config.addinivalue_line(
         "markers", "integration: mark test as an integration test that runs with external dependencies"

tests/io/test_merge_scan_tasks.py

@@ -0,0 +1,69 @@
+from __future__ import annotations
+
+import contextlib
+
+import pytest
+
+import daft
+
+
+@contextlib.contextmanager
+def override_merge_scan_tasks_configs(merge_scan_tasks_min_size_bytes: int, merge_scan_tasks_max_size_bytes: int):
+    config = daft.context.get_context().daft_config
+    original_merge_scan_tasks_min_size_bytes = config.merge_scan_tasks_min_size_bytes
+    original_merge_scan_tasks_max_size_bytes = config.merge_scan_tasks_max_size_bytes
+
+    try:
+        daft.context.set_config(
+            merge_scan_tasks_min_size_bytes=merge_scan_tasks_min_size_bytes,
+            merge_scan_tasks_max_size_bytes=merge_scan_tasks_max_size_bytes,
+        )
+        yield
+    finally:
+        daft.context.set_config(
+            merge_scan_tasks_min_size_bytes=original_merge_scan_tasks_min_size_bytes,
+            merge_scan_tasks_max_size_bytes=original_merge_scan_tasks_max_size_bytes,
+        )
+
+
+@pytest.fixture(scope="function")
+def csv_files(tmpdir):
+    """Writes 3 CSV files, each of 10 bytes in size, to tmpdir and yield tmpdir"""
+
+    for i in range(3):
+        path = tmpdir / f"file.{i}.csv"
+        path.write_text("a,b,c\n1,2,", "utf8")  # 10 bytes
+
+    return tmpdir
+
+
+def test_merge_scan_task_exceed_max(csv_files):
+    with override_merge_scan_tasks_configs(0, 0):
+        df = daft.read_csv(str(csv_files))
+        assert (
+            df.num_partitions() == 3
+        ), "Should have 3 partitions since all merges are more than the maximum (>0 bytes)"
+
+
+def test_merge_scan_task_below_max(csv_files):
+    with override_merge_scan_tasks_configs(1, 20):
+        df = daft.read_csv(str(csv_files))
+        assert (
+            df.num_partitions() == 2
+        ), "Should have 2 partitions [(CSV1, CSV2), (CSV3)] since the second merge is too large (>20 bytes)"
+
+
+def test_merge_scan_task_above_min(csv_files):
+    with override_merge_scan_tasks_configs(0, 40):
+        df = daft.read_csv(str(csv_files))
+        assert (
+            df.num_partitions() == 2
+        ), "Should have 2 partitions [(CSV1, CSV2), (CSV3)] since the first merge is above the minimum (>0 bytes)"
+
+
+def test_merge_scan_task_below_min(csv_files):
+    with override_merge_scan_tasks_configs(35, 40):
+        df = daft.read_csv(str(csv_files))
+        assert (
+            df.num_partitions() == 1
+        ), "Should have 1 partition [(CSV1, CSV2, CSV3)] since both merges are below the minimum and maximum"