fix: set _max_workers in ThreadPoolExecutor with non-None initializer (#1254)

* Add _max_workers definition to ThreadPoolExecutor when arg is not None

* Add test for ThreadPoolExecutor for None and 1 max_workers

* style: pre-commit fixes

---------

Co-authored-by: Nick Manganelli <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

Diff for: src/uproot/source/futures.py

@@ -217,6 +217,8 @@ def __init__(self, max_workers: int | None = None):
                 import multiprocessing
 
                 self._max_workers = multiprocessing.cpu_count()
+        else:
+            self._max_workers = max_workers
 
         self._work_queue = queue.Queue()
         self._workers = []

Diff for: tests/test_1254_test_threadpool_executor_for_dask.py

@@ -0,0 +1,44 @@
+import pytest
+import skhep_testdata
+
+import uproot
+
+pytest.importorskip("pandas")
+
+
+def test_decompression_threadpool_executor_for_dask():
+
+    class TestThreadPoolExecutor(uproot.source.futures.ThreadPoolExecutor):
+        def __init__(self, max_workers=None):
+            super().__init__(max_workers=max_workers)
+            self.submit_count = 0
+
+        def submit(self, task, /, *args, **kwargs):
+            self.submit_count += 1
+            super().submit(task, *args, **kwargs)
+
+    implicitexecutor = TestThreadPoolExecutor(max_workers=None)
+
+    a = uproot.dask(
+        {skhep_testdata.data_path("uproot-sample-6.20.04-uncompressed.root"): "sample"},
+        decompression_executor=implicitexecutor,
+    )
+
+    a["i4"].compute()
+
+    assert implicitexecutor.max_workers > 0
+
+    assert implicitexecutor.submit_count > 0
+
+    explicitexecutor = TestThreadPoolExecutor(max_workers=1)
+
+    b = uproot.dask(
+        {skhep_testdata.data_path("uproot-sample-6.20.04-uncompressed.root"): "sample"},
+        decompression_executor=explicitexecutor,
+    )
+
+    b["i4"].compute()
+
+    assert explicitexecutor.max_workers == 1
+
+    assert explicitexecutor.submit_count > 0