Fixes lost cuts in iterate. (#195)

* Fixes lost cuts in iterate.

* Black and Flake8.

tests/test_0194-fix-lost-cuts-in-iterate.py

@@ -0,0 +1,17 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/uproot4/blob/master/LICENSE
+
+from __future__ import absolute_import
+
+import numpy
+import pytest
+import skhep_testdata
+
+import uproot4
+
+
+def test():
+    with uproot4.open(skhep_testdata.data_path("uproot-Zmumu.root")) as f:
+        for arrays in f["events"].iterate(
+            "px1", step_size=1000, cut="px1 > 0", library="np"
+        ):
+            assert numpy.all(arrays["px1"] > 0)

uproot4/behaviors/TBranch.py

@@ -1375,13 +1375,13 @@ def iterate(
                     self.object_path,
                 )
 
-                expression_context = [
+                minimized_expression_context = [
                     (e, c)
                     for e, c in expression_context
                     if c["is_primary"] and not c["is_cut"]
                 ]
 
-                arrays = library.group(output, expression_context, how)
+                arrays = library.group(output, minimized_expression_context, how)
 
                 next_baskets = {}
                 for branch, basket_num, basket in ranges_or_baskets: