chore: fix ruff warnings (#3010)

* fix: specify dtype for `full`

* fix: drop use of sctype

* fix: don't use quadratic loop

Author: agoose77

src/awkward/_nplikes/typetracer.py

@@ -505,7 +505,7 @@ def _scalar_type_of(obj) -> DType:
     if is_unknown_scalar(obj):
         return obj.dtype
     else:
-        return numpy.obj2sctype(obj)
+        return numpy.array(obj).dtype
 
 
 def try_touch_data(array: Any):

src/awkward/operations/ak_enforce_type.py

@@ -630,7 +630,9 @@ def _recurse_option_any(
         # Converting to an unknown inside an option!
         if isinstance(type_.content, ak.types.UnknownType):
             return ak.contents.IndexedOptionArray(
-                ak.index.Index64(layout.backend.index_nplike.full(layout.length, -1)),
+                ak.index.Index64(
+                    layout.backend.index_nplike.full(layout.length, -1, dtype=np.int64)
+                ),
                 ak.forms.from_type(type_.content).length_zero_array(
                     backend=layout.backend
                 ),
@@ -696,7 +698,9 @@ def _recurse_any_option(
     # Converting to an unknown inside an option!
     if isinstance(type_.content, ak.types.UnknownType):
         return ak.contents.IndexedOptionArray(
-            ak.index.Index64(layout.backend.index_nplike.full(layout.length, -1)),
+            ak.index.Index64(
+                layout.backend.index_nplike.full(layout.length, -1, dtype=np.int64)
+            ),
             ak.forms.from_type(type_.content).length_zero_array(backend=layout.backend),
         )
     else:
@@ -1125,7 +1129,9 @@ def _recurse_record_any(
                 next_contents.append(
                     ak.contents.IndexedOptionArray(
                         ak.index.Index64(
-                            layout.backend.index_nplike.full(layout.length, -1)
+                            layout.backend.index_nplike.full(
+                                layout.length, -1, dtype=np.int64
+                            )
                         ),
                         ak.forms.from_type(next_type.content).length_zero_array(
                             backend=layout.backend
@@ -1170,7 +1176,9 @@ def _recurse_record_any(
                 next_contents.append(
                     ak.contents.IndexedOptionArray(
                         ak.index.Index64(
-                            layout.backend.index_nplike.full(layout.length, -1)
+                            layout.backend.index_nplike.full(
+                                layout.length, -1, dtype=np.int64
+                            )
                         ),
                         ak.forms.from_type(field_type.content).length_zero_array(
                             backend=layout.backend

src/awkward/operations/ak_to_dataframe.py

@@ -2,6 +2,9 @@
 
 from __future__ import annotations
 
+from functools import reduce
+from operator import iconcat
+
 import awkward as ak
 from awkward._dispatch import high_level_function
 from awkward._nplikes.numpy import Numpy
@@ -193,7 +196,8 @@ def recurse(layout, row_arrays, col_names):
             if layout.is_union:
                 return [(ak.operations.to_numpy(layout), row_arrays, col_names)]
             else:
-                return sum(
+                return reduce(
+                    iconcat,
                     (
                         recurse(layout._getitem_field(n), row_arrays, (*col_names, n))
                         for n in layout.fields
@@ -202,7 +206,8 @@ def recurse(layout, row_arrays, col_names):
                 )
 
         elif isinstance(layout, ak.contents.RecordArray):
-            return sum(
+            return reduce(
+                iconcat,
                 (
                     recurse(layout._getitem_field(n), row_arrays, (*col_names, n))
                     for n in layout.fields