fix: complain about CannotBeAwkward earlier, before reading data.

src/uproot/_util.py

@@ -570,6 +570,14 @@ def awkward_form(model, file, context):
                 _primitive_awkward_form[model] = awkward.forms.from_json('"float32"')
             elif model == numpy.dtype(numpy.float64):
                 _primitive_awkward_form[model] = awkward.forms.from_json('"float64"')
+            elif model.fields is not None:
+                fields = []
+                contents = []
+                for field, (dtype, _) in model.fields.items():
+                    fields.append(field)
+                    contents.append(awkward_form(dtype, file, context))
+                # directly return; don't cache RecordForms in _primitive_awkward_form
+                return awkward.forms.RecordForm(contents, fields)
             else:
                 raise AssertionError(f"{model!r}: {type(model)}")
 

src/uproot/behaviors/TBranch.py

@@ -2479,6 +2479,26 @@ def postprocess(self, chunk, cursor, context, file):
 
         return self
 
+    def _awkward_check(self, interpretation):
+        try:
+            interpretation.awkward_form(self.file)
+        except uproot.interpretation.objects.CannotBeAwkward as err:
+            raise ValueError(
+                """cannot produce Awkward Arrays for interpretation {} because
+
+    {}
+
+instead, try library="np" instead of library="ak" or globally set uproot.default_library
+
+in file {}
+in object {}""".format(
+                    repr(interpretation),
+                    err.because,
+                    self.file.file_path,
+                    self.object_path,
+                )
+            ) from err
+
     def debug(
         self,
         entry,
@@ -2986,6 +3006,7 @@ def _ranges_or_baskets_to_arrays(
     range_args = {}
     range_original_index = {}
     original_index = 0
+    branchid_to_branch = {}
 
     for cache_key in branchid_interpretation:
         branchid_num_baskets[cache_key] = 0
@@ -3006,12 +3027,20 @@ def _ranges_or_baskets_to_arrays(
 
         original_index += 1
 
+        branchid_to_branch[branch.cache_key] = branch
+
     for cache_key, interpretation in branchid_interpretation.items():
         if branchid_num_baskets[cache_key] == 0 and cache_key not in arrays:
             arrays[cache_key] = interpretation.final_array(
                 {}, 0, 0, [0], library, None, interp_options
             )
 
+        # check for CannotBeAwkward errors on the main thread before reading any data
+        if isinstance(library, uproot.interpretation.library.Awkward) and isinstance(
+            interpretation, uproot.interpretation.objects.AsObjects
+        ):
+            branchid_to_branch[cache_key]._awkward_check(interpretation)
+
     hasbranches._file.source.chunks(ranges, notifications=notifications)
 
     def replace(ranges_or_baskets, original_index, basket):

src/uproot/interpretation/library.py

@@ -568,27 +568,9 @@ def finalize(self, array, branch, interpretation, entry_start, entry_stop, optio
             return _awkward_add_doc(awkward, awkward.Array(layout), branch, ak_add_doc)
 
         elif isinstance(interpretation, uproot.interpretation.objects.AsObjects):
-            try:
-                form = json.loads(
-                    interpretation.awkward_form(interpretation.branch.file).to_json()
-                )
-            except uproot.interpretation.objects.CannotBeAwkward as err:
-                raise ValueError(
-                    """cannot produce Awkward Arrays for interpretation {} because
-
-    {}
-
-instead, try library="np" instead of library="ak" or globally set uproot.default_library
-
-in file {}
-in object {}""".format(
-                        repr(interpretation),
-                        err.because,
-                        interpretation.branch.file.file_path,
-                        interpretation.branch.object_path,
-                    )
-                ) from err
-
+            form = json.loads(
+                interpretation.awkward_form(interpretation.branch.file).to_json()
+            )
             unlabeled = awkward.from_iter(
                 (_object_to_awkward_json(form, x) for x in array), highlevel=False
             )