avoid copy when not necessary

Author: samuelgarcia

src/spikeinterface/curation/auto_merge.py

@@ -195,7 +195,19 @@ def compute_merge_unit_groups(
             raise ValueError(f"preset must be one of {list(_compute_merge_presets.keys())}")
         steps = _compute_merge_presets[preset]
 
-    if force_copy:
+    # check at least one extension is needed
+    at_least_one_extension_to_compute = False
+    for step in steps:
+        assert step in _default_step_params, f"{step} is not a valid step"
+        if step in _required_extensions:
+            for ext in _required_extensions[step]:
+                if sorting_analyzer.has_extension(ext):
+                    continue
+                if not compute_needed_extensions:
+                    raise ValueError(f"{step} requires {ext} extension")
+                at_least_one_extension_to_compute = True
+
+    if force_copy and at_least_one_extension_to_compute:
         # To avoid erasing the extensions of the user
         sorting_analyzer = sorting_analyzer.copy()
 
@@ -205,14 +217,10 @@ def compute_merge_unit_groups(
 
     for step in steps:
 
-        assert step in _default_step_params, f"{step} is not a valid step"
-
         if step in _required_extensions:
             for ext in _required_extensions[step]:
                 if sorting_analyzer.has_extension(ext):
                     continue
-                if not compute_needed_extensions:
-                    raise ValueError(f"{step} requires {ext} extension")
 
                 # special case for templates
                 if ext == "templates" and not sorting_analyzer.has_extension("random_spikes"):