fix(dspy): keep fields when copying an Example with no visible fields

Example.__init__ guarded the base branch with `if base and ...`. Since
__len__ skips dspy_* keys, an Example that only holds dspy_* metadata or
an input/label split (e.g. `Example().with_inputs("question")`) is falsy,
so copy(), without() and Example(base=...) silently dropped its store and
input keys. Check the type only, as the isinstance test already rules
out None.

Author: Zhuoxi2000

dspy/primitives/example.py

@@ -109,7 +109,7 @@ def __init__(self, base=None, **kwargs):
         self._input_keys = None
 
         # Initialize from a base Example if provided
-        if base and isinstance(base, type(self)):
+        if isinstance(base, type(self)):
             self._store = base._store.copy()
             # Preserve the input/label split so copy()/without()/Example(base=...)
             # don't silently break .inputs()/.labels() on the new instance.
@@ -119,7 +119,7 @@ def __init__(self, base=None, **kwargs):
                 self._input_keys = set(base_input_keys)
 
         # Initialize from a dict if provided
-        elif base and isinstance(base, dict):
+        elif isinstance(base, dict):
             self._store = base.copy()
 
         # Update with provided kwargs

tests/primitives/test_example.py

@@ -160,6 +160,19 @@ def test_example_copy_preserves_input_keys():
     assert Example(base=example)._input_keys == {"question"}
 
 
+def test_example_copy_preserves_fields_when_example_has_no_visible_fields():
+    # An Example whose only contents are `dspy_*` metadata or an input/label split has len() == 0.
+    # Copying it must still carry over its store and input keys.
+    template = Example().with_inputs("question")
+    filled = template.copy(question="q", answer="a")
+    assert filled._input_keys == {"question"}
+    assert filled.inputs().toDict() == {"question": "q"}
+
+    metadata_only = Example(dspy_uuid="u1", dspy_split="train")
+    assert metadata_only.copy().toDict() == {"dspy_uuid": "u1", "dspy_split": "train"}
+    assert Example(base=metadata_only).toDict() == {"dspy_uuid": "u1", "dspy_split": "train"}
+
+
 def test_prediction_copy_does_not_require_input_keys():
     # Example subclasses (e.g. Prediction) don't keep _input_keys; copy() must not crash.
     import dspy