Improve error message on invalid lm setup (#8378)

Author: chenmoneygithub

dspy/predict/predict.py

@@ -99,7 +99,22 @@ def _forward_preprocess(self, **kwargs):
 
         # Get the right LM to use.
         lm = kwargs.pop("lm", self.lm) or settings.lm
-        assert isinstance(lm, BaseLM), "No LM is loaded."
+
+        if lm is None:
+            raise ValueError(
+                "No LM is loaded. Please configure the LM using `dspy.configure(lm=dspy.LM(...))`. e.g, "
+                "`dspy.configure(lm=dspy.LM('openai/gpt-4o-mini'))`"
+            )
+
+        if isinstance(lm, str):
+            # Many users mistakenly use `dspy.configure(lm="openai/gpt-4o-mini")` instead of
+            # `dspy.configure(lm=dspy.LM("openai/gpt-4o-mini"))`, so we are providing a specific error message.
+            raise ValueError(
+                f"LM must be an instance of `dspy.BaseLM`, not a string. Instead of using a string like "
+                f"'dspy.configure(lm=\"{lm}\")', please configure the LM like 'dspy.configure(lm=dspy.LM(\"{lm}\"))'"
+            )
+        elif not isinstance(lm, BaseLM):
+            raise ValueError(f"LM must be an instance of `dspy.BaseLM`, not {type(lm)}. Received `lm={lm}`.")
 
         # If temperature is unset or <=0.15, and n > 1, set temperature to 0.7 to keep randomness.
         temperature = config.get("temperature") or lm.kwargs.get("temperature")

tests/predict/test_predict.py

@@ -582,6 +582,28 @@ def test_positional_arguments():
     )
 
 
+def test_error_message_on_invalid_lm_setup():
+    # No LM is loaded.
+    with pytest.raises(ValueError, match="No LM is loaded"):
+        Predict("question -> answer")(question="Why did a chicken cross the kitchen?")
+
+    # LM is a string.
+    dspy.configure(lm="openai/gpt-4o-mini")
+    with pytest.raises(ValueError) as e:
+        Predict("question -> answer")(question="Why did a chicken cross the kitchen?")
+
+    assert "LM must be an instance of `dspy.BaseLM`, not a string." in str(e.value)
+
+    def dummy_lm():
+        pass
+
+    # LM is not an instance of dspy.BaseLM.
+    dspy.configure(lm=dummy_lm)
+    with pytest.raises(ValueError) as e:
+        Predict("question -> answer")(question="Why did a chicken cross the kitchen?")
+    assert "LM must be an instance of `dspy.BaseLM`, not <class 'function'>." in str(e.value)
+
+
 @pytest.mark.parametrize("adapter_type", ["chat", "json"])
 def test_field_constraints(adapter_type):
     class SpyLM(dspy.LM):