datacollector: Add warning when returning empty dataframe with no rep…

…orters defined

With this commit the get_model_vars_dataframe() and get_agent_vars_dataframe() raise warnings when no model_reporters or agent_reporteres are defined, and it thus returns an empty dataframe. This warning makes it clearer why it's returning an empty dataframe.

Two of my students were stuck on this quite a while, since they were requesting the get_model_vars_dataframe() while having defined only agent reporters.

mesa/datacollection.py

@@ -216,6 +216,12 @@ def get_model_vars_dataframe(self):
         The DataFrame has one column for each model variable, and the index is
         (implicitly) the model tick.
         """
+        # Check if self.model_reporters dictionary is empty, if so raise warning
+        if not self.model_reporters:
+            raise UserWarning(
+                "No model reporters have been defined in the DataCollector, returning empty DataFrame."
+            )
+
         return pd.DataFrame(self.model_vars)
 
     def get_agent_vars_dataframe(self):
@@ -224,6 +230,12 @@ def get_agent_vars_dataframe(self):
         The DataFrame has one column for each variable, with two additional
         columns for tick and agent_id.
         """
+        # Check if self.agent_reporters dictionary is empty, if so raise warning
+        if not self.agent_reporters:
+            raise UserWarning(
+                "No agent reporters have been defined in the DataCollector, returning empty DataFrame."
+            )
+
         all_records = itertools.chain.from_iterable(self._agent_records.values())
         rep_names = list(self.agent_reporters)