MlflowLogger limit parameter value length to 250 char (#5893)

pytorch_lightning/loggers/mlflow.py

@@ -150,6 +150,12 @@ class MLFlowLogger(LightningLoggerBase):
         params = self._convert_params(params)
         params = self._flatten_dict(params)
         for k, v in params.items():
+            if len(str(v)) > 250:
+                rank_zero_warn(
+                    f"Mlflow only allows parameters with up to 250 characters. Discard {k}={v}", RuntimeWarning
+                )
+                continue
+
             self.experiment.log_param(self.run_id, k, v)
 
     @rank_zero_only

tests/loggers/test_mlflow.py

@@ -184,3 +184,18 @@ def test_mlflow_logger_with_unexpected_characters(client, mlflow, tmpdir):
 
     with pytest.warns(RuntimeWarning, match='special characters in metric name'):
         logger.log_metrics(metrics)
+
+
+@mock.patch('pytorch_lightning.loggers.mlflow.mlflow')
+@mock.patch('pytorch_lightning.loggers.mlflow.MlflowClient')
+def test_mlflow_logger_with_long_param_value(client, mlflow, tmpdir):
+    """
+    Test that the logger raises warning with special characters not accepted by MLFlow.
+    """
+    logger = MLFlowLogger('test', save_dir=tmpdir)
+    value = 'test' * 100
+    key = 'test_param'
+    params = {key: value}
+
+    with pytest.warns(RuntimeWarning, match=f'Discard {key}={value}'):
+        logger.log_hyperparams(params)