Deprecate prefix argument in ModelCheckpoint

CHANGELOG.md

@@ -50,6 +50,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Deprecated
 
+- Deprecated `prefix` argument in `ModelCheckpoint` ([#4765](https://github.com/PyTorchLightning/pytorch-lightning/pull/4765))
 
 
 ### Removed

pytorch_lightning/callbacks/model_checkpoint.py

@@ -75,6 +75,10 @@ class ModelCheckpoint(Callback):
             saved (``model.save_weights(filepath)``), else the full model
             is saved (``model.save(filepath)``).
         period: Interval (number of epochs) between checkpoints.
+        prefix: A string to put at the beginning of checkpoint filename.
+
+            .. warning::
+               This argument has been deprecated in v1.1 and will be removed in v1.3
 
         dirpath: directory to save the model file.
 
@@ -167,6 +171,12 @@ def __init__(
         if save_top_k is None and monitor is not None:
             self.save_top_k = 1
 
+        if prefix:
+            rank_zero_warn(
+                'Argument `prefix` is deprecated in v1.1 and will be removed in v1.3.'
+                ' Please prepend your prefix in `filename` instead.', DeprecationWarning
+            )
+
         self.__init_monitor_mode(monitor, mode)
         self.__init_ckpt_dir(filepath, dirpath, filename, save_top_k)
         self.__validate_init_configuration()
@@ -380,7 +390,11 @@ def _format_checkpoint_name(
                 if name not in metrics:
                     metrics[name] = 0
             filename = filename.format(**metrics)
-        return cls.CHECKPOINT_JOIN_CHAR.join([txt for txt in (prefix, filename) if txt])
+
+        if prefix:
+            filename = cls.CHECKPOINT_JOIN_CHAR.join([prefix, filename])
+
+        return filename
 
     def format_checkpoint_name(
         self, epoch: int, step: int, metrics: Dict[str, Any], ver: Optional[int] = None

tests/test_deprecated.py

@@ -19,6 +19,10 @@ def test_tbd_remove_in_v1_3_0(tmpdir):
         callback = ModelCheckpoint()
         Trainer(checkpoint_callback=callback, callbacks=[], default_root_dir=tmpdir)
 
+    # Deprecate prefix
+    with pytest.deprecated_call(match='will be removed in v1.3'):
+        callback = ModelCheckpoint(prefix='temp')
+
 
 def test_tbd_remove_in_v1_2_0():
     with pytest.deprecated_call(match='will be removed in v1.2'):