Fix iterating over a DummyLogger when fast_dev_run > 0 (#10232)

Author: awaelchli

CHANGELOG.md

@@ -677,6 +677,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed an issue with `pl.utilities.seed.reset_seed` converting the `PL_SEED_WORKERS` environment variable to `bool` ([#10099](https://github.com/PyTorchLightning/pytorch-lightning/pull/10099))
 
 
+- Fixed iterating over a logger collection when `fast_dev_run > 0` ([#10232](https://github.com/PyTorchLightning/pytorch-lightning/pull/10232))
+
+
+
 ## [1.4.9] - 2021-09-30
 
 - Fixed `lr_find` to generate same results on multiple calls ([#9704](https://github.com/PyTorchLightning/pytorch-lightning/pull/9704))

pytorch_lightning/loggers/base.py

@@ -510,6 +510,10 @@ def __getitem__(self, idx) -> "DummyLogger":
         # enables self.logger[0].experiment.add_image(...)
         return self
 
+    def __iter__(self):
+        # if DummyLogger is substituting a logger collection, pretend it is empty
+        yield from ()
+
 
 def merge_dicts(
     dicts: Sequence[Mapping],

tests/loggers/test_base.py

@@ -227,6 +227,13 @@ def test_dummylogger_support_indexing():
     assert logger[0] == logger
 
 
+def test_dummylogger_empty_iterable():
+    """Test that DummyLogger represents an empty iterable."""
+    logger = DummyLogger()
+    for _ in logger:
+        assert False
+
+
 def test_dummylogger_noop_method_calls():
     """Test that the DummyLogger methods can be called with arbitrary arguments."""
     logger = DummyLogger()