Docs and Tests for "gpus" Trainer Argument (#593)

* add table for gpus argument

* fix typo in error message

* tests for supported values

* tests for unsupported values

* fix typo

* add table for gpus argument

* fix typo in error message

* tests for supported values

* tests for unsupported values

* fix typo

* fix typo list->str

* fix travis warning "line too long"

pytorch_lightning/trainer/distrib_parts.py

@@ -164,6 +164,38 @@ Make sure you're on a GPU machine. You can set as many GPUs as you want.
     # RECOMMENDED use DistributedDataParallel
     trainer = Trainer(gpus=8, distributed_backend='ddp')
 
+Custom device selection
+-----------------------
+
+The number of GPUs can also be selected with a list of indices or a string containing
+a comma separated list of GPU ids.
+The table below lists examples of possible input formats and how they are interpreted by Lightning.
+Note in particular the difference between `gpus=0`, `gpus=[0]` and `gpus="0"`.
+
++---------------+-----------+---------------------+---------------------------------+
+| `gpus`        | Type      | Parsed              | Meaning                         |
++===============+===========+=====================+=================================+
+| None          | NoneType  | None                | CPU                             |
++---------------+-----------+---------------------+---------------------------------+
+| 0             | int       | None                | CPU                             |
++---------------+-----------+---------------------+---------------------------------+
+| 3             | int       | [0, 1, 2]           | first 3 GPUs                    |
++---------------+-----------+---------------------+---------------------------------+
+| -1            | int       | [0, 1, 2, ...]      | all available GPUs              |
++---------------+-----------+---------------------+---------------------------------+
+| [0]           | list      | [0]                 | GPU 0                           |
++---------------+-----------+---------------------+---------------------------------+
+| [1, 3]        | list      | [1, 3]              | GPUs 1 and 3                    |
++---------------+-----------+---------------------+---------------------------------+
+| "0"           | str       | [0]                 | GPU 0                           |
++---------------+-----------+---------------------+---------------------------------+
+| "3"           | str       | [3]                 | GPU 3                           |
++---------------+-----------+---------------------+---------------------------------+
+| "1, 3"        | str       | [1, 3]              | GPUs 1 and 3                    |
++---------------+-----------+---------------------+---------------------------------+
+| "-1"          | str       | [0, 1, 2, ...]      | all available GPUs              |
++---------------+-----------+---------------------+---------------------------------+
+
 
 Multi-node
 ----------
@@ -531,7 +563,7 @@ def parse_gpu_ids(gpus):
     gpus = sanitize_gpu_ids(gpus)
 
     if not gpus:
-        raise MisconfigurationException("GPUs requested but non are available.")
+        raise MisconfigurationException("GPUs requested but none are available.")
     return gpus
 
 

tests/test_gpu_models.py

@@ -351,9 +351,14 @@ test_parse_gpu_ids_data = [
     pytest.param(None, None),
     pytest.param(0, None),
     pytest.param(1, [0]),
-    pytest.param(-1, list(range(PRETEND_N_OF_GPUS)), id="-1 - use all gpus"),
-    pytest.param('-1', list(range(PRETEND_N_OF_GPUS)), id="'-1' - use all gpus"),
     pytest.param(3, [0, 1, 2]),
+    pytest.param(-1, list(range(PRETEND_N_OF_GPUS)), id="-1 - use all gpus"),
+    pytest.param([0], [0]),
+    pytest.param([1, 3], [1, 3]),
+    pytest.param('0', [0]),
+    pytest.param('3', [3]),
+    pytest.param('1, 3', [1, 3]),
+    pytest.param('-1', list(range(PRETEND_N_OF_GPUS)), id="'-1' - use all gpus"),
 ]
 
 
@@ -363,6 +368,33 @@ def test_parse_gpu_ids(mocked_device_count, gpus, expected_gpu_ids):
     assert parse_gpu_ids(gpus) == expected_gpu_ids
 
 
+test_parse_gpu_invalid_inputs_data = [
+    pytest.param(0.1),
+    pytest.param(-2),
+    pytest.param(False),
+    pytest.param([]),
+    pytest.param([-1]),
+    pytest.param([None]),
+    pytest.param(['0']),
+    pytest.param((0, 1)),
+]
+
+
+@pytest.mark.gpus_param_tests
+@pytest.mark.parametrize(['gpus'], test_parse_gpu_invalid_inputs_data)
+def test_parse_gpu_fail_on_unsupported_inputs(mocked_device_count, gpus):
+    with pytest.raises(MisconfigurationException):
+        parse_gpu_ids(gpus)
+
+
+@pytest.mark.gpus_param_tests
+@pytest.mark.parametrize("gpus", [''])
+def test_parse_gpu_fail_on_empty_string(mocked_device_count, gpus):
+    # This currently results in a ValueError instead of MisconfigurationException
+    with pytest.raises(ValueError):
+        parse_gpu_ids(gpus)
+
+
 @pytest.mark.gpus_param_tests
 @pytest.mark.parametrize("gpus", [[1, 2, 19], -1, '-1'])
 def test_parse_gpu_fail_on_non_existant_id(mocked_device_count_0, gpus):