From 4f22ae1feefa83eb54dae6f089f7920bc7b7a000 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Wed, 2 Mar 2022 17:31:53 +0530
Subject: [PATCH 01/21] Add AcceleratorRegistry

---
 pytorch_lightning/accelerators/__init__.py |  8 +++
 pytorch_lightning/accelerators/registry.py | 78 ++++++++++++++++++++++
 2 files changed, 86 insertions(+)
 create mode 100644 pytorch_lightning/accelerators/registry.py

diff --git a/pytorch_lightning/accelerators/__init__.py b/pytorch_lightning/accelerators/__init__.py
index 1c9e0024f39bd..d58defbfeb460 100644
--- a/pytorch_lightning/accelerators/__init__.py
+++ b/pytorch_lightning/accelerators/__init__.py
@@ -10,8 +10,16 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from pathlib import Path
+
 from pytorch_lightning.accelerators.accelerator import Accelerator  # noqa: F401
 from pytorch_lightning.accelerators.cpu import CPUAccelerator  # noqa: F401
 from pytorch_lightning.accelerators.gpu import GPUAccelerator  # noqa: F401
 from pytorch_lightning.accelerators.ipu import IPUAccelerator  # noqa: F401
+from pytorch_lightning.accelerators.registry import AcceleratorRegistry, register_accelerators  # noqa: F401
 from pytorch_lightning.accelerators.tpu import TPUAccelerator  # noqa: F401
+
+FILE_ROOT = Path(__file__).parent
+ACCELERATORS_BASE_MODULE = "pytorch_lightning.accelerators"
+
+register_accelerators(FILE_ROOT, ACCELERATORS_BASE_MODULE)
diff --git a/pytorch_lightning/accelerators/registry.py b/pytorch_lightning/accelerators/registry.py
new file mode 100644
index 0000000000000..62dd1d4e8a381
--- /dev/null
+++ b/pytorch_lightning/accelerators/registry.py
@@ -0,0 +1,78 @@
+# Copyright The PyTorch Lightning team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import importlib
+from inspect import getmembers, isclass
+from pathlib import Path
+from typing import Any, List, Optional
+
+from pytorch_lightning.accelerators.accelerator import Accelerator
+from pytorch_lightning.utilities.exceptions import MisconfigurationException
+
+
+class _AcceleratorRegistry(dict):
+    def register(self, accelerator: Accelerator, name: Optional[str] = None, override: bool = False) -> Accelerator:
+        """Registers an accelerator mapped to a name.
+
+        Args:
+            accelerator: the accelerator to be mapped.
+            name: the name that identifies the provided accelerator.
+            override: Whether to override an existing key.
+        """
+        if name is None:
+            name = accelerator.name()
+        elif not isinstance(name, str):
+            raise TypeError(f"`name` must be a str, found {name}")
+
+        if name in self and not override:
+            raise MisconfigurationException(f"'{name}' is already present in the registry. HINT: Use `override=True`.")
+        self[name] = accelerator
+        return accelerator
+
+    def get(self, name: str, default: Optional[Any] = None) -> Any:
+        """Calls the registered Accelerator and returns the Accelerator object.
+
+        Args:
+            name (str): the name that identifies a Accelerator, e.g. "tpu"
+        """
+        if name in self:
+            accelerator = self[name]
+            return accelerator()
+
+        if default is not None:
+            return default
+
+        err_msg = "'{}' not found in registry. Available names: {}"
+        available_names = ", ".join(sorted(self.keys())) or "none"
+        raise KeyError(err_msg.format(name, available_names))
+
+    def remove(self, name: str) -> None:
+        """Removes the registered accelerator by name."""
+        self.pop(name)
+
+    def available_accelerators(self) -> List:
+        """Returns a list of registered accelerators."""
+        return list(self.keys())
+
+    def __str__(self) -> str:
+        return "Registered Accelerators: {}".format(", ".join(self.keys()))
+
+
+AcceleratorRegistry = _AcceleratorRegistry()
+
+
+def register_accelerators(root: Path, base_module: str) -> None:
+    module = importlib.import_module(base_module)
+    for _, mod in getmembers(module, isclass):
+        if issubclass(mod, Accelerator) and mod is not Accelerator:
+            AcceleratorRegistry.register(mod)

From ec17fbed9e6f0cab4a6042fe7c0a877fac91d8f4 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Wed, 2 Mar 2022 17:51:09 +0530
Subject: [PATCH 02/21] Add tests & update changelog

---
 CHANGELOG.md                                  |  3 +
 .../accelerators/test_accelerator_registry.py | 57 +++++++++++++++++++
 2 files changed, 60 insertions(+)
 create mode 100644 tests/accelerators/test_accelerator_registry.py

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b90cf41b0736b..031b573461302 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -164,6 +164,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Added `Callback.state_dict()` and `Callback.load_state_dict()` methods ([#12232](https://github.com/PyTorchLightning/pytorch-lightning/pull/12232))
 
 
+- Added `AcceleratorRegistry` ([#12180](https://github.com/PyTorchLightning/pytorch-lightning/pull/12180))
+
+
 ### Changed
 
 - Drop PyTorch 1.7 support ([#12191](https://github.com/PyTorchLightning/pytorch-lightning/pull/12191))
diff --git a/tests/accelerators/test_accelerator_registry.py b/tests/accelerators/test_accelerator_registry.py
new file mode 100644
index 0000000000000..4ecb945a46c3b
--- /dev/null
+++ b/tests/accelerators/test_accelerator_registry.py
@@ -0,0 +1,57 @@
+# Copyright The PyTorch Lightning team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from pytorch_lightning import Trainer
+from pytorch_lightning.accelerators import Accelerator, AcceleratorRegistry
+
+
+def test_accelerator_registry_with_new_accelerator():
+
+    accelerator_name = "custom_accelerator"
+
+    class TestAccelerator(Accelerator):
+        @staticmethod
+        def parse_devices(devices):
+            return devices
+
+        @staticmethod
+        def get_parallel_devices(devices):
+            return ["foo"] * devices
+
+        @staticmethod
+        def auto_device_count():
+            return 3
+
+        @staticmethod
+        def is_available():
+            return True
+
+        @staticmethod
+        def name():
+            return accelerator_name
+
+    AcceleratorRegistry.register(TestAccelerator)
+
+    assert accelerator_name in AcceleratorRegistry
+    assert isinstance(AcceleratorRegistry.get(accelerator_name), TestAccelerator)
+
+    trainer = Trainer(accelerator=TestAccelerator(), devices="auto")
+    assert isinstance(trainer.accelerator, TestAccelerator)
+    assert trainer._accelerator_connector.parallel_devices == ["foo"] * 3
+
+    AcceleratorRegistry.remove(accelerator_name)
+    assert accelerator_name not in AcceleratorRegistry
+
+
+def test_available_accelerators_in_registry():
+    assert AcceleratorRegistry.available_accelerators() == ["cpu", "gpu", "ipu", "tpu"]

From 25aca8ad28b5b63c696af7895322ca8fdf05d134 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Wed, 2 Mar 2022 17:58:25 +0530
Subject: [PATCH 03/21] Update accelerator connector

---
 .../connectors/accelerator_connector.py       | 20 +++++++++----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/pytorch_lightning/trainer/connectors/accelerator_connector.py b/pytorch_lightning/trainer/connectors/accelerator_connector.py
index 0d2013c1606cf..8415c615e0b8f 100644
--- a/pytorch_lightning/trainer/connectors/accelerator_connector.py
+++ b/pytorch_lightning/trainer/connectors/accelerator_connector.py
@@ -23,6 +23,7 @@
 from pytorch_lightning.accelerators.cpu import CPUAccelerator
 from pytorch_lightning.accelerators.gpu import GPUAccelerator
 from pytorch_lightning.accelerators.ipu import IPUAccelerator
+from pytorch_lightning.accelerators.registry import AcceleratorRegistry
 from pytorch_lightning.accelerators.tpu import TPUAccelerator
 from pytorch_lightning.plugins import (
     ApexMixedPrecisionPlugin,
@@ -486,27 +487,24 @@ def _choose_accelerator(self) -> str:
         return "cpu"
 
     def _set_parallel_devices_and_init_accelerator(self) -> None:
-        ACCELERATORS = {
-            "cpu": CPUAccelerator,
-            "gpu": GPUAccelerator,
-            "tpu": TPUAccelerator,
-            "ipu": IPUAccelerator,
-        }
         if isinstance(self._accelerator_flag, Accelerator):
             self.accelerator: Accelerator = self._accelerator_flag
         else:
             assert self._accelerator_flag is not None
             self._accelerator_flag = self._accelerator_flag.lower()
-            if self._accelerator_flag not in ACCELERATORS:
+            if self._accelerator_flag not in AcceleratorRegistry:
                 raise MisconfigurationException(
                     "When passing string value for the `accelerator` argument of `Trainer`,"
-                    f" it can only be one of {list(ACCELERATORS)}."
+                    f" it can only be one of {AcceleratorRegistry.available_accelerators()}."
                 )
-            accelerator_class = ACCELERATORS[self._accelerator_flag]
-            self.accelerator = accelerator_class()  # type: ignore[abstract]
+            self.accelerator = AcceleratorRegistry.get(self._accelerator_flag)
 
         if not self.accelerator.is_available():
-            available_accelerator = [acc_str for acc_str in list(ACCELERATORS) if ACCELERATORS[acc_str].is_available()]
+            available_accelerator = [
+                acc_str
+                for acc_str in AcceleratorRegistry.available_accelerators()
+                if AcceleratorRegistry[acc_str].is_available()
+            ]
             raise MisconfigurationException(
                 f"{self.accelerator.__class__.__qualname__} can not run on your system"
                 f" since {self.accelerator.name().upper()}s are not available."

From ad81525f7e2b55850268c9d23ff56f9265b20873 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Wed, 2 Mar 2022 18:07:24 +0530
Subject: [PATCH 04/21] Update test

---
 .../trainer/connectors/accelerator_connector.py           | 8 +++-----
 tests/accelerators/test_accelerator_registry.py           | 2 +-
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/pytorch_lightning/trainer/connectors/accelerator_connector.py b/pytorch_lightning/trainer/connectors/accelerator_connector.py
index 8415c615e0b8f..aadd8ef92d65c 100644
--- a/pytorch_lightning/trainer/connectors/accelerator_connector.py
+++ b/pytorch_lightning/trainer/connectors/accelerator_connector.py
@@ -158,7 +158,7 @@ def __init__(
         # 1. Parsing flags
         # Get registered strategies, built-in accelerators and precision plugins
         self._registered_strategies = StrategyRegistry.available_strategies()
-        self._accelerator_types = ("tpu", "ipu", "gpu", "cpu")
+        self._accelerator_types = AcceleratorRegistry.available_accelerators()
         self._precision_types = ("16", "32", "64", "bf16", "mixed")
 
         # Raise an exception if there are conflicts between flags
@@ -495,15 +495,13 @@ def _set_parallel_devices_and_init_accelerator(self) -> None:
             if self._accelerator_flag not in AcceleratorRegistry:
                 raise MisconfigurationException(
                     "When passing string value for the `accelerator` argument of `Trainer`,"
-                    f" it can only be one of {AcceleratorRegistry.available_accelerators()}."
+                    f" it can only be one of {self._accelerator_types}."
                 )
             self.accelerator = AcceleratorRegistry.get(self._accelerator_flag)
 
         if not self.accelerator.is_available():
             available_accelerator = [
-                acc_str
-                for acc_str in AcceleratorRegistry.available_accelerators()
-                if AcceleratorRegistry[acc_str].is_available()
+                acc_str for acc_str in self._accelerator_types if AcceleratorRegistry[acc_str].is_available()
             ]
             raise MisconfigurationException(
                 f"{self.accelerator.__class__.__qualname__} can not run on your system"
diff --git a/tests/accelerators/test_accelerator_registry.py b/tests/accelerators/test_accelerator_registry.py
index 4ecb945a46c3b..6d7617cc2ef41 100644
--- a/tests/accelerators/test_accelerator_registry.py
+++ b/tests/accelerators/test_accelerator_registry.py
@@ -45,7 +45,7 @@ def name():
     assert accelerator_name in AcceleratorRegistry
     assert isinstance(AcceleratorRegistry.get(accelerator_name), TestAccelerator)
 
-    trainer = Trainer(accelerator=TestAccelerator(), devices="auto")
+    trainer = Trainer(accelerator="custom_accelerator", devices="auto")
     assert isinstance(trainer.accelerator, TestAccelerator)
     assert trainer._accelerator_connector.parallel_devices == ["foo"] * 3
 

From ad3c87e0814446f89ed79dc34d2e53cfd417b661 Mon Sep 17 00:00:00 2001
From: Kaushik B <45285388+kaushikb11@users.noreply.github.com>
Date: Tue, 22 Mar 2022 15:33:26 +0530
Subject: [PATCH 05/21] Update tests/accelerators/test_accelerator_registry.py

Co-authored-by: Rohit Gupta <rohitgr1998@gmail.com>
---
 tests/accelerators/test_accelerator_registry.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/accelerators/test_accelerator_registry.py b/tests/accelerators/test_accelerator_registry.py
index 6d7617cc2ef41..5cd3577cfe8d0 100644
--- a/tests/accelerators/test_accelerator_registry.py
+++ b/tests/accelerators/test_accelerator_registry.py
@@ -45,7 +45,7 @@ def name():
     assert accelerator_name in AcceleratorRegistry
     assert isinstance(AcceleratorRegistry.get(accelerator_name), TestAccelerator)
 
-    trainer = Trainer(accelerator="custom_accelerator", devices="auto")
+    trainer = Trainer(accelerator=accelerator_name, devices="auto")
     assert isinstance(trainer.accelerator, TestAccelerator)
     assert trainer._accelerator_connector.parallel_devices == ["foo"] * 3
 

From 84379ef7d7a53bd114128a1210c82dab21d6b818 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Wed, 23 Mar 2022 02:25:02 +0530
Subject: [PATCH 06/21] Implement logic for register_accelerators

---
 pytorch_lightning/accelerators/__init__.py    |  7 +-
 pytorch_lightning/accelerators/accelerator.py |  7 +-
 pytorch_lightning/accelerators/cpu.py         | 11 ++-
 pytorch_lightning/accelerators/gpu.py         | 11 ++-
 pytorch_lightning/accelerators/ipu.py         | 11 ++-
 pytorch_lightning/accelerators/registry.py    | 82 ++++++++++++++-----
 pytorch_lightning/accelerators/tpu.py         | 11 ++-
 pytorch_lightning/strategies/__init__.py      |  5 +-
 .../strategies/strategy_registry.py           | 21 +----
 pytorch_lightning/utilities/registry.py       | 27 ++++++
 10 files changed, 127 insertions(+), 66 deletions(-)
 create mode 100644 pytorch_lightning/utilities/registry.py

diff --git a/pytorch_lightning/accelerators/__init__.py b/pytorch_lightning/accelerators/__init__.py
index d58defbfeb460..fe9ae0a120cfb 100644
--- a/pytorch_lightning/accelerators/__init__.py
+++ b/pytorch_lightning/accelerators/__init__.py
@@ -10,16 +10,13 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-from pathlib import Path
-
 from pytorch_lightning.accelerators.accelerator import Accelerator  # noqa: F401
 from pytorch_lightning.accelerators.cpu import CPUAccelerator  # noqa: F401
 from pytorch_lightning.accelerators.gpu import GPUAccelerator  # noqa: F401
 from pytorch_lightning.accelerators.ipu import IPUAccelerator  # noqa: F401
-from pytorch_lightning.accelerators.registry import AcceleratorRegistry, register_accelerators  # noqa: F401
+from pytorch_lightning.accelerators.registry import AcceleratorRegistry, call_register_accelerators  # noqa: F401
 from pytorch_lightning.accelerators.tpu import TPUAccelerator  # noqa: F401
 
-FILE_ROOT = Path(__file__).parent
 ACCELERATORS_BASE_MODULE = "pytorch_lightning.accelerators"
 
-register_accelerators(FILE_ROOT, ACCELERATORS_BASE_MODULE)
+call_register_accelerators(ACCELERATORS_BASE_MODULE)
diff --git a/pytorch_lightning/accelerators/accelerator.py b/pytorch_lightning/accelerators/accelerator.py
index ad0779d88b96c..f63545a1e42e1 100644
--- a/pytorch_lightning/accelerators/accelerator.py
+++ b/pytorch_lightning/accelerators/accelerator.py
@@ -75,7 +75,6 @@ def auto_device_count() -> int:
     def is_available() -> bool:
         """Detect if the hardware is available."""
 
-    @staticmethod
-    @abstractmethod
-    def name() -> str:
-        """Name of the Accelerator."""
+    @classmethod
+    def register_accelerators(cls, accelerator_registry: Dict) -> None:
+        raise NotImplementedError(f"`register_accelerators` is not implemented for {cls.__name__}")
diff --git a/pytorch_lightning/accelerators/cpu.py b/pytorch_lightning/accelerators/cpu.py
index a027e7db6e209..3d28a4d80f682 100644
--- a/pytorch_lightning/accelerators/cpu.py
+++ b/pytorch_lightning/accelerators/cpu.py
@@ -63,7 +63,10 @@ def is_available() -> bool:
         """CPU is always available for execution."""
         return True
 
-    @staticmethod
-    def name() -> str:
-        """Name of the Accelerator."""
-        return "cpu"
+    @classmethod
+    def register_accelerators(cls, accelerator_registry: Dict) -> None:
+        accelerator_registry.register(
+            "cpu",
+            cls,
+            description=f"{cls.__class__.__name__}",
+        )
diff --git a/pytorch_lightning/accelerators/gpu.py b/pytorch_lightning/accelerators/gpu.py
index 529d067025f97..1f74da7da3f4e 100644
--- a/pytorch_lightning/accelerators/gpu.py
+++ b/pytorch_lightning/accelerators/gpu.py
@@ -93,10 +93,13 @@ def auto_device_count() -> int:
     def is_available() -> bool:
         return torch.cuda.device_count() > 0
 
-    @staticmethod
-    def name() -> str:
-        """Name of the Accelerator."""
-        return "gpu"
+    @classmethod
+    def register_accelerators(cls, accelerator_registry: Dict) -> None:
+        accelerator_registry.register(
+            "gpu",
+            cls,
+            description=f"{cls.__class__.__name__}",
+        )
 
 
 def get_nvidia_gpu_stats(device: _DEVICE) -> Dict[str, float]:
diff --git a/pytorch_lightning/accelerators/ipu.py b/pytorch_lightning/accelerators/ipu.py
index 1e8b2bc27fe57..b5110e58028a5 100644
--- a/pytorch_lightning/accelerators/ipu.py
+++ b/pytorch_lightning/accelerators/ipu.py
@@ -47,7 +47,10 @@ def auto_device_count() -> int:
     def is_available() -> bool:
         return _IPU_AVAILABLE
 
-    @staticmethod
-    def name() -> str:
-        """Name of the Accelerator."""
-        return "ipu"
+    @classmethod
+    def register_accelerators(cls, accelerator_registry: Dict) -> None:
+        accelerator_registry.register(
+            "ipu",
+            cls,
+            description=f"{cls.__class__.__name__}",
+        )
diff --git a/pytorch_lightning/accelerators/registry.py b/pytorch_lightning/accelerators/registry.py
index 62dd1d4e8a381..80a8784e1ddb2 100644
--- a/pytorch_lightning/accelerators/registry.py
+++ b/pytorch_lightning/accelerators/registry.py
@@ -13,41 +13,85 @@
 # limitations under the License.
 import importlib
 from inspect import getmembers, isclass
-from pathlib import Path
-from typing import Any, List, Optional
+from typing import Any, Callable, Dict, List, Optional
 
 from pytorch_lightning.accelerators.accelerator import Accelerator
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
+from pytorch_lightning.utilities.registry import _is_register_method_overridden
 
 
 class _AcceleratorRegistry(dict):
-    def register(self, accelerator: Accelerator, name: Optional[str] = None, override: bool = False) -> Accelerator:
-        """Registers an accelerator mapped to a name.
+    """This class is a Registry that stores information about the Accelerators.
+
+    The Accelerators are mapped to strings. These strings are names that identify
+    an accelerator, e.g., "gpu". It also returns Optional description and
+    parameters to initialize the Accelerator, which were defined during the
+    registration.
+
+    The motivation for having a AcceleratorRegistry is to make it convenient
+    for the Users to try different accelerators by passing mapped aliases
+    to the accelerator flag to the Trainer.
+
+    Example::
+
+        @AcceleratorRegistry.register("sota", description="Custom sota accelerator", a=1, b=True)
+        class SOTAAccelerator(Accelerator):
+            def __init__(self, a, b):
+                ...
+
+        or
+
+        AcceleratorRegistry.register("sota", SOTAAAccelerator, description="Custom sota accelerator", a=1, b=True)
+    """
+
+    def register(
+        self,
+        name: str,
+        accelerator: Optional[Callable] = None,
+        description: Optional[str] = None,
+        override: bool = False,
+        **init_params: Any,
+    ) -> Callable:
+        """Registers a accelerator mapped to a name and with required metadata.
 
         Args:
-            accelerator: the accelerator to be mapped.
-            name: the name that identifies the provided accelerator.
-            override: Whether to override an existing key.
+            name : the name that identifies a accelerator, e.g. "gpu"
+            accelerator : accelerator class
+            description : accelerator description
+            override : overrides the registered accelerator, if True
+            init_params: parameters to initialize the accelerator
         """
-        if name is None:
-            name = accelerator.name()
-        elif not isinstance(name, str):
+        if not (name is None or isinstance(name, str)):
             raise TypeError(f"`name` must be a str, found {name}")
 
         if name in self and not override:
             raise MisconfigurationException(f"'{name}' is already present in the registry. HINT: Use `override=True`.")
-        self[name] = accelerator
-        return accelerator
+
+        data: Dict[str, Any] = {}
+        data["description"] = description if description is not None else ""
+
+        data["init_params"] = init_params
+
+        def do_register(name: str, accelerator: Callable) -> Callable:
+            data["accelerator"] = accelerator
+            data["accelerator_name"] = name
+            self[name] = data
+            return accelerator
+
+        if accelerator is not None:
+            return do_register(name, accelerator)
+
+        return do_register
 
     def get(self, name: str, default: Optional[Any] = None) -> Any:
-        """Calls the registered Accelerator and returns the Accelerator object.
+        """Calls the registered accelerator with the required parameters and returns the accelerator object.
 
         Args:
-            name (str): the name that identifies a Accelerator, e.g. "tpu"
+            name (str): the name that identifies a accelerator, e.g. "gpu"
         """
         if name in self:
-            accelerator = self[name]
-            return accelerator()
+            data = self[name]
+            return data["accelerator"](**data["init_params"])
 
         if default is not None:
             return default
@@ -71,8 +115,8 @@ def __str__(self) -> str:
 AcceleratorRegistry = _AcceleratorRegistry()
 
 
-def register_accelerators(root: Path, base_module: str) -> None:
+def call_register_accelerators(base_module: str) -> None:
     module = importlib.import_module(base_module)
     for _, mod in getmembers(module, isclass):
-        if issubclass(mod, Accelerator) and mod is not Accelerator:
-            AcceleratorRegistry.register(mod)
+        if issubclass(mod, Accelerator) and _is_register_method_overridden(mod, Accelerator, "register_accelerators"):
+            mod.register_accelerators(AcceleratorRegistry)
diff --git a/pytorch_lightning/accelerators/tpu.py b/pytorch_lightning/accelerators/tpu.py
index dfdc950e70124..fa8bd007cb25f 100644
--- a/pytorch_lightning/accelerators/tpu.py
+++ b/pytorch_lightning/accelerators/tpu.py
@@ -65,7 +65,10 @@ def auto_device_count() -> int:
     def is_available() -> bool:
         return _TPU_AVAILABLE
 
-    @staticmethod
-    def name() -> str:
-        """Name of the Accelerator."""
-        return "tpu"
+    @classmethod
+    def register_accelerators(cls, accelerator_registry: Dict) -> None:
+        accelerator_registry.register(
+            "tpu",
+            cls,
+            description=f"{cls.__class__.__name__}",
+        )
diff --git a/pytorch_lightning/strategies/__init__.py b/pytorch_lightning/strategies/__init__.py
index f06edfa53ec7a..a4cd57a50ac1d 100644
--- a/pytorch_lightning/strategies/__init__.py
+++ b/pytorch_lightning/strategies/__init__.py
@@ -11,8 +11,6 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-from pathlib import Path
-
 from pytorch_lightning.strategies.bagua import BaguaStrategy  # noqa: F401
 from pytorch_lightning.strategies.ddp import DDPStrategy  # noqa: F401
 from pytorch_lightning.strategies.ddp2 import DDP2Strategy  # noqa: F401
@@ -31,7 +29,6 @@
 from pytorch_lightning.strategies.strategy_registry import call_register_strategies, StrategyRegistry  # noqa: F401
 from pytorch_lightning.strategies.tpu_spawn import TPUSpawnStrategy  # noqa: F401
 
-FILE_ROOT = Path(__file__).parent
 STRATEGIES_BASE_MODULE = "pytorch_lightning.strategies"
 
-call_register_strategies(FILE_ROOT, STRATEGIES_BASE_MODULE)
+call_register_strategies(STRATEGIES_BASE_MODULE)
diff --git a/pytorch_lightning/strategies/strategy_registry.py b/pytorch_lightning/strategies/strategy_registry.py
index 17e08acb23bcc..7dee7146d415d 100644
--- a/pytorch_lightning/strategies/strategy_registry.py
+++ b/pytorch_lightning/strategies/strategy_registry.py
@@ -12,13 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import importlib
-import inspect
 from inspect import getmembers, isclass
-from pathlib import Path
 from typing import Any, Callable, Dict, List, Optional
 
 from pytorch_lightning.strategies.strategy import Strategy
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
+from pytorch_lightning.utilities.registry import _is_register_method_overridden
 
 
 class _StrategyRegistry(dict):
@@ -116,22 +115,8 @@ def __str__(self) -> str:
 StrategyRegistry = _StrategyRegistry()
 
 
-def is_register_strategies_overridden(strategy: type) -> bool:
-
-    method_name = "register_strategies"
-    strategy_attr = getattr(strategy, method_name)
-    previous_super_cls = inspect.getmro(strategy)[1]
-
-    if issubclass(previous_super_cls, Strategy):
-        super_attr = getattr(previous_super_cls, method_name)
-    else:
-        return False
-
-    return strategy_attr.__code__ is not super_attr.__code__
-
-
-def call_register_strategies(root: Path, base_module: str) -> None:
+def call_register_strategies(base_module: str) -> None:
     module = importlib.import_module(base_module)
     for _, mod in getmembers(module, isclass):
-        if issubclass(mod, Strategy) and is_register_strategies_overridden(mod):
+        if issubclass(mod, Strategy) and _is_register_method_overridden(mod, Strategy, "register_strategies"):
             mod.register_strategies(StrategyRegistry)
diff --git a/pytorch_lightning/utilities/registry.py b/pytorch_lightning/utilities/registry.py
new file mode 100644
index 0000000000000..ff0d5e8a94413
--- /dev/null
+++ b/pytorch_lightning/utilities/registry.py
@@ -0,0 +1,27 @@
+# Copyright The PyTorch Lightning team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import inspect
+from typing import Callable
+
+
+def _is_register_method_overridden(mod: type, base_cls: Callable, method: str) -> bool:
+    mod_attr = getattr(mod, method)
+    previous_super_cls = inspect.getmro(mod)[1]
+
+    if issubclass(previous_super_cls, base_cls):
+        super_attr = getattr(previous_super_cls, method)
+    else:
+        return False
+
+    return mod_attr.__code__ is not super_attr.__code__

From 5d736d9a06f8119dedfded774626f34077c3bff2 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Wed, 23 Mar 2022 02:34:55 +0530
Subject: [PATCH 07/21] Update tests

---
 .../accelerators/test_accelerator_registry.py | 31 ++++++++++++++-----
 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/tests/accelerators/test_accelerator_registry.py b/tests/accelerators/test_accelerator_registry.py
index 5cd3577cfe8d0..57eeb907b1c3b 100644
--- a/tests/accelerators/test_accelerator_registry.py
+++ b/tests/accelerators/test_accelerator_registry.py
@@ -18,8 +18,14 @@
 def test_accelerator_registry_with_new_accelerator():
 
     accelerator_name = "custom_accelerator"
+    accelerator_description = "Custom Accelerator"
+
+    class CustomAccelerator(Accelerator):
+        def __init__(self, param1, param2):
+            self.param1 = param1
+            self.param2 = param2
+            super().__init__()
 
-    class TestAccelerator(Accelerator):
         @staticmethod
         def parse_devices(devices):
             return devices
@@ -36,17 +42,28 @@ def auto_device_count():
         def is_available():
             return True
 
-        @staticmethod
-        def name():
-            return accelerator_name
+        @classmethod
+        def register_accelerators(cls, accelerator_registry) -> None:
+            accelerator_registry.register(
+                accelerator_name,
+                cls,
+                description=f"{cls.__class__.__name__}",
+            )
 
-    AcceleratorRegistry.register(TestAccelerator)
+    AcceleratorRegistry.register(
+        accelerator_name, CustomAccelerator, description=accelerator_description, param1="abc", param2=123
+    )
 
     assert accelerator_name in AcceleratorRegistry
-    assert isinstance(AcceleratorRegistry.get(accelerator_name), TestAccelerator)
+
+    assert AcceleratorRegistry[accelerator_name]["description"] == accelerator_description
+    assert AcceleratorRegistry[accelerator_name]["init_params"] == {"param1": "abc", "param2": 123}
+    assert AcceleratorRegistry[accelerator_name]["accelerator_name"] == accelerator_name
+
+    assert isinstance(AcceleratorRegistry.get(accelerator_name), CustomAccelerator)
 
     trainer = Trainer(accelerator=accelerator_name, devices="auto")
-    assert isinstance(trainer.accelerator, TestAccelerator)
+    assert isinstance(trainer.accelerator, CustomAccelerator)
     assert trainer._accelerator_connector.parallel_devices == ["foo"] * 3
 
     AcceleratorRegistry.remove(accelerator_name)

From 42061984f78fc2bce41dde83a4abdfa124c3fa95 Mon Sep 17 00:00:00 2001
From: Kaushik B <45285388+kaushikb11@users.noreply.github.com>
Date: Wed, 23 Mar 2022 02:26:01 +0530
Subject: [PATCH 08/21] Update pytorch_lightning/accelerators/accelerator.py

---
 pytorch_lightning/accelerators/accelerator.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pytorch_lightning/accelerators/accelerator.py b/pytorch_lightning/accelerators/accelerator.py
index f63545a1e42e1..2e2414cd71c10 100644
--- a/pytorch_lightning/accelerators/accelerator.py
+++ b/pytorch_lightning/accelerators/accelerator.py
@@ -77,4 +77,4 @@ def is_available() -> bool:
 
     @classmethod
     def register_accelerators(cls, accelerator_registry: Dict) -> None:
-        raise NotImplementedError(f"`register_accelerators` is not implemented for {cls.__name__}")
+        raise NotImplementedError(f"`register_accelerators` is not implemented for {cls.__class__.__name__}")

From f7f5eda4addd91e4d4b2cb41b77704cee2bcff4b Mon Sep 17 00:00:00 2001
From: Kaushik B <45285388+kaushikb11@users.noreply.github.com>
Date: Wed, 23 Mar 2022 02:26:26 +0530
Subject: [PATCH 09/21] Update pytorch_lightning/accelerators/registry.py

---
 pytorch_lightning/accelerators/registry.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pytorch_lightning/accelerators/registry.py b/pytorch_lightning/accelerators/registry.py
index 80a8784e1ddb2..090ae653b0eeb 100644
--- a/pytorch_lightning/accelerators/registry.py
+++ b/pytorch_lightning/accelerators/registry.py
@@ -41,7 +41,7 @@ def __init__(self, a, b):
 
         or
 
-        AcceleratorRegistry.register("sota", SOTAAAccelerator, description="Custom sota accelerator", a=1, b=True)
+        AcceleratorRegistry.register("sota", SOTAAccelerator, description="Custom sota accelerator", a=1, b=True)
     """
 
     def register(

From ef188a14591b0d98c9ac74cbd17a9ae5db6a7858 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Wed, 23 Mar 2022 02:58:26 +0530
Subject: [PATCH 10/21] Fix tests

---
 pytorch_lightning/trainer/connectors/accelerator_connector.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pytorch_lightning/trainer/connectors/accelerator_connector.py b/pytorch_lightning/trainer/connectors/accelerator_connector.py
index aadd8ef92d65c..15e7e06571aa1 100644
--- a/pytorch_lightning/trainer/connectors/accelerator_connector.py
+++ b/pytorch_lightning/trainer/connectors/accelerator_connector.py
@@ -501,7 +501,7 @@ def _set_parallel_devices_and_init_accelerator(self) -> None:
 
         if not self.accelerator.is_available():
             available_accelerator = [
-                acc_str for acc_str in self._accelerator_types if AcceleratorRegistry[acc_str].is_available()
+                acc_str for acc_str in self._accelerator_types if AcceleratorRegistry.get(acc_str).is_available()
             ]
             raise MisconfigurationException(
                 f"{self.accelerator.__class__.__qualname__} can not run on your system"

From eb22e015681c428ae2f60b870d959be685d4b377 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Wed, 23 Mar 2022 14:29:49 +0530
Subject: [PATCH 11/21] Update exception message

---
 .../trainer/connectors/accelerator_connector.py             | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pytorch_lightning/trainer/connectors/accelerator_connector.py b/pytorch_lightning/trainer/connectors/accelerator_connector.py
index 15e7e06571aa1..e1cf4c6232f90 100644
--- a/pytorch_lightning/trainer/connectors/accelerator_connector.py
+++ b/pytorch_lightning/trainer/connectors/accelerator_connector.py
@@ -505,9 +505,9 @@ def _set_parallel_devices_and_init_accelerator(self) -> None:
             ]
             raise MisconfigurationException(
                 f"{self.accelerator.__class__.__qualname__} can not run on your system"
-                f" since {self.accelerator.name().upper()}s are not available."
-                " The following accelerator(s) is available and can be passed into"
-                f" `accelerator` argument of `Trainer`: {available_accelerator}."
+                " since the accelerator is not available. The following accelerator(s)"
+                " is available and can be passed into `accelerator` argument of"
+                f" `Trainer`: {available_accelerator}."
             )
 
         self._set_devices_flag_if_auto_passed()

From cdb0572ca741943256b0cc07826542c4bd97f8d3 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Wed, 23 Mar 2022 16:16:56 +0530
Subject: [PATCH 12/21] Update test

---
 tests/accelerators/test_accelerator_connector.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tests/accelerators/test_accelerator_connector.py b/tests/accelerators/test_accelerator_connector.py
index fce6a7fae2502..794cb9b2922cd 100644
--- a/tests/accelerators/test_accelerator_connector.py
+++ b/tests/accelerators/test_accelerator_connector.py
@@ -498,7 +498,8 @@ def test_accelerator_cpu(_):
     with pytest.raises(MisconfigurationException, match="You requested gpu:"):
         trainer = Trainer(gpus=1)
     with pytest.raises(
-        MisconfigurationException, match="GPUAccelerator can not run on your system since GPUs are not available."
+        MisconfigurationException,
+        match="GPUAccelerator can not run on your system since the accelerator is not available.",
     ):
         trainer = Trainer(accelerator="gpu")
     with pytest.raises(MisconfigurationException, match="You requested gpu:"):

From c8ea7f21b783e776d7c1fbab5f0a05a16c52bef9 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Wed, 23 Mar 2022 16:26:56 +0530
Subject: [PATCH 13/21] Remove NotImplementedError

---
 pytorch_lightning/accelerators/accelerator.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pytorch_lightning/accelerators/accelerator.py b/pytorch_lightning/accelerators/accelerator.py
index 2e2414cd71c10..526cec3e47319 100644
--- a/pytorch_lightning/accelerators/accelerator.py
+++ b/pytorch_lightning/accelerators/accelerator.py
@@ -77,4 +77,4 @@ def is_available() -> bool:
 
     @classmethod
     def register_accelerators(cls, accelerator_registry: Dict) -> None:
-        raise NotImplementedError(f"`register_accelerators` is not implemented for {cls.__class__.__name__}")
+        pass

From 8c96d9c440fe54f29a97950c9e2fb05cc9e0bfa0 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Wed, 23 Mar 2022 16:28:14 +0530
Subject: [PATCH 14/21] Update test

---
 tests/accelerators/test_accelerator_registry.py | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/tests/accelerators/test_accelerator_registry.py b/tests/accelerators/test_accelerator_registry.py
index 57eeb907b1c3b..b21cd95e33cbd 100644
--- a/tests/accelerators/test_accelerator_registry.py
+++ b/tests/accelerators/test_accelerator_registry.py
@@ -42,14 +42,6 @@ def auto_device_count():
         def is_available():
             return True
 
-        @classmethod
-        def register_accelerators(cls, accelerator_registry) -> None:
-            accelerator_registry.register(
-                accelerator_name,
-                cls,
-                description=f"{cls.__class__.__name__}",
-            )
-
     AcceleratorRegistry.register(
         accelerator_name, CustomAccelerator, description=accelerator_description, param1="abc", param2=123
     )

From d18091d0a8e37f34fc44242101573c0f86fcb322 Mon Sep 17 00:00:00 2001
From: Kaushik B <45285388+kaushikb11@users.noreply.github.com>
Date: Thu, 24 Mar 2022 00:32:10 +0530
Subject: [PATCH 15/21] Update pytorch_lightning/accelerators/registry.py

Co-authored-by: ananthsub <ananth.subramaniam@gmail.com>
---
 pytorch_lightning/accelerators/registry.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pytorch_lightning/accelerators/registry.py b/pytorch_lightning/accelerators/registry.py
index 090ae653b0eeb..ba5fc71f925a0 100644
--- a/pytorch_lightning/accelerators/registry.py
+++ b/pytorch_lightning/accelerators/registry.py
@@ -48,7 +48,7 @@ def register(
         self,
         name: str,
         accelerator: Optional[Callable] = None,
-        description: Optional[str] = None,
+        description: str = "",
         override: bool = False,
         **init_params: Any,
     ) -> Callable:

From a5925d9effa507804d96936aa3e8e2e48676c7a1 Mon Sep 17 00:00:00 2001
From: Kaushik B <45285388+kaushikb11@users.noreply.github.com>
Date: Thu, 24 Mar 2022 00:32:15 +0530
Subject: [PATCH 16/21] Update pytorch_lightning/accelerators/registry.py

Co-authored-by: ananthsub <ananth.subramaniam@gmail.com>
---
 pytorch_lightning/accelerators/registry.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/pytorch_lightning/accelerators/registry.py b/pytorch_lightning/accelerators/registry.py
index ba5fc71f925a0..a9b5ac5cf9f0e 100644
--- a/pytorch_lightning/accelerators/registry.py
+++ b/pytorch_lightning/accelerators/registry.py
@@ -68,7 +68,6 @@ def register(
             raise MisconfigurationException(f"'{name}' is already present in the registry. HINT: Use `override=True`.")
 
         data: Dict[str, Any] = {}
-        data["description"] = description if description is not None else ""
 
         data["init_params"] = init_params
 

From 75884c6e1d02f72d1e5b75313a67ee194c8ca7ff Mon Sep 17 00:00:00 2001
From: Kaushik B <45285388+kaushikb11@users.noreply.github.com>
Date: Thu, 24 Mar 2022 00:32:45 +0530
Subject: [PATCH 17/21] Update pytorch_lightning/accelerators/registry.py

Co-authored-by: ananthsub <ananth.subramaniam@gmail.com>
---
 pytorch_lightning/accelerators/registry.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pytorch_lightning/accelerators/registry.py b/pytorch_lightning/accelerators/registry.py
index a9b5ac5cf9f0e..4a16e6ca545eb 100644
--- a/pytorch_lightning/accelerators/registry.py
+++ b/pytorch_lightning/accelerators/registry.py
@@ -103,7 +103,7 @@ def remove(self, name: str) -> None:
         """Removes the registered accelerator by name."""
         self.pop(name)
 
-    def available_accelerators(self) -> List:
+    def available_accelerators(self) -> List[str]:
         """Returns a list of registered accelerators."""
         return list(self.keys())
 

From 197dbaa3d53796c52ddaf6b95cc0cf96662a7fac Mon Sep 17 00:00:00 2001
From: Kaushik B <45285388+kaushikb11@users.noreply.github.com>
Date: Thu, 24 Mar 2022 00:32:57 +0530
Subject: [PATCH 18/21] Update pytorch_lightning/accelerators/registry.py

Co-authored-by: ananthsub <ananth.subramaniam@gmail.com>
---
 pytorch_lightning/accelerators/registry.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pytorch_lightning/accelerators/registry.py b/pytorch_lightning/accelerators/registry.py
index 4a16e6ca545eb..b7e305c7eeedd 100644
--- a/pytorch_lightning/accelerators/registry.py
+++ b/pytorch_lightning/accelerators/registry.py
@@ -96,7 +96,7 @@ def get(self, name: str, default: Optional[Any] = None) -> Any:
             return default
 
         err_msg = "'{}' not found in registry. Available names: {}"
-        available_names = ", ".join(sorted(self.keys())) or "none"
+        available_names = self.available_accelerators()
         raise KeyError(err_msg.format(name, available_names))
 
     def remove(self, name: str) -> None:

From b0e11200a938fbeb14e8ecd6357e8f58157c7c7e Mon Sep 17 00:00:00 2001
From: Kaushik B <45285388+kaushikb11@users.noreply.github.com>
Date: Thu, 24 Mar 2022 00:33:03 +0530
Subject: [PATCH 19/21] Update pytorch_lightning/accelerators/registry.py

Co-authored-by: ananthsub <ananth.subramaniam@gmail.com>
---
 pytorch_lightning/accelerators/registry.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pytorch_lightning/accelerators/registry.py b/pytorch_lightning/accelerators/registry.py
index b7e305c7eeedd..f6bd3aa83328b 100644
--- a/pytorch_lightning/accelerators/registry.py
+++ b/pytorch_lightning/accelerators/registry.py
@@ -108,7 +108,7 @@ def available_accelerators(self) -> List[str]:
         return list(self.keys())
 
     def __str__(self) -> str:
-        return "Registered Accelerators: {}".format(", ".join(self.keys()))
+        return "Registered Accelerators: {}".format(", ".join(self.available_accelerators()))
 
 
 AcceleratorRegistry = _AcceleratorRegistry()

From d1de49ec2c56dbb5877aa5b17d2137d8ee31f0a7 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Thu, 24 Mar 2022 02:03:29 +0530
Subject: [PATCH 20/21] Fix tests

---
 pytorch_lightning/accelerators/registry.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/pytorch_lightning/accelerators/registry.py b/pytorch_lightning/accelerators/registry.py
index f6bd3aa83328b..992fa34b02aee 100644
--- a/pytorch_lightning/accelerators/registry.py
+++ b/pytorch_lightning/accelerators/registry.py
@@ -69,6 +69,7 @@ def register(
 
         data: Dict[str, Any] = {}
 
+        data["description"] = description
         data["init_params"] = init_params
 
         def do_register(name: str, accelerator: Callable) -> Callable:

From 096afb937ce96f4068c0b926f26d89f72f075b19 Mon Sep 17 00:00:00 2001
From: Kaushik B <kaushikbokka@gmail.com>
Date: Thu, 24 Mar 2022 02:08:13 +0530
Subject: [PATCH 21/21] Fix mypy

---
 pytorch_lightning/utilities/registry.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pytorch_lightning/utilities/registry.py b/pytorch_lightning/utilities/registry.py
index ff0d5e8a94413..83970e885bdcd 100644
--- a/pytorch_lightning/utilities/registry.py
+++ b/pytorch_lightning/utilities/registry.py
@@ -12,10 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import inspect
-from typing import Callable
+from typing import Any
 
 
-def _is_register_method_overridden(mod: type, base_cls: Callable, method: str) -> bool:
+def _is_register_method_overridden(mod: type, base_cls: Any, method: str) -> bool:
     mod_attr = getattr(mod, method)
     previous_super_cls = inspect.getmro(mod)[1]