Remove abc inheritance from Serializable (#8254)

Currently the Serializable class provides `serialize` and `deserialize` as `abstractmethod`s via the mechanisms afforded by inheritance from `abc.ABC`. Since this class is purely internal to `cudf` and is not describing an abstract interface in a manner useful to consumers of our code, the benefits of the abstract base class concept are outweighed by the performance and maintenance costs. In particular, `isinstance` checks on subclasses of `abc.ABC` are much more expensive than for normal classes (due to an expensive implementation of `__instancecheck__`), and (for better or worse) our code base currently makes use of these checks extensively. In addition, in certain places we can benefit from the use of custom metaclasses in `cudf`, but their usage becomes more cumbersome with `ABC` because metaclasses then also have to inherit from `ABCMeta` (which brings along any associated complexities). This PR removes that inheritance, replacing it with a much simpler approach that simply implements `serialize` and `deserialize` as raising `NotImplementedError`.

Authors:
  - Vyas Ramasubramani (https://github.com/vyasr)

Approvers:
  - Ashwin Srinath (https://github.com/shwina)
  - https://github.com/brandon-b-miller

URL: #8254

python/cudf/cudf/core/abc.py

@@ -1,9 +1,7 @@
 # Copyright (c) 2020-2021, NVIDIA CORPORATION.
 """Common abstract base classes for cudf."""
 
-import abc
 import sys
-from abc import abstractmethod
 
 import rmm
 
@@ -18,7 +16,7 @@
     import pickle  # type: ignore
 
 
-class Serializable(abc.ABC):
+class Serializable:
     """A serializable object composed of device memory buffers.
 
     This base class defines a standard serialization protocol for objects
@@ -32,7 +30,6 @@ class Serializable(abc.ABC):
     latter converts back from that representation into an equivalent object.
     """
 
-    @abstractmethod
     def serialize(self):
         """Generate an equivalent serializable representation of an object.
 
@@ -53,10 +50,11 @@ def serialize(self):
 
         :meta private:
         """
-        pass
+        raise NotImplementedError(
+            "Subclasses of Serializable must implement serialize"
+        )
 
     @classmethod
-    @abstractmethod
     def deserialize(cls, header, frames):
         """Generate an object from a serialized representation.
 
@@ -80,7 +78,9 @@ class can be constructed from a serialized representation generalized
 
         :meta private:
         """
-        pass
+        raise NotImplementedError(
+            "Subclasses of Serializable must implement deserialize"
+        )
 
     def device_serialize(self):
         """Serialize data and metadata associated with device memory.