Experiment w/ dataclasses (including Py36) (#3423)

* [ci] Also run test_examples in py37

(will revert at the end of the experiment)

* InputExample: use immutable dataclass

* [deps] Install dataclasses for Py<3.7

* [skip ci] Revert "[ci] Also run test_examples in py37"

This reverts commit d29afd9959786b77759b0b8fa4e6b4335b952015.

setup.py

@@ -97,6 +97,8 @@ setup(
     install_requires=[
         "numpy",
         "tokenizers == 0.5.2",
+        # dataclasses for Python versions that don't have it
+        "dataclasses;python_version<'3.7'",
         # accessing files from S3 directly
         "boto3",
         # filesystem locks e.g. to prevent parallel downloads

src/transformers/data/processors/utils.py

@@ -16,8 +16,11 @@
 
 import copy
 import csv
+import dataclasses
 import json
 import logging
+from dataclasses import dataclass
+from typing import Optional
 
 from ...file_utils import is_tf_available, is_torch_available
 
@@ -25,7 +28,8 @@ from ...file_utils import is_tf_available, is_torch_available
 logger = logging.getLogger(__name__)
 
 
-class InputExample(object):
+@dataclass(frozen=True)
+class InputExample:
     """
     A single training/test example for simple sequence classification.
 
@@ -39,23 +43,14 @@ class InputExample(object):
             specified for train and dev examples, but not for test examples.
     """
 
-    def __init__(self, guid, text_a, text_b=None, label=None):
-        self.guid = guid
-        self.text_a = text_a
-        self.text_b = text_b
-        self.label = label
-
-    def __repr__(self):
-        return str(self.to_json_string())
-
-    def to_dict(self):
-        """Serializes this instance to a Python dictionary."""
-        output = copy.deepcopy(self.__dict__)
-        return output
+    guid: str
+    text_a: str
+    text_b: Optional[str] = None
+    label: Optional[str] = None
 
     def to_json_string(self):
         """Serializes this instance to a JSON string."""
-        return json.dumps(self.to_dict(), indent=2, sort_keys=True) + "\n"
+        return json.dumps(dataclasses.asdict(self), indent=2, sort_keys=True) + "\n"
 
 
 class InputFeatures(object):