pretty-format-json: ignore original newline presence or absence

.pre-commit-config.yaml

@@ -4,6 +4,7 @@ repos:
     hooks:
     -   id: trailing-whitespace
     -   id: end-of-file-fixer
+        exclude: testing/resources/pretty_formatted_json_no_endline.json
     -   id: check-yaml
     -   id: debug-statements
     -   id: double-quote-string-fixer

pre_commit_hooks/pretty_format_json.py

@@ -4,6 +4,7 @@
 import json
 import sys
 from difflib import unified_diff
+from os import linesep
 from typing import Mapping
 from typing import Sequence
 
@@ -27,7 +28,8 @@ def pairs_first(pairs: Sequence[tuple[str, str]]) -> Mapping[str, str]:
         indent=indent,
         ensure_ascii=ensure_ascii,
     )
-    return f'{json_pretty}\n'
+    finisher = linesep if contents.endswith(linesep) else ''
+    return f'{json_pretty}{finisher}'
 
 
 def _autofix(filename: str, new_contents: str) -> None:

testing/resources/pretty_formatted_json_no_endline.json

@@ -0,0 +1,9 @@
+{
+  "alist": [
+    2,
+    34,
+    234
+  ],
+  "blah": null,
+  "foo": "bar"
+}

tests/pretty_format_json_test.py

@@ -23,6 +23,7 @@ def test_parse_num_to_int():
         ('unsorted_pretty_formatted_json.json', 1),
         ('non_ascii_pretty_formatted_json.json', 1),
         ('pretty_formatted_json.json', 0),
+        ('pretty_formatted_json_no_endline.json', 0),
     ),
 )
 def test_main(filename, expected_retval):
@@ -36,6 +37,7 @@ def test_main(filename, expected_retval):
         ('unsorted_pretty_formatted_json.json', 0),
         ('non_ascii_pretty_formatted_json.json', 1),
         ('pretty_formatted_json.json', 0),
+        ('pretty_formatted_json_no_endline.json', 0),
     ),
 )
 def test_unsorted_main(filename, expected_retval):
@@ -49,6 +51,7 @@ def test_unsorted_main(filename, expected_retval):
         ('unsorted_pretty_formatted_json.json', 1),
         ('non_ascii_pretty_formatted_json.json', 1),
         ('pretty_formatted_json.json', 1),
+        ('pretty_formatted_json_no_endline.json', 1),
         ('tab_pretty_formatted_json.json', 0),
     ),
 )