Using first line to determine EOL

pre_commit_hooks/end_of_file_fixer.py

@@ -20,10 +20,15 @@ def fix_file(file_obj: IO[bytes]) -> int:
     last_character = file_obj.read(1)
     # last_character will be '' for an empty file
     if last_character not in {LF, CR} and last_character != b'':
-        # Check if file uses CRLF endings
+        # Look at first line to determine line ending
         file_obj.seek(0, os.SEEK_SET)
-        content = file_obj.read()
-        ending = CRLF if CRLF in content else LF
+        first_line = file_obj.readline()
+        if CRLF in first_line:
+            ending = CRLF
+        elif CR in first_line:
+            ending = CR
+        else:
+            ending = LF
         # Needs this seek for windows, otherwise IOError
         file_obj.seek(0, os.SEEK_END)
         file_obj.write(ending)

tests/end_of_file_fixer_test.py

@@ -19,6 +19,7 @@ TESTS = (
     (b'\xe2\x98\x83', 1, b'\xe2\x98\x83\n'),
     (b'foo\r\n', 0, b'foo\r\n'),
     (b'foo\r\nbar', 1, b'foo\r\nbar\r\n'),
+    (b'foo\rbar', 1, b'foo\rbar\r'),
     (b'foo\r\n\r\n\r\n', 1, b'foo\r\n'),
     (b'foo\r', 0, b'foo\r'),
     (b'foo\r\r\r\r', 1, b'foo\r'),