Don't write the file unless there are changes

autopep8.py

@@ -142,8 +142,7 @@ class documentation for more information.
 MAX_PYTHON_FILE_DETECTION_BYTES = 1024
 
 
-def open_with_encoding(filename,
-                       encoding=None, mode='r', limit_byte_check=-1):
+def open_with_encoding(filename, mode='r', encoding=None, limit_byte_check=-1):
     """Return opened file with a specific encoding."""
     if not encoding:
         encoding = detect_encoding(filename, limit_byte_check=limit_byte_check)
@@ -159,7 +158,7 @@ def detect_encoding(filename, limit_byte_check=-1):
             from lib2to3.pgen2 import tokenize as lib2to3_tokenize
             encoding = lib2to3_tokenize.detect_encoding(input_file.readline)[0]
 
-        with open_with_encoding(filename, encoding) as test_file:
+        with open_with_encoding(filename, encoding=encoding) as test_file:
             test_file.read(limit_byte_check)
 
         return encoding
@@ -3431,12 +3430,11 @@ def fix_file(filename, options=None, output=None, apply_config=False):
             output.flush()
         return diff
     elif options.in_place:
-        fp = open_with_encoding(filename, encoding=encoding, mode='w')
-        fp.write(fixed_source)
-        fp.close()
         original = "".join(original_source).splitlines()
         fixed = fixed_source.splitlines()
         if original != fixed:
+            with open_with_encoding(filename, 'w', encoding=encoding) as fp:
+                fp.write(fixed_source)
             return fixed_source
         else:
             return ''

test/test_autopep8.py

@@ -4956,6 +4956,18 @@ def test_in_place(self):
                 self.assertEqual(fixed, f.read())
             self.assertEqual(p.returncode, autopep8.EXIT_CODE_OK)
 
+    def test_in_place_no_modifications_no_writes(self):
+        with temporary_file_context('import os\n') as filename:
+            # ensure that noops do not do writes by making writing an error
+            os.chmod(filename, 0o444)
+            p = Popen(
+                list(AUTOPEP8_CMD_TUPLE) + [filename, '--in-place'],
+                stderr=PIPE,
+            )
+            _, err = p.communicate()
+            self.assertEqual(err, b'')
+            self.assertEqual(p.returncode, autopep8.EXIT_CODE_OK)
+
     def test_in_place_with_exit_code_option(self):
         line = "'abc'  \n"
         fixed = "'abc'\n"