Not require to rewrite full cfg-file when bumping the version

README.md

@@ -105,6 +105,13 @@ tag = True
 If no `.bumpversion.cfg` exists, `bump2version` will also look into
 `setup.cfg` for configuration.
 
+Beware: the default operation of bump2version is rewriting the complete file, causing
+the loss of comments and custom formatting. To avoid this, add the cfg-file itself to
+be bumped by bump2version. Like:
+```ini
+[bumpversion:file:.bumpversion.cfg]
+```
+
 ### Configuration file -- Global configuration
 
 General configuration is grouped in a `[bumpversion]` section.

bumpversion/cli.py

@@ -121,14 +121,19 @@ def main(original_args=None):
         for file_name
         in (file_names or positionals[1:])
     )
+
+    write_config = (not args.dry_run) and config_file_exists
+    # do not update the config-file, when the updating should be handled by bump2version
+    if config_file in [f.path for f in files]:
+        logger.info("Not rewriting '%s' because it is handled by bump2version", config_file)
+        write_config = False
+
     _check_files_contain_version(files, current_version, context)
     _replace_version_in_files(files, current_version, new_version, args.dry_run, context)
     _log_list(config, args.new_version)
 
     # store the new version
-    _update_config_file(
-        config, config_file, config_newlines, config_file_exists, args.new_version, args.dry_run,
-    )
+    _update_config_file(config, config_file, config_newlines, args.new_version, write_config)
 
     # commit and tag
     if vcs:
@@ -631,24 +636,20 @@ def _log_list(config, new_version):
     config.remove_option("bumpversion", "new_version")
 
 
-def _update_config_file(
-        config, config_file, config_newlines, config_file_exists, new_version, dry_run,
-):
+def _update_config_file(config, config_file, config_newlines, new_version, write_file):
     config.set("bumpversion", "current_version", new_version)
     new_config = io.StringIO()
     try:
-        write_to_config_file = (not dry_run) and config_file_exists
-
         logger.info(
             "%s to config file %s:",
-            "Would write" if not write_to_config_file else "Writing",
+            "Would write" if not write_file else "Writing",
             config_file,
         )
 
         config.write(new_config)
         logger.info(new_config.getvalue())
 
-        if write_to_config_file:
+        if write_file:
             with open(config_file, "wt", encoding="utf-8", newline=config_newlines) as f:
                 f.write(new_config.getvalue().strip() + "\n")
 

tests/test_cli.py

@@ -2292,6 +2292,24 @@ def test_no_configured_files_still_file_args_work(tmpdir, vcs):
     assert "1.1.2" == tmpdir.join("please_update_me.txt").read()
 
 
+def test_cfg_files_not_rewritten_when_specified(tmpdir):
+    """ when the .bumpversion.cfg is specified as a file to be bumped, the
+    config should not be rewritten (i.e., losing custom foratting + comments). """
+    tmpdir.chdir()
+    initial_config = """[bumpversion]
+current_version = 500.0.0
+# specify the .bumpversion.cfg here to avoid unintentionally rewriting it
+# for example comments are removed or trailing whitespaces
+[bumpversion:file:.bumpversion.cfg]   
+
+"""
+
+    tmpdir.join(".bumpversion.cfg").write(initial_config, mode='w')
+    main(['major'])
+    expected_new_config = initial_config.replace('500', '501')
+    assert expected_new_config == tmpdir.join(".bumpversion.cfg").read(mode='r')
+
+
 class TestSplitArgsInOptionalAndPositional:
 
     def test_all_optional(self):