Append to .gitignore instead of overwriting it

Help ensure that the `.gitignore` file is not destroyed, if an
environment is created in a directory that already exists.

Author: pradyunsg

Lib/test/test_venv.py

@@ -787,6 +787,20 @@ def test_scm_ignore_files_git(self):
         file_lines = self.get_text_file_contents('.gitignore').splitlines()
         self.assertIn('*', file_lines)
 
+    @requireVenvCreate
+    def test_scm_ignore_files_git_appends_to_existing_file(self):
+        """Test that an existing .gitignore file is appended to."""
+        gitignore_path = self.get_env_file('.gitignore')
+        with open(gitignore_path, 'w', encoding='utf-8') as fp:
+            fp.write("# Existing comment\n")
+
+        self.run_with_capture(venv.create, self.env_dir,
+                              scm_ignore_files={'git'})
+
+        file_lines = self.get_text_file_contents('.gitignore').splitlines()
+        self.assertIn('# Existing comment', file_lines)
+        self.assertIn('*', file_lines)
+
     @requireVenvCreate
     def test_create_scm_ignore_files_multiple(self):
         """

Lib/venv/__init__.py

@@ -293,8 +293,8 @@ def create_git_ignore_file(self, context):
         ignored by git.
         """
         gitignore_path = os.path.join(context.env_dir, '.gitignore')
-        with open(gitignore_path, 'w', encoding='utf-8') as file:
-            file.write('# Created by venv; '
+        with open(gitignore_path, 'a', encoding='utf-8') as file:
+            file.write('# Added by venv; '
                        'see https://docs.python.org/3/library/venv.html\n')
             file.write('*\n')