Add --override/--no-override flag to "dotenv run"

This makes it possible to not override previously defined environment
variables when running `dotenv run`.  It defaults to `--override` for
compatibility with the previous behavior.

CHANGELOG.md

@@ -7,7 +7,9 @@ project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
-_There are no unreleased changes at this time._
+### Added
+
+- Add `--override`/`--no-override` option to `dotenv run` (#312 by [@zueve] and [@bbc2]).
 
 ## [0.16.0] - 2021-03-27
 
@@ -242,6 +244,7 @@ _There are no unreleased changes at this time._
 [@venthur]: https://github.com/venthur
 [@x-yuri]: https://github.com/x-yuri
 [@yannham]: https://github.com/yannham
+[@zueve]: https://github.com/zueve
 
 [Unreleased]: https://github.com/theskumar/python-dotenv/compare/v0.16.0...HEAD
 [0.16.0]: https://github.com/theskumar/python-dotenv/compare/v0.15.0...v0.16.0

src/dotenv/cli.py

@@ -107,17 +107,26 @@ def unset(ctx, key):
 
 @cli.command(context_settings={'ignore_unknown_options': True})
 @click.pass_context
+@click.option(
+    "--override/--no-override",
+    default=True,
+    help="Override variables from the environment file with those from the .env file.",
+)
 @click.argument('commandline', nargs=-1, type=click.UNPROCESSED)
-def run(ctx, commandline):
-    # type: (click.Context, List[str]) -> None
+def run(ctx, override, commandline):
+    # type: (click.Context, bool, List[str]) -> None
     """Run command with environment variables present."""
     file = ctx.obj['FILE']
     if not os.path.isfile(file):
         raise click.BadParameter(
             'Invalid value for \'-f\' "%s" does not exist.' % (file),
             ctx=ctx
         )
-    dotenv_as_dict = {to_env(k): to_env(v) for (k, v) in dotenv_values(file).items() if v is not None}
+    dotenv_as_dict = {
+        to_env(k): to_env(v)
+        for (k, v) in dotenv_values(file).items()
+        if v is not None and (override or to_env(k) not in os.environ)
+    }
 
     if not commandline:
         click.echo('No command given.')

tests/test_cli.py

@@ -138,6 +138,34 @@ def test_run(tmp_path):
     assert result == "b\n"
 
 
+def test_run_with_existing_variable(tmp_path):
+    sh.cd(str(tmp_path))
+    dotenv_file = str(tmp_path / ".env")
+    with open(dotenv_file, "w") as f:
+        f.write("a=b")
+
+    result = sh.dotenv("run", "printenv", "a", _env={"LANG": "en_US.UTF-8", "a": "c"})
+
+    assert result == "b\n"
+
+
+def test_run_with_existing_variable_not_overridden(tmp_path):
+    sh.cd(str(tmp_path))
+    dotenv_file = str(tmp_path / ".env")
+    with open(dotenv_file, "w") as f:
+        f.write("a=b")
+
+    result = sh.dotenv(
+        "run",
+        "--no-override",
+        "printenv",
+        "a",
+        _env={"LANG": "en_US.UTF-8", "a": "c"},
+    )
+
+    assert result == "c\n"
+
+
 def test_run_with_none_value(tmp_path):
     sh.cd(str(tmp_path))
     dotenv_file = str(tmp_path / ".env")