From d4887816c3209a92d730b9ee6b817bb7726d241e Mon Sep 17 00:00:00 2001
From: zueve <zueves@gmail.com>
Date: Thu, 28 Jan 2021 12:33:48 +0200
Subject: [PATCH] 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      |  5 ++++-
 src/dotenv/cli.py | 15 ++++++++++++---
 tests/test_cli.py | 28 ++++++++++++++++++++++++++++
 3 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 14f10d37..5a5b276b 100644
--- a/CHANGELOG.md
+++ b/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
diff --git a/src/dotenv/cli.py b/src/dotenv/cli.py
index e17d248f..51f25e8d 100644
--- a/src/dotenv/cli.py
+++ b/src/dotenv/cli.py
@@ -107,9 +107,14 @@ 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):
@@ -117,7 +122,11 @@ def run(ctx, commandline):
             '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.')
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 23404e70..a048ef3b 100644
--- a/tests/test_cli.py
+++ b/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")