Add option not to override existing env variables

The reason the default behavior of this plugin is what it is, is because
it has origins during the time when there were a lot of users migrating
away from [Pipenv](https://github.com/pypa/pipenv), many switching to
Poetry. [Pipenv had built-in `.env`
loading](https://pipenv.pypa.io/en/latest/shell/#automatic-loading-of-env)
which [did override by default (and still
does)](https://github.com/pypa/pipenv/blob/98bdb5f8b2f08a435a825915d7d7d215a7aaec19/pipenv/utils/environment.py#L37).
In order to support the many people coming from `pipenv` the goal was to
make the fewest changes from that toolset possible in regards to `.env`
loading. That is also why there are other some [analogous environment
variables between this project and `pipenv` with a section about them in
the
README](https://github.com/mpeteuil/poetry-dotenv-plugin#coming-from-pipenv).

If you're coming from vanilla `python-dotenv` then this is unexpected.
That needs to be reconciled with the expectations of anyone coming from
`pipenv`. Here we add a system environment
variable—`POETRY_DOTENV_DONT_OVERRIDE`. I think it should solve most
cases. This is because most users who want it to [behave like
`python-dotenv`'s
defaults](https://github.com/theskumar/python-dotenv#getting-started)
probably want that everywhere and the same for anyone who wants it to
behave like `pipenv`.

Author: mpeteuil

README.md

@@ -34,3 +34,9 @@ Pipenv env var | Poetry env var
 -------------- | ----------------------
 PIPENV_DOTENV_LOCATION | POETRY_DOTENV_LOCATION
 PIPENV_DONT_LOAD_ENV | POETRY_DONT_LOAD_ENV
+
+### Overriding existing environment variables
+
+By default, this plugin will override existing environment variables. This is because this plugin was built to make onboarding for users coming from `pipenv` as seamless as possible. If you want to prevent existing environment variables from being overridden, you can set the `POETRY_DOTENV_DONT_OVERRIDE` environment variable to `true`.[^1]
+
+[^1]: See [#16](https://github.com/mpeteuil/poetry-dotenv-plugin/pull/16) for background.

poetry_dotenv_plugin/dotenv_plugin.py

@@ -29,4 +29,9 @@ def load_dotenv(
             io.write_line("<debug>Loading environment variables.</debug>")
 
         path = POETRY_DOTENV_LOCATION or dotenv.find_dotenv(usecwd=True)
-        dotenv.load_dotenv(dotenv_path=path, override=True)
+        POETRY_DOTENV_DONT_OVERRIDE = os.environ.get("POETRY_DOTENV_DONT_OVERRIDE", "")
+        DOTENV_OVERRIDE = not POETRY_DOTENV_DONT_OVERRIDE.lower() in (
+            "true",
+            "1",
+        )
+        dotenv.load_dotenv(dotenv_path=path, override=DOTENV_OVERRIDE)

tests/test_system.sh

@@ -56,6 +56,46 @@ function test_end_to_end_system_with_dotenv_location_override() {
   fi
 }
 
+function test_end_to_end_system_with_default_env_overrides() {
+  # Setup
+  local expected
+  expected='foo'
+  create_dotenv_file
+
+  local output
+  output=$(export MY_ENV_VAR='bar' && poetry run python -c "import os; print(os.environ['MY_ENV_VAR'])")
+
+  # Cleanup
+  delete_dotenv_file
+
+  if [ "$expected" = "$output" ]; then
+    printf "test_end_to_end_system_with_default_env_overrides: PASSED\n"
+  else
+    printf "Expected '$expected', but got '%s'.\n" "$output"
+    exit 1
+  fi
+}
+
+function test_end_to_end_system_without_env_overrides() {
+  # Setup
+  local expected
+  expected='bar'
+  create_dotenv_file
+
+  local output
+  output=$(export MY_ENV_VAR='bar' POETRY_DOTENV_DONT_OVERRIDE=true && poetry run python -c "import os; print(os.environ['MY_ENV_VAR'])")
+
+  # Cleanup
+  delete_dotenv_file
+
+  if [ "$expected" = "$output" ]; then
+    printf "test_end_to_end_system_without_env_overrides: PASSED\n"
+  else
+    printf "Expected '$expected', but got '%s'.\n" "$output"
+    exit 1
+  fi
+}
+
 function test_end_to_end_system_without_loading_dotenv_file() {
   # Setup
   local expected
@@ -78,4 +118,6 @@ function test_end_to_end_system_without_loading_dotenv_file() {
 
 test_end_to_end_system_with_default_dotenv_file
 test_end_to_end_system_with_dotenv_location_override
+test_end_to_end_system_with_default_env_overrides
+test_end_to_end_system_without_env_overrides
 test_end_to_end_system_without_loading_dotenv_file