[BUG] Environment variables from .env file ignored in Dash.run

[jonasjancarik]

Describe your context
Please provide us your environment, so we can easily reproduce the issue.

• replace the result of pip list | grep dash below

dash                      2.17.1
dash-bootstrap-components 1.6.0
dash-core-components      2.0.0
dash-html-components      2.0.0
dash-table                5.0.0

Describe the bug

Environment variables HOST, PORT and DASH_PROXY are ignored if defined in a .env file and loaded using python-dotenv.

In Dash.run, environment variables used as default argument values are evaluated at the time the method is defined, not when it is called. This can lead to discrepancies if the environment variables change between definition and execution.

Specifically, this means that environment variables specified in a .env file and loaded using the python-dotenv package will be ignored if they are loaded after the method definition. Since load_dotenv() is typically called after importing Dash, this issue affects many standard usage patterns.

The docs for app.run say "If a parameter can be set by an environment variable, that is listed too. Values provided here take precedence over environment variables." It doesn't mention that an .env file cannot be used.

Expected behavior

Arguments which can be set using environment variables should reflect variables from the .env file (loaded using python-dotenv).

Proposed solution

In Dash.app (dash.py), set host, port and proxy to None by default.

   def run(
        self,
        host=None,
        port=None,
        proxy=None,
        ...

Then use

        host = host or os.getenv("HOST", "127.0.0.1")
        port = port or os.getenv("PORT", "8050")
        proxy = proxy or os.getenv("DASH_PROXY", None)

at the start of the function itself to read the env vars when the method is called.