Pick up app name from server instance (#771)

[Batalex]

Start with a description of this PR. Then edit the list below to the items that make sense for your PR scope, and check off the boxes as you go!

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • use flask server name for dash app if name not passed in args. Fix [Feature Request] Pick up app name from server instance #771
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follow
  • this github #PR number updates the dash docs
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Details

I changed the default name arg value from __main__ to None in order to distinguish "not passing a name" from "setting explicitly the name to __main__ ". I figured it would be the best course of action to avoid unwanted renaming cases.

[alexcjohnson]

Looks great, thanks! Would you mind adding a test, perhaps in https://github.com/plotly/dash/blob/master/tests/unit/test_configs.py, that just instantiates an app in various ways and checks that app.config.name has the desired value?

[alexcjohnson]

I changed the default name arg value from __main__ to None in order to distinguish "not passing a name" from "setting explicitly the name to __main__ ". I figured it would be the best course of action to avoid unwanted renaming cases.

💯 I don't know why a user would do this, but it should definitely behave the way you've done it!

[Batalex]

@alexcjohnson I see that https://github.com/plotly/dash/blob/master/tests/unit/test_configs.py still use unitest, am I free to import pytest? pytest.mark.parametrize would be useful for this PR -> multiple cases to test

[alexcjohnson]

Yes, you are more than welcome to use pytest! We'll be converting this and the others the rest of the way soon, just haven't gotten to this file yet.

[Batalex]

It should be good now. To keep a narrow scope for this PR I added the bare minimum for the test but I would be glad to continue this work in a new PR with a full convertion to pytest

[alexcjohnson]

Beautiful test, thanks! 💃

And yes, you're right that it's best not to mix in other topics like converting more tests to pytest into this PR. But if you're interested in helping that process in another PR we would love to have your contribution! If you haven't seen it, https://github.com/plotly/dash-docs/pull/520 describes the new pytest fixtures we (@byronz) made, which are in master here but have not yet been released.