Why does this work?

Anvil Q&A
1

In this other post you gave a great tip on how to show a “Please wait” notification. Works perfectly.

My question is how/why does this work?

with Notification("Unit tests running..."):
  import test_me
  test_me.run_tests()

You don’t show() the notification or anything. How does it even display? And why does it disappear when the block ends?

EDIT - I just read this in the documentation, I suppose I don’t really need to know how it works. I shall just accept the magic :slight_smile:

If you use a notification in a with block, it will be displayed as long as the body of the block is still running. It will then disappear.

1 Like
2

The magic is a really cool Python feature called “context managers” (alternative name: “Things you can put in a with block”). A with block provides a way of automatically tidying up something at the end of a block of code, whether you exited normally, by returning, or by throwing an exception. In Anvil, we use them for things like auto-closing Notifications, and atomic transactions in data tables.

If you want to dig into them a little bit more, here’s a page with a more detailed exploration of how they work (and even how to build your own): http://effbot.org/zone/python-with-statement.htm

3

Very interesting, thanks.

So the Notification box magically showing and hiding - is that something you do in the background in the __enter__ and __exit__ methods?

4

Yes! On a Notification, we show on __enter__ and hide on __exit__.

5

Very cool.

Are there any other controls that exhibit interesting traits when used in this (or any other unusual) way, that might not be immediately obvious to us?

6

We try to make sure it’s all in the documentation!

https://anvil.works/doc

7

Yes, that is very true.
My issue is I don’t always recognise what I’m looking at. I saw that “with” paragraph but didn’t make the connection that it could be used in that way. I’m getting old, you know …

I’m putting together a little “Anvil Cookbook” of what I consider useful snippets and techniques. I’ll stuff it in the appropriate section of this forum for anyone who’s interested (and for those cleverer than me to correct/improve/snigger at).

8

Thank you for posting this its perfect just what I needed :slight_smile: