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Does Pulumi re-read your program between the preview and update steps?

Yes - it does.

To address what might be your follow up question- see <https://github.com/pulumi/pulumi/issues/2318>. 

Ok great. I ran a `pulumi up -y` and switched branches. The branch had quite old pulumi code which essentially caused almost everything in the stack to break. That fix seems extremely valuable.

Agreed - this is an important feature to add.  Thanks for adding details of your scenario to the issue - and sorry about the trouble this caused. 

Of course. Thanks for the quick responses!

Perhaps as a short term workaround pulumi could detect if the file has changed since the initial pulumi run and error if it has.

That's a reasonable suggestion - mind adding a note to that issue to suggest this?