What is a practical use example of `hg copy file file2` using Mercurial?

Question

Mercurial has an

hg copy file file2

command and the change can propagate change at the first merge. The O'Reilly Mercurial book says that Mercurial is the only source control system that does that.

What is a practical use of this? The book mentioned making a copy of the file and do bug fix, so the bug fix can propagate back to the original file, but with version control, don't we usually edit the file directly, and if the bug fix works, then directly commit that file? Or even if for some reason we need to make a copy, we can cp file file2, test the fix, and mv file2 file to move that file back to the original file, and commit the file. What is a good example of using the hg copy feature?

Answer 1

Let's say you have a base class in a file. You want to make a derived class so you do an hg copy and customize the derived class by deleting some methods and making changes to others. Now your colleague in another branch has fixed a bug in the base class. When you do a merge, that bug fix will be merged into your derived class as well as your base class.

Answer 2

Here's one example I've used personally. Software package with complex configuration files sometimes provide examples with names like universe-wsgi.ini.sample (hi, galaxy), and as part of installation you're supposed to copy the .ini.sample file over to being a .ini file. If you do that using:

hg copy universe-wsgi.ini.sample universe-wsgi.ini

then whenever you update the software with hg pull ; hg update the new settings available in the sample will be added with their defaults to your customized version.