[Python-Dev] of branches and heads

[Antoine Pitrou]

On Sat, 26 Feb 2011 10:40:03 -0800
Daniel Stutzbach <stutzbach at google.com> wrote:
> On Sat, Feb 26, 2011 at 9:55 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:
> 
> > There is no such thing as an "unnamed branch". What would "hg branches"
> > show? An empty space?
> 
> I understand now why I was confused.  I had previously read the sentence
> "Both Git and Mercurial support unnamed local branches." at
> http://mercurial.selenic.com/wiki/BranchingExplained
> 
> But as I dig deeper, I see that there is only one unnamed branch, and it
> actually does have an implicit name: "default".


Ok, so beware, the term "branch" can conflate two concepts:
- a path in the topology (or line of development)
- a "named branch" in hg terminology

So, actually, hg promotes a slightly different terminology:
- a "head" is a changeset without a child in the topology
- a "branch" usually means a "named branch": a set of changesets
  bearing the same label (e.g. "default"); that label is freely chosen
  by the committer at any point, and enforces no topological
  characteristic (even though in practice it will have, since it's the
  whole point from the user's perspective, and also because hg's
  default behaviour and concept of a "current branch" encourages it)

A (named) branch can have zero, one, or several heads:
- zero head: if all branch-local heads have a child in another named
  branch (for example, "trunk" is linearly followed by "2.7")
- several heads: if several lines of development were started in this
  branch without bothering to give them separate names

When you have several heads on a branch, you can merge them together if
you want to reconcile the lines of development they represent.

When you have several branches with at least one head each, you can
also merge them together: you must be careful to choose which named 
branch the merge changeset will be part of (for example, if you want
to merge "3.1" into "3.2", you will certainly want the merge changeset
to be part of "3.2", otherwise "3.1" will get a lot of unwanted
features ;-)).

Note: a branch with zero head is marked "inactive" in "hg branches".
This basically means that it has already been merged in another branch.
(of course, you can still develop in that branch, which will certainly
create a new head as soon as you commit your first new changeset)

Regards

Antoine.