copier/docs/updating.md

Updating a project

The best way to update a project from its template is when all of these conditions are true:

1. The template includes a valid .copier-answers.yml file.
2. The template is versioned with Git (with tags).
3. The destination folder is versioned with Git.

If that's your case, then just enter the destination folder, make sure git status shows it clean, and run:

copier update

This will read all available Git tags, will compare them using PEP 440, and will check out the latest one before updating. To update to the latest commit, add --vcs-ref=HEAD. You can use any other Git ref you want.

When updating, Copier will do its best to respect your project evolution by using the answers you provided when copied last time. However, sometimes it's impossible for Copier to know what to do with a diff code hunk. In those cases, you will find *.rej files that contain the unresolved diffs. You should review those manually before committing.

You probably don't want *.rej files in your Git history, but if you add them to .gitignore, some important changes could pass unnoticed to you. That's why the recommended way to deal with them is to not add them to add a pre-commit (or equivalent) hook that forbids them, just like this:

repos:
    - repo: local
      hooks:
          - id: forbidden-files
            name: forbidden files
            entry: found Copier update rejection files; review them and remove them
            language: fail
            files: "\\.rej$"

Never change the answers file manually

!!! important

**Never** update `.copier-answers.yml` manually.

This will trick Copier, making it believe that those modified answers produced the
current subproject, while it was produced by a different answers set. This will
produce unpredictable behavior of the smart diff algorithm used for updates, which
may work under certain circumstances, but not always.

**This is an unsupported way to update**. Please do not open issues if you updated
this way.

The correct process to update a subproject is:

1. Run copier update.
2. Answer to the questions. They'll default to your answers on your last update.

If you want to just reuse all previous answers:

copier --force update

If you want to change just one question, and leave all others untouched, and don't want to go through the whole questionary again:

copier --force --data updated_question="my new answer" update

How the update works

To understand how the updating process works, take a look at this diagram:

graph TD

%% nodes ----------------------------------------------------------
template_repo("template repository")
template_current("/tmp/template<br>(current tag)")
template_latest("/tmp/template<br>(latest tag)")

project_regen("/tmp/project<br>(fresh, current version)")
project_current("current project")
project_half("half migrated<br>project")
project_updated("updated project")
project_applied("updated project<br>(diff applied)")
project_full("fully updated<br>and migrated project")

update["update current<br>project in-place<br>(prompting)<br>+ run tasks again"]
compare["compare to get diff"]
apply["apply diff"]

diff("diff")

%% edges ----------------------------------------------------------
        template_repo --> |git clone| template_current
        template_repo --> |git clone| template_latest

     template_current --> |generate and run tasks| project_regen
      project_current --> compare
      project_current --> |apply pre-migrations| project_half
        project_regen --> compare
         project_half --> update
      template_latest --> update
               update --> project_updated
              compare --> diff
                 diff --> apply
      project_updated --> apply
                apply --> project_applied
      project_applied --> |apply post-migrations| project_full

%% style ----------------------------------------------------------
classDef blackborder stroke:#000;
class compare,update,apply blackborder;

As you can see here, copier does several things:

• it regenerates a fresh project from the current template version
• then it compares both version, to get the diff from "fresh project" to "current project"
• now it applies pre-migrations to your project, and updates the current project with the latest template changes (asking confirmation)
• finally, it re-applies the previously obtained diff, and then run the post-migrations

!!! important

The diff obtained by comparing the fresh, regenerated project to your
current project can cancel the modifications applied by the update from the latest
template version. During the process, `copier` will ask you confirmation to overwrite or
skip modifications, but in the end, it is possible that nothing has changed (except for
the version in `.copier-answers.yml` of course). This is not a bug: although it can be
quite surprising, this behavior is correct.

Migration across Copier major versions

When there's a new major release of Copier (for example from Copier 5.x to 6.x), there are chances that there's something that changed. Maybe your template will not work as it did before.

[As explained above][how-the-update-works], Copier needs to make a copy of the template in its old state with its old answers so it can actually produce a diff with the new state and answers and apply the smart update to the project. However, how can I be sure that Copier will produce the same "old state" if I copied the template with an older Copier major release?. Good question.

We will do our best to respect older behaviors for at least one extra major release cycle, but the simpler answer is that you can't be sure of that.

How to overcome that situation?

1. You can write good [migrations][].
2. Then you can test them on your template's CI on a matrix against several Copier versions.
3. Or you can just [recopy the project][regenerating-a-project] when you update to a newer Copier major release.