# Frequently Asked Questions

## How can I alter the context before rendering the project?

Similar questions:

-   **How can I add/remove variables to/from the rendering context?**
-   **How to infer context variables based on the users answers, without prompting
    users?**

Answer:

**Use the [`ContextHook` extension][context-hook].**

The [`ContextHook` extension][context-hook] lets you modify the context used to render
templates, so that you can add, change or remove variables.

[context-hook]:
    https://github.com/copier-org/copier-templates-extensions#context-hook-extension

In order for Copier to be able to load and use the extension when generating a project,
it must be installed alongside Copier itself. More details in the
[`jinja_extensions` docs](#jinja_extensions).

You can then configure your Jinja extensions in Copier's configuration file:

```yaml title="copier.yaml"
_jinja_extensions:
    - copier_templates_extensions.TemplateExtensionLoader
    - extensions/context.py:ContextUpdater
```

Following this example, you are supposed to provide a `context.py` file in the
`extensions` folder at the root of your template to modify the context. If for example
your `copier.yaml` contains a multiple-choice variable like this:

```yaml title="copier.yaml"
flavor:
    type: str
    choices:
        - docker
        - instances
        - kubernetes
        - none
```

The `context.py` file contains your context hook which could look like:

```python title="extensions/context.py"
from copier_templates_extensions import ContextHook


class ContextUpdater(ContextHook):
    def hook(self, context):
        flavor = context["flavor"]  # user's answer to the "flavor" question
        return {
            "isDocker": flavor == "docker"
            "isK8s": flavor == "kubernetes"
            "isInstances": flavor == "instances"
            "isLite": flavor == "none"
            "isNotDocker": flavor != "docker"
            "isNotK8s": flavor != "kubernetes"
            "isNotInstances": flavor != "instances"
            "isNotLite": flavor != "none"
            "hasContainers": flavor in {"docker", "kubernetes"}
        }
```

Before rendering each templated file/folder, the context will be updated with this new
context object that you return from the hook. If you wish to update the context in-place
rather than update it, set the `update` class attribute to false:

```python title="extensions/context.py"
from copier_templates_extensions import ContextHook


class ContextUpdater(ContextHook):
    update = False

    def hook(self, context):
        flavor = context["flavor"]  # user's answer to the "flavor" question

        context["isDocker"] = flavor == "docker"
        context["isK8s"] = flavor == "kubernetes"
        context["isInstances"] = flavor == "instances"
        context["isLite"] = flavor == "none"

        context["isNotDocker"] = flavor != "docker"
        context["isNotK8s"] = flavor != "kubernetes"
        context["isNotInstances"] = flavor != "instances"
        context["isNotLite"] = flavor != "none"

        context["hasContainers"] = context["isDocker"] or context["isK8s"]

        # you can now actually remove items from the context
        del context["flavor"]
```

Now you can use these added variables in your Jinja templates, and in files and folders
names!