lektor-website/content/docs/plugins/howto/contents.lr

title: How To
---
summary: Covers the most common things plugins would do and how to implement it.
---
sort_key: 30
---
body:

So you want to build a plugin but it's not quite clear how you would structure
a plugin to accomplish this?  This part of the documentation should help you
find answers to those questions.

## Expose Functionality

Most plugins will provide functionality of sorts.  There are two places where
functionality is typically needed: in templates or for other plugins to use.
Plugins can import from each other just like this, but functionality exposed
into templates should follow some basic rules:

* modify `jinja_env.globals` or `jinja_env.filters` and do not use
  `process-template-context` unless you absolutely know what you are doing.
* expose as few global variables as possible.  If you have a lot you want to
  provide then consider registering an object with the templates and to
  attach multiple attributes on it.
* use clear names for template globals.  This is a shared space that all
  plugins and Lektor itself modify so do not provide a function called
  `get_script` but for instance call it `get_my_plugin_script`.

A simple example of a plugin that implements [Gravatar
:ext](https://en.gravatar.com/) support:

```python
from hashlib import md5
from werkzeug.urls import url_encode
from lektor.pluginsystem import Plugin

BASE_URL = 'https://secure.gravatar.com/avatar/'

def get_gravatar(email, **options):
    fn = md5(email.lower().strip().encode('utf-8')).hexdigest()
    return '%s/%s?%s' % (BASE_URL, fn, url_encode(options))

class GravatarPlugin(Plugin):
    name = 'Gravatar'
    def on_setup_env(self, **extra):
        env.jinja_env.filters['gravatar'] = get_gravatar
```

## Configure Plugins

Plugins can come with their own config files and it's encouraged that plugins
take advantage of this.  Each plugin has exactly one INI file called
`<plugin-id>.ini` inside the `configs` folder.

To get access to the plugin the `get_config` function can be used which
returns a dict like object to access the ini file.

```python
config = self.get_config()
value = config.get('section.key', 'default_value')
```

This would correspond to this config in `configs/my-plugin.ini`:

```ini
[section]
key = the value
```

## Dependency Tracking

While a lot of dependencies are tracked automatically, when you develop a
plugin you probably will discover that sometimes you need to track your own
ones.  There are different ways in which dependency tracking can work and
depending on the situation you might have to use two different mechanisms.

1. **Dependency tracking by association**: while a build of a source object
   into an artifact is active more dependencies for that artifact can be
   registered with the [record_dependency :ref](../../api/build/context/record-dependency/)
   method of the context.  It takes a filename that should be recorded as
   additional dependency for the current artifact
2. **Dependency tracking as artifact source**: if you build your own artifact
   you need to define the source files that make up the artifact (if you have
   such a thing).  For instance if you build a thumbnail you will need to
   track those source files that are the source images.  This can be done
   through the [sub_artifact :ref](../../api/build/context/sub-artifact/)
   method which declares a new artifact.

Here examples for both of those in one:

```python
import os
from flask import json
from lektor.pluginsystem import Plugin

def dump_exif(image):
    ctx = get_ctx()
    path = posixpath.join(image.path, '-exif.json')
    @ctx.sub_artifact(path, sources=[image.source_filename])
    def include_file(artifact):
        ctx.record_dependency(__file__)
        with artifact.open('wb') as f:
            json.dump(image.exif.to_dict(), f)
    return path

class ExifDumpPlugin(Plugin):
    def setup_env(self, **extra):
        self.env.jinja_env.globals['dump_exif'] = dump_exif
```

This dumps out the EXIF data into a JSON file and returns the artifact name.
The source image is tracked as direct source for the artifact and within the
function we also track the plugin's filename to rebuild if the plugin changes.

## Adding New Field Types

Let's say you want to add a "datetime" type to the set of
[field types :ref](../../api/db/types/) used in your models, because the
built-in "date" type has no time component. You might update `blog-post.ini`
from the [blog guide :ref](../../guides/blog) like so:

```ini
[fields.pub_date]
label = Publication date
type = datetime  # Custom "datetime" type.
```

In a blog post's `contents.lr`, specify its publication date like:

```
pub_date: 2016-01-02 12:34:56
```

You can add your "datetime" type to Lektor with a plugin:

```python
from datetime import datetime
from lektor.pluginsystem import Plugin
from lektor.types import Type

class DatetimeType(Type):
    def value_from_raw(self, raw):
        return datetime.strptime(raw.value, '%Y-%m-%d %H:%M:%S')

def strftime_filter(dt, fmt='%B %-d, %Y'):
    return dt.strftime(fmt)

class DatetimeTypePlugin(Plugin):
    name = u'datetime type'
    description = u'Adds a new field type, "datetime".'

    def on_setup_env(self, **extra):
        self.env.types['datetime'] = DatetimeType
        self.env.jinja_env.filters['strftime'] = strftime_filter
```

This plugin also adds a `strftime` filter to Jinja.
Render a datetime with the default format in a template like:

```html+jinja
Published on {{ this.pub_date|strftime }}
```

Or a custom format:

```html+jinja
Published on {{ this.pub_date|strftime('%Y-%m-%d') }}
```

Python datetime objects implement the standard comparison protocol, so
ordering still works without modifying `blog.ini`:

```ini
[children]
model = blog-post
order_by = -pub_date, title
```