Merge pull request #240 from fabaff/tutorial

Development tutorial

source/_cookbook/python_component_basic_state.markdown

@@ -10,34 +10,116 @@ footer: true
 ha_category: Custom Python Component Examples
 ---
 
-This is a simple hello world example to show the basics for setting a state. To use this example, create the file `<config dir>/custom_components/hello_state.py` and copy the below example code.
+This is a simple tutorial/example on how to write a component for [Home Assistant](https://home-assistant.io/). We will work on a component called "hello_state" to beginn with. The purpose of this component is to display a given text in the frontend. 
+
+The setup of a development environment is described in the [Developers section](/developers/#starting-development) of the documentation.
+
+## {% linkable_title Component %}
+
+To get started, create the file `<config dir>/custom_components/hello_state.py` and copy the below example code.
 
 ```python
-# The domain of your component. Should be equal to the name of your component.
+"""
+Support for showing text in the frontend.
+
+For more details about this component, please refer to the documentation at
+https://home-assistant.io/components/hello_state/
+"""
+import logging
+
+_LOGGER = logging.getLogger(__name__)
+
 DOMAIN = 'hello_state'
+DEPENDENCIES = []
 
-CONF_NAME = 'name'
+def setup(hass, config=None):
-DEFAULT_NAME = 'World'
+    """Setup the Hello State component. """
+    _LOGGER.info("The 'hello state' component is ready!")
 
-
-def setup(hass, config):
-    """Setup is called when Home Assistant is loading our component."""
-
-    # Get the name from the configuration. Use DEFAULT_NAME if no name provided.
-    name = config[DOMAIN].get(CONF_NAME, DEFAULT_NAME)
-
-    # States are in the format DOMAIN.OBJECT_ID
-    hass.states.set('hello_state.hello', name)
-
-    # Return boolean to indicate that initialization was successfully.
     return True
 ```
 
-Load the component by adding the following to your `configuration.yaml`:
+1. In the file header we decided to add some details: A short description and the link to the documentation. 
+2. We want to do some logging. This means that we import the Python logging module and create an alias.
+3. The component name is equal to the domain name.
+4. At the moment this component has no dependencies. For detail check [dependencies](/developers/creating_components/#dependencies) section.
+5. The `setup` function will take care of the initialization of our component.
+   The component will only write a log message. Keep in mind for later that you have several options for the severity:
+
+   - `_LOGGER.info(msg)`
+   - `_LOGGER.warning(msg)`
+   - `_LOGGER.error(msg)`
+   - `_LOGGER.critical(msg)`
+   - `_LOGGER.exception(msg)`
+
+7. We return `True` if everything is ok.
+
+Add the component to your `configuration.yaml` file.
 
 ```yaml
-# configuration.yaml entry
 hello_state:
-  # optional
-  name: Paulus
 ```
+
+After a start or a restart of Home Assistant the component will create an entry in the log.
+
+```bash
+16-03-12 14:16:42 INFO (MainThread) [custom_components.hello_state] The 'hello state' component is ready!
+```
+
+The next step is the introduction of configuration options. Most configuration details are coming out of the `configuration.yaml` file. To do that we need to update the `def setup()` method to accept configuration information and access the configuration variable in the `setup` method.
+
+More details about this topic can be found in the [User given configuration](/developers/creating_components/#config-user-given-configuration) section.
+
+```python
+import logging
+
+_LOGGER = logging.getLogger(__name__)
+
+DOMAIN = 'hello_state'
+DEPENDENCIES = []
+
+CONF_TEXT = 'text'
+DEFAULT_TEXT = 'No text!'
+
+def setup(hass, config):
+    """Setup the Hello State component. """
+    # Get the text from the configuration. Use DEFAULT_TEXT if no name is provided.
+    text = config[DOMAIN].get(CONF_TEXT, DEFAULT_TEXT)
+
+    # States are in the format DOMAIN.OBJECT_ID
+    hass.states.set('hello_state.Hello_State', text)
+
+    return True
+```
+
+To add the latest feature of our component, update the entry in your `configuration.yaml` file.
+
+```yaml
+information:
+  text: 'Hello, World!'
+```
+
+Thanks to `DEFAULT_TEXT` variable the component will launch even if no `text:` field is used in the `configuration.yaml` file. Quite often there are variables which are required. It's important to check if all mandatory configuration variables are provided. If not, the setup should fail. We will use the `validate_config` function as a helper to achive this. The next listing shows the essential parts.
+
+```python
+from homeassistant.helpers import validate_config
+[...]
+    if not validate_config(config, {DOMAIN: [CONF_TEXT]}, _LOGGER):
+        return False
+```
+
+If `text:` is missing, there will be a warning in the log file.
+
+```bash
+16-03-12 14:37:37 ERROR (MainThread) [custom_components.hello_state] Missing required configuration items in hello_state: text
+16-03-12 14:37:37 ERROR (MainThread) [homeassistant.bootstrap] component hello_state failed to initialize
+```
+
+After a start or a restart of Home Assistant the component will be visible in the frontend if the `configuration.yaml` file is up-to-date.
+
+<p class='img'>
+<img src='/images/screenshots/create-component01.png' />
+</p>
+
+To get your component included in the Home Assistant releases, follow the steps described in the [Submitting improvements](https://home-assistant.io/developers/#submitting-improvements) section. Basically you only need to move your component in the `homeassistant/component/` directory of your fork and create a Pull Request.
+