layout: post title: YAML for Non-programmers subtitle: and programmers tags: [homeassistant] It's been a while... ### Introduction I've been fiddling with home automation for a few years, but only recently found my way to [http://www.home-assistant.io](homeassistant) - a project with great developers and a great community. I've been hanging about the official [Discord Chat Server](https://discord.gg/c5DvZ4e), and try to give something back by helping people when I can. One thing I noticed is that people often struggle with is the fact that the configuration is made through YAML. A strange choice for the kind of quasi-programming you may want to do when automating your home appliances. In this post I have tried to describe how YAML works, and how I think about the way it represents basic data structures. I hope it can be useful to someone... ### Dictionaries and lists To understand YAML, you need to understand what it's describing. There are two main concepts: *Dictionaries* and *Lists*. **Lists** are quite simple. It's just a list - an ordered collection of things. There are two things you need to remember though. Let me show them to you in a list :) - Lists are ordered. If you make a list with a dog, a cat and an elephant, the dog will be first, the elephant will be last and the cat will be in between. - Lists can be lists of anything; strings, integers, booleans, or even dictionaries or lists. A list of lists might sound weird, but just think about the lists you have in your home. - A shopping list - A todo list - A list of things to pack for the vacation next week - The list of passwords on a post-it under your keyboard **Dictionaries** are not much more complicated. You've probably seen it in some way or another, but perhaps with a different name. In different programming languages they can be called *dictionaries*, *hashes*, *maps*, *hash tables*, *tables*, *collections* or even just *objects*. The technical name is *Associative Array*. Regardless of name, the concept is simple. A dictionary is a collection of key-value pairs. That is, a Name for something, and that Something. The name - the *key* must be unique. The same key can not be used twice in the same dictionary. The Something - the *value* can be anything at all, just like the items in a list. Let's look at some sample dictionaries: ```yaml Monday: Sausage and beans Tuesday: Fish Wednesday: French onion soup Thursday: Pea soup and pancakes Friday: Pizza ``` Each day is labeled by a key, and has a value - what are you going to eat that day. Note that while you could add another `Wednesday` to the end of the list, it wouldn't really make sense. Thus keys have to be unique. The values doesn't however. It would make perfect sense to eat pizza again on Saturday. Since keys are unique, their order is not important: ```yaml Wednesday: French onion soup Friday: Pizza Monday: Sausage and beans Tuesday: Fish Thursday: Pea soup and pancakes ``` This dictionary contains exactly the same data as the one above. A clear difference from a list, where the order itself is a part of the data. Another dictionary: ```yaml Name: Thomas Lovén Email: thomasloven@example.com ``` That's a dictionary. Looks kind of like a database of a sort, doesn't it? Like the address book in your email program? Ah! But don't get fooled. The address book is a **list**, not a **dictionary**. However - the *items* in the list are *dictionaries*. Let's add on to that dictionary: ```yaml Name: Thomas Lovén Email: thomasloven@example.com Hobbies: singing, woodworking, home automation ``` Now we added an entry to the dictionary where the value is a list. I have three hobbies. This illustrates that the value of a dictionary key-value pair can be anything. Even a dictionary: ```yaml Name: Thomas Lovén Email: thomasloven@example.com Hobbies: singing, woodworking, home automation Phones: Home: +46 (0)XX XXXXXX, Work: +40 (0)XX XXXXXX ``` And remember that lists can contain dictionaries too... ```yaml Name: Thomas Lovén Email: thomasloven@example.com Hobbies: singing, woodworking, home automation Phones: Home: +46 (0)XX XXXXXX, Work: +40 (0)XX XXXXXX Children: Name: N, Age: 3 ; Name: H, Age: 1 ``` But that this point things are getting advanced. It's hard to keep track of what is a dictionary and what is a list and what contains what... If only there was a language to describe these concepts... a sort of Markup Language, if you will... but who needs Yet Another one of those? Let me tell you about ### JSON Javascript Structured Object Notation - [JSON](https://www.json.org/). You thought I was going to say YAML, didn't you? We'll get there too. JSON is a simple way of writing down the concepts described above which can be easily understood both by humans and by computers. Basically, there are *objects* and *arrays*, but let's call them *dictionaries* and *lists* instead. *lists* are surrounded by square brackets and contain items separated by commas. The items can be strings, numbers, dictionaries, lists or any of the magic values `true`, `false`, or `null`. *Dictionaries* are surrounded by curly braces and contain key-value pairs separated by commas. Each key-value pair has the key, a colon and the value. Keys must be strings, but values can be anything that can be in a list. Let's look at our dictionary from above in JSON format: ```json { Name: "Thomas Lovén", Email: "thomasloven@example.com", Hobbies: [ "singing", "woodworking", "home automation" ] Phones: { Home: "+46 (0)XX XXXXXX", Work: "+46 (0)XX XXXXXX" } Children: [ { Name: "N", Age: 3}, { Name: "H", Age: 1} ] } ``` I added some line breaks and indentations to make it more pretty, but this is much easier to read. Even the last key-value pair about my children. Two things to note - You don't need quotes around the keys, but you do around values that are strings. If you want whitespace in a key (which is entirely OK) it must be quoted, though. - The indentations and newlines I added, and in fact any whitespace not in quotes, is ignored. OK. Now you understand one markup language. Let's learn something different. ### YAML [YAML](http://yaml.org/) Ain't a Markup Language - but it's pretty darn close, to be honest. While probably not historically accurate, YAML can be seen as an evolution of JSON. In fact, any valid JSON is also valid YAML. That might be important to remember. There are some notable differences, though. First of all, YAML does away with the braces. Instead items in lists are separated by newlines where each item starts with a dash: ```yaml - Item 1 - Item 2 - Item 3 is a long one that stretches over multiple lines. The new item won't start until we get to a line that starts with a dash, like the one below this one. - Item 4 - - Item 5a - Item 5b - Item 5c - Item 5 ``` Some things to note: - There are no quotes. In YAML quotes are pretty much optional.This can be a blessing and a curse. For example `"true"` is a string, but `true` is a boolean value. - Indentation is important. Item 3 in the list stretches over multiple lines. Each line after the first one is indented (with an equal number of spaces, *NOT* tabs). The same is true for Item 5, which is a list. Each item in the sub-list is indented with an equal number of spaced. - The items of the list are not of the same type. Most are strings, but item 5 is a list. Dictionaries are also separated on lines with the key, a colon and the value: ```yaml Name: Thomas Lovén Email: thomasloven@example.com Hobbies: - singing - woodworking - home automation Phones: Home: +46 (0)XX XXXXXX Work: +46 (0)XX XXXXXX Children: - Name: N Age: 3 - Name: H Age: 1 ``` Things to note: - The value corresponding to the key `Hobbies` is a list. Like above, each line of the value is indented by an equal number of spaces. - The value corresponding to the key `Phones` is a dictionary. The same indentation rules apply. - The value corresponding to the key "Children" is a list where each item is a dictionary. So each line in each dictionary is indented twice. - The second entry in the list of children uses a contracted form, where the first key-value pair of the dictionary is put on the same line that signifies the list item. More on this later. And that's all the basics of data representation using YAML. ### On indentation As I've been trying to help people with their configurations on the homeassistant Discord server, I have found one problem which occurs more than any other. Indentation errors. *Indentation is important* It *must* be correct, or the YAML won't be accepted by the parser, or it will describe something entirely different from what you intended. The only advice I can give is to think carefully about the structure of the data you are trying to represent. What is your object? Is it a dictionary or a list? Where is it contained? Is it freestanding? Is it the value of a dictionary key-value pair? Is it an item in a list? What is it's parent? What are it's children? What are it's siblings? It it's a complex structure, it might help to make a drawing on actual paper. In the YAML dictionary sample above, I used a contracted for in my list of dictionaries. This is common practice, but may be a bit confusing at first since it makes the indentation unclear. If might be easier to understand the structure of the document if you use the expanded form: ```yaml Children: - Name: N Age: 3 - Name: H Age: 1 ``` ### Advanced topics #### Comments Adding comments to your code makes it easier to understand. Both to other people, and - more importantly - to you when you return to it in six months because something stopped working. In YAML, comments begin with a number sign `#`, last until the end of the line and are ignored by the parser. ```yaml # A dictionary about me Name: Thomas Lovén Email: thomasloven@example.com # This isn't really my email Hobbies: # Just some of the ways I like to waste time - singing # choir, mostly - woodworking ``` #### Spaces and colons As mentioned, YAML doesn't require quotes around strings, but they are allowed. Quotes can be useful to tweak the parsing. Imagine for example the following list: ```yaml - Halflife 1 - Halflife 2 - Halflife 2: Episode Two ``` This is a list of strings, right? Wrong. The third entry is a dictionary with the key "Halflife 2" and the value "Episode Two" (keys can contain spaces, by the way). To fix this, you can use quotes: ```yaml - Halflife 1 - Halflife 2 - "Halflife 2: Episode Two" ``` #### Using JSON There's a reason I went through JSON to explain YAML. As I said, all JSON is also valid YAML. This allows for compact notation: ```yaml Name: Thomas Lovén Email: thomasloven@example.com Hobbies: [singing, woodworking, home automation] Phones: Home: +46 (0)XX XXXXXX Work: +46 (0)XX XXXXXX Children: - {Name: N, Age: 3} - {Name: H, Age: 1} ``` I mention this because you just might run into it sometime. I like to use it in my configurations to bring down the line count, but it's easy to go overboard and make the data hard to read instead. In the end it's a matter of taste. Note that there are still no quotes. That's OK as long as you don't want commas, } or ] in the value. #### Merging Dictionaries can be merged using the key: `<<`. For example: ```yaml a key: a value b key: b value <<: {d key: d value, e key: e value} ``` will be parsed as ```yaml a key: a value b key: b value c key: c value d key: d value ``` and so will ```yaml a key: a value b key: b value <<: c key: c value d key: d value ``` In short, the `<<` key takes a dictionary as its value, and merges it into the parent dictionary. #### Node anchors Merging is very convenient when used in combination with node anchors. Node anchors are a way of saving a dictionary, and reusing it later ```yaml my_dict: &my_dict a: 1 b: 2 c: 3 ``` In this case `&my_dict` is NOT the value corresponding to the key `my_dict`, but a node anchor - as signified by the ampersand `&`. The anchor saves the value for later reuse and can be recalled any number of times using an asterisk `*`: ```yaml a dictionary: &saved a: 1 b: 2 c: 3 another dictionary: *saved a list: - *saved - *saved ``` This will be parsed as: ```yaml a dictionary: a: 1 b: 2 c: 3 another dictionary: a: 1 b: 2 c: 3 a list: - a: 1 b: 2 c: 3 - a: 1 b: 2 c: 3 ``` You can also merge an anchor if you want to add more entries to the dict: ```yaml base: &base a: 1 b: 2 extended version: <<: *base c: 3 ``` At this point, understanding how this will parse shouldn't be a problem to you. Now, for my final trick: #### Merging while defining. The problem with the above examples is that you need to put the definition somewhere. The YAML snippets above will have the dictionary keys `a dictionary`and `base` defined and set no matter what. Sometimes that's impractical, which is why you often see the following in homeassistant packages: ```yaml homeassistant: customize: package.node_anchors: common: &common key1: val1 key2: val2 sensor.my_sensor: <<: *common icon: mdi:temp sensor.another_sensor: <<: *common icon: mdi:home ``` The `package.node_anchors` key in the `customize` dictionary contains a dictionary of stuff that is simply ignored. Anything you put there will have no effect on the package, so it's a great place to define anchors. Another possibility is to put the definition in the first place it is used, and merge it immediately: ```yaml homeassistant: customize: sensor.my_sensor: <<: &common {key1: val1, key2: val2} icon: mdi:temp sensor.another_sensor: <<: *common icon: mdi:temp ``` Not all YAML parsers allow this, but it seems to work with homeassistant.