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 that people often struggle with is the YAML configuration files. 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. YAML works with two main data structures - *Dictionaries* and *Lists*. **Lists** are easy to understand. They're just a list - ordered collections 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** (or *dicts* for short) are not much more complicated. You've probably encountered them 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 an unordered collection of entries - where each entry has a *Key* and a *Value*. The Key can be thought of as a name or title for the entry, and the Value is the data of the entry. Remember: - Keys are unique. Two entries in the same dict can not have the same key. - Values can be anything, just like the items in a list - Strings, integers, booleans, dictionaries or lists. 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 dict: ```yaml Name: Thomas Lovén Email: thomasloven@example.com Hobbies: singing, woodworking, home automation ``` Now we added an entry to the dict where the value is a list. I have three hobbies. This illustrates that the value of a dictionary entry 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 at this point things are getting advanced. It's hard to keep track of what is a dict 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? Soon... JSON is a simple way of writing down the concepts described above, which can be easily understood both by humans and by computers. JSON describes *object* and *arrays*, 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 entries separated by commas. The key and value of each entry is separated by a colon. Keys must be strings, but values can be anything that can be in a list. Let's look at our dict 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 entry with the nested dict-list-dict about my children. Two things to note: - You don't need quotes around the keys, but you do around string values. 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 or superset of JSON. In fact, any valid JSON is also valid YAML. That might be useful to remember. There are some notable differences, though. First of all, YAML doesn't require 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. I'll just take the opportunity to say this again: Indentation is important. Probably the most important concept of YAML. Slightly simplified one can say that by indenting line, those line become a continuation of the line above, until the next de-dented line. Dictionaries are also written one entry per line, as `key: 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 space after the colon is important. - The value of `Hobbies` is a list. Like above, each line of the value is indented by an equal number of spaces. - The value of `Phones` is a dictionary. The same indentation rules apply. - The value of `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 either won't be accepted by the parser or 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 an entry in a dict? Is it an item in a list? What is it's parent? What are it's children? What are it's siblings? If you have a complex structure - say some nested cards in lovelace - it might help to make a drawing on actual paper of what you want. In the YAML dictionary sample above, I used a contracted form in my list of dicts. 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 ``` Also, remember that dictionary entries are unordered. The YAML below describes the same thing as that above, but might be more confusing. ```yaml Children: - Name: N Age: 3 - Age: 1 Name: H ``` Often software generated YAML (such as what's produced by [lovelace-gen](https://github.com/thomasloven/homeassistant-lovelace-gen)) write the entries out in such a way that the keys are ordered alphabetically. ### Advanced topics #### Comments Adding comments to your code makes it easier to understand. Both to other people, and - more importantly - to your self 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 simply 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 ``` If you want a number sign in your entry, the entry must be quoted. #### 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 a single entry "Halflife 2" with 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. Further, YAML allows single line comma separated lists without braces. ```yaml Hobbies: singing, woodworking, home automation ``` This also means that a string containing a comma must be quoted. #### Merging Dictionaries can be merged using the merging operator: `<<`. For example: ```yaml a key: a value b key: b value <<: {c key: c value, d key: d 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 `<<` operator 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 of `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 ``` You should be able to guess how this parses out. 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 entries `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` entry 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.