This question tries to collect information spread over questions about different languages and YAML implementations in a mostly language-agnostic manner.
Suppose I have a YAML file like this:
first:
- foo: {a: "b"}
- "bar": [1, 2, 3]
second: | # some comment
some long block scalar value
I want to load this file into an native data structure, possibly change or add some values, and dump it again. However, when I dump it, the original formatting is not preserved:
The scalars are formatted differently, e.g. "b" loses its quotation marks, the value of second is not a literal block scalar anymore, etc.
The collections are formatted differently, e.g. the mapping value of foo is written in block style instead of the given flow style, similarly the sequence value of "bar" is written in block style
The order of mapping keys (e.g. first/second) changes
The comment is gone
The indentation level differs, e.g. the items in first are not indented anymore.
How can I preserve the formatting of the original file?
Preface: Throughout this answer, I mention some popular YAML implementations. Those mentions are never exhaustive since I do not know all YAML implementations out there.
I will use YAML terms for data structures: Atomic text content (even numbers) is a scalar. Item sequences, known elsewhere as arrays or lists, are sequences. A collection of key-value pairs, known elsewhere as dictionary or hash, is a mapping.
If you are using Python, using ruamel will help you preserve quite some formatting since it implements round-tripping up to native structures. However, it isn't perfect and cannot preserve all formatting.
Background
The process of loading YAML is also a process of losing information. Let's have a look at the process of loading/dumping YAML, as given in the spec:
When you are loading a YAML file, you are executing some or all of the steps in the Load direction, starting at the Presentation (Character Stream). YAML implementations usually promote their most high-level APIs, which load the YAML file all the way to Native (Data Structure). This is true for most common YAML implementations, e.g. PyYAML/ruamel, SnakeYAML, go-yaml, and Ruby's YAML module. Other implementations, such as libyaml and yaml-cpp, only provide deserialization up to the Representation (Node Graph), possibly due to restrictions of their implementation languages (loading into native data structures requires either compile-time or runtime reflection on types).
The important information for us is what is contained in those boxes. Each box mentions information which is not available anymore in the box left to it. So this means that styles and comments, according to the YAML specification, are only present in the actual YAML file content, but are discarded as soon as the YAML file is parsed. For you, this means that once you have loaded a YAML file to a native data structure, all information about how it originally looked in the input file is gone. Which means that when you dump the data, the YAML implementation chooses a representation it deems useful for your data. Some implementations let you give general hints/options, e.g. that all scalars should be quoted, but that doesn't help you restore the original formatting.
Thankfully, this diagram only describes the logical process of loading YAML; a conforming YAML implementation does not need to slavishly conform to it. Most implementations actually preserve data longer than they need to. This is true for PyYAML/ruamel, SnakeYAML, go-yaml, yaml-cpp, libyaml and others. In all these implementations, the style of scalars, sequences and mappings is remembered up until the Representation (Node Graph) level.
On the other hand, comments are discarded rather early since they do not belong to an event or node (the exceptions here is ruamel which links comments to the following event, and go-yaml which remembers comments before, at and after the line that created a node). Some YAML implementations (libyaml, SnakeYAML) provide access to a token stream which is even more low-level than the Event Tree. This token stream does contain comments, however it is only usable for doing things like syntax highlighting, since the APIs do not contain methods for consuming the token stream again.
So what to do?
Loading & Dumping
If you need to only load your YAML file and then dump it again, use one of the lower-level APIs of your implementation to only load the YAML up until the Representation (Node Graph) or Serialization (Event Tree) level. The API functions to search for are compose/parse and serialize/present respectively.
It is preferable to use the Event Tree instead of the Node Graph as some implementations already forget the original order of mapping keys (due to internally using hashmaps) when composing. This question, for example, details loading / dumping events with SnakeYAML.
Information that is already lost in the event stream of your implementation, for example comments in most implementations, is impossible to preserve. Also impossible to preserve is scalar layout, like in this example:
"1 \x2B 1"
This loads as string "1 + 1" after resolving the escape sequence. Even in the event stream, the information about the escape sequence has already been lost in all implementations I know. The event only remembers that it was a double-quoted scalar, so writing it back will result in:
"1 + 1"
Similarly, a folded block scalar (starting with >) will usually not remember where line breaks in the original input have been folded into space characters.
To sum up, loading to the Event Tree and dumping again will usually preserve:
Style: unquoted/quoted/block scalars, flow/block collections (sequences & mappings)
Order of keys in mappings
YAML tags and anchors
You will usually lose:
Information about escape sequences and line breaks in flow scalars
Indentation and non-content spacing
Comments – unless the implementation specifically supports putting them in events and/or nodes
If you use the Node Graph instead of the Event Tree, you will likely lose anchor representations (i.e. that &foo may be written out as &a later with all aliases referring to it using *a instead of *foo). You might also lose key order in mappings. Some APIs, like go-yaml, don't provide access to the Event Tree, so you have no choice but to use the Node Graph instead.
Modifying Data
If you want to modify data and still preserve what you can of the original formatting, you need to manipulate your data without loading it to a native structure. This usually means that you operate on YAML scalars, sequences and mappings, instead of strings, numbers, lists or whatever structures the target programming language provides.
You have the option to either process the Event Tree or the Node Graph (assuming your API gives you access to it). Which one is better usually depends on what you want to do:
The Event Tree is usually provided as stream of events. It may be better for large data since you do not need to load the complete data in memory; instead you inspect each event, track your position in the input structure, and place your modifications accordingly. The answer to this question shows how to append items giving a path and a value to a given YAML file with PyYAML's event API.
The Node Graph is better for highly structured data. If you use anchors and aliases, they will be resolved there but you will probably lose information about their names (as explained above). Unlike with events, where you need to track the current position yourself, the data is presented as complete graph here, and you can just descend into the relevant sections.
In any case, you need to know a bit about YAML type resolution to work with the given data correctly. When you load a YAML file into a declared native structure (typical in languages with a static type system, e.g. Java or Go), the YAML processor will map the YAML structure to the target type if that's possible. However, if no target type is given (typical in scripting languages like Python or Ruby, but also possible in Java), types are deduced from node content and style.
Since we are not working with native loading because we need to preserve formatting information, this type resolution will not be executed. However, you need to know how it works in two cases:
When you need to decide on the type of a scalar node or event, e.g. you have a scalar with content 42 and need to know whether that is a string or integer.
When you need to create a new event or node that should later be loaded as a specific type. E.g. if you create a scalar containing 42, you might want to control whether that it is loaded as integer 42 or string "42" later.
I won't discuss all the details here; in most cases, it suffices to know that if a string is encoded as a scalar but looks like something else (e.g. a number), you should use a quoted scalar.
Depending on your implementation, you may come in touch with YAML tags. Seldom used in YAML files (they look like e.g. !!str, !!map, !!int and so on), they contain type information about a node which can be used in collections with heterogeneous data. More importantly, YAML defines that all nodes without an explicit tag will be assigned one as part of type resolution. This may or may not have already happened at the Node Graph level. So in your node data, you may see a node's tag even when the original node does not have one.
Tags starting with two exclamation marks are actually shorthands, e.g. !!str is a shorthand for tag:yaml.org,2002:str. You may see either in your data, since implementations handle them quite differently.
Important for you is that when you create a node or event, you may be able and may also need to assign a tag. If you don't want the output to contain an explicit tag, use the non-specific tags ! for non-plain scalars and ? for everything else on event level. On node level, consult your implementation's documentation about whether you need to supply resolved tags. If not, same rule for the non-specific tags applies. If the documentation does not mention it (few do), try it out.
So to sum up: You modify data by loading either the Event Tree or the Node Graph, you add, delete or modify events or nodes in the data you get, and then you present the modified data as YAML again. Depending on what you want to do, it may help you to create the data you want to add to your YAML file as native structure, serialize it to YAML and then load it again as Node Graph or Event Tree. From there, you can include it in the structure of the YAML file you want to modify.
Conclusion / TL;DR
YAML has not been designed for this task. In fact, it has been defined as a serialization language, assuming that your data is authored as native data structures in some programming language and from there dumped to YAML. However, in reality, YAML is used a lot for configuration, meaning that you typically write YAML by hand and then load it into native data structures.
This contrast is the reason why it is so difficult to modify YAML files while preserving formatting: The YAML format has been designed as transient data format, to be written by one application, and then to be loaded by another (or the same) application. In that process, preserving formatting does not matter. It does, however, for data that is checked-in to version control (you want your diff to only contain the line(s) with data you actually changed), and other situations where you write your YAML by hand, because you want to keep style consistent.
There is no perfect solution for changing exactly one data item in a given YAML file and leaving everything else intact. Loading a YAML file does not give you a view of the YAML file, it gives you the content it describes. Therefore, everything that is not part of the described content – most importantly, comments and whitespace – is extremely hard to preserve.
If format preservation is important to you and you can't live with the compromises made by the suggestions in this answer, YAML is not the right tool for you.
I would like to challenge the accepted answer. Whether you can preserve comments, the order of map keys, or other features depends on the YAML parsing library that you use. For starters, the library needs to give you access to the parsed YAML as a YAML Document, which is a collection of YAML nodes. These nodes can contain metadata besides the actual key/value pairs. The kinds of metadata that your library chooses to store will determine how much of the initial YAML document you can preserve. I will not speak for all languages and all libraries, but Golang's most popular YAML parsing library, go-yaml supports parsing YAML into a YAML document tree and serializing YAML document back, and preserves:
comments
the order of keys
anchors and aliases
scalar blocks
However, it does not preserve indentation, insignificant whitespace, and some other minor things. On the plus side, it allows modifying the YAML document and there's another library,
yaml-jsonpath that simplifies browsing the YAML node tree. Example:
import (
"github.com/stretchr/testify/assert"
"gopkg.in/yaml.v3"
"testing"
)
func Test1(t *testing.T) {
var n yaml.Node
y := []byte(`# Comment
t: &t
- x: 1 # anchor
a:
b: *t # alias
b: |
cccc
dddd
`)
err := yaml.Unmarshal(y, &n)
assert.NoError(t, err)
y2, _ := yaml.Marshal(&n)
assert.Equal(t, y, y2)
}
I'm using Azure Search on my e-commerce site, and now want to implement filtering and I can't choose right way to do is. The problem is that I have really different product types, so they have various attributes (properties) and I don't want to create index with 50 attribute fields to faced them all. Another way - I can define few properties (like Attribute1, Attribute2 ... ) and then determine their 'Key' names according to facated values, but it sound not so good too. Is there some common or checked way to build filters on e-commerce sites?
Azure Search will do well if you have 50 fields that have sparse values. Assuming that sparseness comes with relatively low per-facet cardinality, it shouldn’t have a bad performance impact, and by explicitly modeling stuff as fields you get to keep the nice and clean programming model that comes from having explicit fields.
The attribute mapping thing would work assuming all facets are of the same data type or can be grouped easily.
Another thing you can do for string facets is to have a prefix in the facet value (e.g. Cameras/Canon, Memory/MiniSD, etc.). That gives you counts per value within the parent scope. If you also have the parent scope (e.g. Camera, Memory) in a separate field you can filter by the whole scope if needed. This generalizes well into hierarchical facets (as long as they are all strings). Here is a blog post that explores this technique.
New to XSD here.
Has anyone found or written a framework for validating XML with an XML schema in Cocoa/Obj-C?
What I really need is the ability to define permitted types of modifications to an NSXMLDocument, as described in an XSD file. This includes defining sequences of child elements, list of attributes and their permitted values, etc etc. I need to expose these modification rules in my UI. For example:
I want to constrain the names of the new child elements added to an existing NSXMLElement node in my NSOutlineView
If the XSD says that Node A has required child elements (Nodes Aa and Ab) then when the user adds Node A to the XML tree, I want to automatically create Nodes Aa & Ab and add them to the just-created Node A.
etc etc
It seems to me that a good solution would be a Cocoa counterpart of JAXB. XSOM (which doesn't create schema-derived classes, but rather gives an query-able object model of the XSD) would work too.
My question is similar to this one, but I don't want to limit myself to JAXB-like solution. I'm interested in finding out other solutions that people have come up to this problem.
Cheers!!
You can create a DTD and validate against it, or create a recursive parser based on your XSD, such as existing ones for RSS or Atom based on the spec.
I have class called 'Tool'. Tool has properties like: Name, Description and some specific others. Tool is special because Name and others are read only but description can be modified by users.
Count of tools is constant and known at development time. It is not Value Object because I need to query them and show to users where they can update Description. So, it's kind of Entity but users can not create new Tools.
I'm looking for possibility to take Tool like this:
Tool.SomeGreatTool
where SomeGreatTool is Tool with name "Some great tool" and description should be the same like this specified by user.
Jimmy Bogard has solution almost perfect but NHibernate know anything about SomeGreatTool and Description will be null.
How to modify Jimmy's solution or how to do it different way? How to instantiate SomeGreatTool from database?
We still treat these kinds of semi-constant data as a sort of well-known entities. We create value objects/enumeration classes for the tool types, but separate tool types from tools. You still need to go to the persistent store to do something like ToolRepository.Find(ToolType.Screwdriver). Even though there will be only one tool per tool type, you'd still separate these two concepts.
Files and directories could have different namespaces, and still be used to identify specific files, because a file and directory with the same name can be distinguished by being different kinds of things.
Primitive field and reference fields could also have different namespaces (in Java), because if a primitive and a reference field had the same name, they could be identified by being different kinds of things.
Separate namespaces are used elsewhere like this. For example, in Java, you can have a method exampleName() and a field exampleName, and though they have the same name, they are distinguished by being different kinds of things.
First, this question is language specific. In pure OOP languages there's no distinction between atomic and compound elements. Everything is an object. By a similar reason in a pure functional language you can't have function and variable named the same.
Second, if you have polymorphic operations, there's no way to tell which variable did you refer to. For example, you can't have different namespaces for files and directories, because of the polymorphic operations, like
cp foo bar
The cp works on files and dirs, and if you have different namespaces, there's no way to tell what did you mean.
I do not believe that this would be a good idea. I imagine that the reasons involve things like performance and simplicity of the filesystem code. If a directory listing had to go down 2 or 3 or more different paths depending on how many different namespaces you think you should have, this would probably complicate the code.
Additionally, consider the end user confusion that might arise. Currently we have a kind of namespacing available in filesystems by using file extensions. You can have file.txt and file.dll and file.exe all existant in the same directory. What happens when these files are present simultaneously is a matter of concern - this has been one method for virus writers to use a form of social engineering to get you to click on the wrong file. Imagine if you could confuse a directory with a file of the same name as well?
Directories and files aren't necessarily so different. They are both entries in their parent directory, just with a flag to indicate if an entry is a directory or not. You can open a directory and read it just as if it was a file, just that certain other operations are possible on it- symbolic links work the same way. (This description is biased towards the Unix filesystem view, but I think the DOS/Windows view works much the same way). Within any directory, there is a set of names of members, and the filesystem enforces the uniqueness constraint that a directory can only have one member with a given name.
Thinking about Java method names compared to field names--- back in C, you couldn't have a global function and global variable with the same name, because all the symbols in the object file are in a single namespace. But you could with C++, because a function "void foo()" was mapped to mangled symbol name ("foo__vv" or something). So it's not so much that they have a separate namespace, as that the key into the namespace is different for a field "foo" vs a method "foo()". Given that you can't get key clashes, they look like separate namespaces, but is that really how it's implemented?