Quick Start

This section contains a series of hands-on examples that illustrate basic ways of using the Yangson library. The snippets below should be enough to get a moderately experienced Python programmer going. The examples use the YANG data model and instance document from Example 2. Of course, the reader is encouraged to continue experimenting on his or her own.


First, let’s import the necessary Python modules and symbols, and go to the appropriate directory:

>>> import os
>>> import json
>>> import yaml
>>> from yangson import DataModel
>>> from yangson.enumerations import ContentType
>>> os.chdir("examples/ex2")

Initializing the Data Model

Next, we need to initialize the YANG data model. It can be done easily by reading YANG library data [RFC7895] from a file:

>>> dm = DataModel.from_file('yang-library-ex2.json',
... [".", "../../../yang-modules/ietf"])
>>> dm.schema.description
'Data model ID: 9a9b7d2d28d4d78fa42e12348346990e3fb1c1b9'


Distribution directory tools/python contains the script mkylib.py that can help with preparing YANG library data.

This example also uses deviations (see sec. 5.6.3 in [RFC7950]) specified in YANG module example-2-dev:

  • leaf unlucky is removed from the schema

  • a default value is added for the leaf prime.

With the deviations applied, the resulting schema tree looks like this:

>>> print(dm.ascii_tree(), end='')
+--rw example-2:bag
   +--ro bar <boolean>
   +--rw baz? <decimal64>
   +--rw foo# [number]
      +--rw in-words? <string>
      +--rw number <uint64>
      +--rw prime? <boolean>

For the most part, Yangson uses the ASCII tree representation described in [RFC8340], the differences are described in the documentation for DataModel.ascii_tree() method.

As we can see in the ASCII tree, the data model contains both configuration data nodes (rw) and a single leaf bar representing state data (ro).

Loading and Validating Instance Data

Now, we can load a JSON-encoded instance document

>>> with open('example-data.json') as infile:
...   ri = json.load(infile)
>>> inst = dm.from_raw(ri)

and validate it against the data model:

>>> inst.validate(ctype=ContentType.all)

No output means that the validation was successful. Note that we had to use validation for content type all because the instance document contains both configuration and state data. The default content type for the InstanceNode.validate() method is configuration (ContentType.config).

Speaking about content type, it is also worth pointing out that the empty instance is valid as configuration:

>>> empty = dm.from_raw({})
>>> empty.validate()

However, it is not valid as content type all, i.e. combined configuration and state data, because the state data leaf bar is mandatory, which in turn makes the top-level container example-2:bag mandatory (see sec. 3 in [RFC7950]):

>>> empty.validate(ctype=ContentType.all)
Traceback (most recent call last):
yangson.exceptions.SchemaError: {/} missing-data: expected 'example-2:bag'

It is also possible to validate a subtree of instance data against the corresponding schema node. For example:

>>> foo2 = inst['example-2:bag']['foo'][2]
>>> foo2.validate()

We can now print the ASCII tree again, this time without showing the types but instead displaying validation counters that indicate how many times each schema node has been used for validating instances during the previous two validation runs on inst. This is useful for assessing the coverage of instance data with respect to the schema.

>>> print(dm.ascii_tree(no_types=True, val_count=True), end='')
+--rw example-2:bag {1}
   +--ro bar {1}
   +--rw baz? {0}
   +--rw foo# [number] {5}
      +--rw in-words? {5}
      +--rw number {5}
      +--rw prime? {3}

Moving Around and Editing the Data Tree

We can move around the instance data tree, either step by step or directly to any location by using an InstanceRoute. One way to obtain the latter is to parse it from a RESTCONF resource identifier:

>>> irt = dm.parse_resource_id('/example-2:bag/foo=3/in-words')
>>> type(irt)
<class 'yangson.instance.InstanceRoute'>

No we can go straight to the desired spot, see that we are really there, and inspect the value of that instance:

>>> inw = inst.goto(irt)
>>> inw.json_pointer()
>>> inw.value

We can also change the value:

>>> inw2 = inw.update('forty-two')
>>> inw2.value

Instance data is represented as a data structure called zipper [Hue97]. This structure is persistent, which means that invoking the update() method on the inw instance results in a new instance, and inw hasn’t changed at all – it contains the value of three as before:

>>> inw.value

We can move from the new instance inw2 back to the top, thus obtaining an edited version of the original data tree:

>>> inst2 = inw2.top()

We expect the two data trees to differ in the value of in-words leaf that we modified. To verify it, we can once again use the InstanceRoute irt that we compiled previously, this time with the peek() method:

>>> inst.peek(irt)
>>> inst2.peek(irt)

Another nice property of the zipper structure is that the two data trees share their contents to the maximum possible extent – it’s kind of copy on write.

So, the new data tree differs from the original but it is nevertheless still valid:

>>> inst2.validate(ctype=ContentType.all)

Adding Default Values

We can also add default values as specified in the data model to both data trees:

>>> iwd = inst.add_defaults()
>>> i2wd = inst2.add_defaults()

Again, it is worth noting that we get new instances whilst the original ones (inst and inst2) haven’t been touched.

The YANG module example-2 defines a default value of 0 for the baz leaf:

leaf baz {
  when "not(../foo/in-words = 'forty-two')";
  type decimal64 {
    fraction-digits "7";
  default "0";

So let’s see if that default value is in place:

>>> iwd['example-2:bag']['baz'].value

Indeed it is – Decimal('0E-7') is just a fancy way of writing decimal zero.

However, if we try the same for the other data tree, we don’t find the baz instance:

>>> i2wd['example-2:bag']['baz'].value
Traceback (most recent call last):
yangson.exceptions.NonexistentInstance: {/example-2:bag} member 'baz'

Why is that? The reason is also hidden in the above definition of the baz leaf: due to the edit that we made, the when expression becomes False, the baz leaf isn’t therefore valid, and so the default value doesn’t apply.

Breaking the Schema

In order to see validation in action, we will try to violate the data model schema in various ways. First, let’s modify the inw instance as follows:

>>> broken1 = inw.update("six").top()
>>> broken1.validate(ctype=ContentType.all)
Traceback (most recent call last):
yangson.exceptions.SemanticError: {/example-2:bag/foo} data-not-unique: entry 1

This is correct because the values of the in-words leaf are required to be unique among all entries of the foo list, but entry #1 now has the same value as the previous entry, namely six.

Next we modify the name sibling of our inw instance, which happens to be the key of the foo list:

>>> broken2 = inw.sibling('number').update(6).top()
>>> broken2.validate(ctype=ContentType.all)
Traceback (most recent call last):
yangson.exceptions.SemanticError: {/example-2:bag/foo} non-unique-key: 6

In this case, two entries of the foo list have the same key, namely 6, which illegal.

Another thing that YANG doesn’t permit is to install a leaf value that doesn’t conform to the leaf’s type, as in the following example:

>>> inw.update('INFINITY').validate()
Traceback (most recent call last):
yangson.exceptions.YangTypeError: {/example-2:bag/foo[number="3"]/in-words} invalid-type: must be number in words: INFINITY

This is again correct because the new value INFINITY doesn’t match the regular expression pattern in the definition of the in-words leaf. Note that the traceback displays the custom error message that is defined for the pattern.

And note also that validation needn’t be performed only on entire data trees, it can start from any instance node (inw in this case) and check just its subtree.

And finally, we delete a leaf that’s defined as mandatory in the data model:

>>> broken3 = inw.up().up().up().delete_item('bar').top()
>>> broken3.validate(ctype=ContentType.all)
Traceback (most recent call last):
yangson.exceptions.SchemaError: {/example-2:bag} missing-data: expected 'bar'

Instances in YAML Representation

Instance data may alternatively be read from a YAML document:

>>> with open('example-data.yaml') as infile:
...   ri = yaml.load(infile, Loader=yaml.SafeLoader)
>>> inst = dm.from_raw(ri)
>>> inst.validate(ctype=ContentType.all)
>>> inst.peek(irt)

This approach parses YAML data into a raw value using the Python module PyYAML, and relies on the close relationship between JSON and YAML. However, it hasn’t been heavily tested and may fail for some corner cases. For example, the PyYAML parser interprets unquoted strings yes and no as bool values True and False.