Opinionated Python bindings for the libyang library

Python bindings and packaging of libyang. We're focusing on parsing, validating and accessing YANG-modeled JSON data trees. Essentially, just enough to get gnpy going. Want more? Patches welcome.

Compared to the CFFI libyang bindings, this wrapper takes care of low-level memory management. This means no more node.free() and ctx.destroy(). We also produce prebuilt binary wheels to make installation very simple.

Usage

Loading YANG data

import oopt_gnpy_libyang as ly

c = ly.Context('tests/yang', ly.ContextOptions.RefImplemented | ly.ContextOptions.NoYangLibrary)
for m in ('iana-if-type', 'ietf-interfaces', 'ietf-ip'):
    c.load_module(m)
blob = '''{
  "ietf-interfaces:interfaces": {
    "interface": [
      {
        "name": "lo",
        "type": "iana-if-type:softwareLoopback",
        "ietf-ip:ipv4": {
          "address": [
            {
              "ip": "127.0.0.1",
              "prefix-length": 8
            }
          ]
        },
        "ietf-ip:ipv6": {
          "address": [
            {
              "ip": "::1",
              "prefix-length": 128
            }
          ]
        }
      },
      {
        "name": "eth0",
        "type": "iana-if-type:ethernetCsmacd"
      }
    ]
  }
}'''

data = c.parse_data(blob,
    ly.DataFormat.JSON, ly.ParseOptions.Strict | ly.ParseOptions.Ordered,
    ly.ValidationOptions.Present | ly.ValidationOptions.NoState)

Working with data

Libyang works with forests (sets of trees), this is how to process all the data:

for x in data.siblings():
    print(f'a sibling: {x.path}')
    for xx in x.children_dfs():
        print(f' {"term " if xx.is_term else "child"}: {xx.path}')
        if xx.is_term:
            print(f'  {xx.as_term()} {" (default)" if xx.as_term().has_default_value else ""}')

Data can be accessed via their known paths, of course. Either as a full, multi-level XPath:

data["interface[name='lo']/ietf-ip:ipv6/address[ip='::1']/prefix-length"].as_term().value == 128

Or individually, one item per index:

data["interface[name='lo']"]["ietf-ip:ipv6"]["address[ip='::1']"]["prefix-length"].as_term().value

Everything is an XPath, so it's possible to take a shortcut and skip specifying keys for single-element lists:

data["interface[name='lo']"]["ietf-ip:ipv6"]["address"]["prefix-length"].as_term().value == 128

The data are provided as native Python types:

type(data["interface[name='lo']"]["ietf-ip:ipv6"]["address"]["prefix-length"]
    .as_term().value) == int

Absolute paths and generic XPath expressions can be used to retrieve arbitrary parts of the data forest, and to iterate over them:

for iface in data.find("/ietf-interfaces:interfaces/interface"):
    print iface["name"].as_term().value

Relative XPath conditions can be also used at the root level (which is represented as NULL in the C level):

for iface in search_at_root(data)("ietf-interfaces:interfaces/interface"):
    print iface["name"].as_term().value

New values can be created; use None for non-terminals, or str when a value is needed:

node = ctx.create("/ietf-interfaces:interfaces/interface[name='666']")
another = ctx.create("/ietf-interfaces:interfaces/interface[name='666']/ietf-ip:ipv6/enabled", "true")
data["interface[name='lo']"]["ietf-ip:ipv6"]["address"]["prefix-length"] = "64"

Validation errors

In libyang, if an operation fails, error details are available via context.errors():

import json
wrong = json.loads(blob)
wrong["ietf-interfaces:interfaces"]["interface"][0]\
    ["ietf-ip:ipv6"]["address"][0]["prefix-length"] = 666
try:
    data = c.parse_data(json.dumps(wrong),
        ly.DataFormat.JSON, ly.ParseOptions.Strict | ly.ParseOptions.Ordered,
        ly.ValidationOptions.Present | ly.ValidationOptions.NoState)
    assert False
except ly.Error:
    for error in c.errors():
        assert error.schema_path is None
        assert error.data_path == "/ietf-interfaces:interfaces/interface[name='lo']/ietf-ip:ipv6/address[ip='::1']/prefix-length"
        assert error.line > 0
        assert error.message == 'Value "666" is out of type uint8 min/max bounds.'

Installing

We're producing wheels for many popular platforms. The installation is as simple as:

$ pip install oopt-gnpy-libyang

Building from source

Since this library is a Python wrapper around a C++ wrapper around a C library, source-based builds are more complex. They require:

• a C++20 compiler (e.g., GCC 10+, clang 10+, MSVC 17.2+)
• libyang and its dependencies
• libyang-cpp and its dependencies
• CMake 3.21+

Unlike the wheels already bundle all the required libraries, when building from source, libyang, libyang-cpp and all their dependencies will have to be installed first. Also, in a from-source build these won't be bundled into the resulting package. For an inspiration, consult our GitHub packaging recipes.

License

Copyright © 2021-2023 Telecom Infra Project and GNPy contributors. Licensed under the 3-clause BSD license.