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Validate json documents using a Python dictionary defining keys and types.

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In Greek mythology, the Colchian Dragon guarded the Golden Fleece. Jason was sent on a quest to obtain the Golden Fleece, to prove himself worthy as king - but the Colchian Dragon was the final obstacle (after many others) stopping Jason from obtaining the fleece.

The colchian package contains the Colchian class, which can be used to validate .json documents, or rather the Python dict resulting from loading a .json file with json.load().

Colchian was developed with validation of .json configurations in mind, specifically those provided by the Conffu ( package, but will work for any reasonably sized .json file (no testing was performed for large documents, nor has the code been optimised for performance).


Colchian is available from PyPI:

pip install colchian


A very simple example:

from colchian import Colchian
from json import load, dumps

type_dict = {
    "an integer": int,
    "some strings": [str]

with open('my.json') as f:
    data = load(f)
        valid_data = Colchian.validated(data, type_dict)
    except SyntaxError as e:
        print(f'Something is wrong: {e}')

A valid my.json:

  "an integer": 42,
  "some strings": ["vastly", "hugely", "mind-bogglingly", "big"]

To use Colchian:

  • create a dictionary that defines the structure and types of a valid data dictionary (from a .json document);
  • call Colchian.validated() with the data and the type dictionary;
  • the result will be the same data, with some data casted to the appropriate type, if strict=False is passed to the .validated() method;
  • a SyntaxError exception will be raised if the data is not valid according top the type_dict.

A few more tricks:

  • use the Python typing module to use special types like Union, Any, List or Optional;
  • use wildcards (keys starting with *) to define elements that may appear with any name (and repeatedly);
  • instead of typing.Union, you can also use a tuple to indicate multiple options (for example (int, float) for a number field);
  • assign a function as a type to perform custom validation or transformation on elements.

These two type dictionaries function identically:

type_dict1 = {
    'words': [str],
    'secret': (bool, None)

type_dict2 = {
    'words': List[str],
    'secret': Optional[bool]

When setting functions as a type, make sure they match this signature:

def func(x: Any, *args, strict: bool, keys: List[str]) -> Any:

That is, the function should expect the value to validate as the first positional parameter, followed by any extra parameters you may define in the type dict, with two required keyword parameters, strict (which will tell your function if the validation should be strict) and keys (which will tell your function where the value sits in the .json document).

To keep things easily organised, you should probably define such functions as methods on child of the Colchian class. For example:

from typing import List
from colchian import Colchian

class MyColchian(Colchian):
    def fizzbuzz(xs: List[int], fizz: str, buzz: str, strict: bool, keys: List[str]):
        return [fizz*(i % 3 == 0)+buzz*(i % 5 == 0) or i for i in xs]

data = {
    'xs': range(15)

type_dict = {
    'xs': (MyColchian.fizzbuzz, 'fizz', 'buzz')

print(MyColchian.validated(data, type_dict))

You can pass extra parameters to your functions like this:

type_dict = {
    'xs': (MyColchian.dt_str, '%Y-%m-%d %H:%M')

The validation will interpret the rest of the tuple as positional parameters after the value, instead of type options.

Note: this means that you can't pass two functions as the only options for a type:

# causes problems:
type_dict = {
    'xs': (MyColchian.method1, MyColchian.method2)

This will cause values for key 'xs' to be validated by calling MyColchian.method1(value, MyColchian.method2, strict, keys), instead of trying each function separately as you might expect. If you need this functionality, you can instead do this:

# no problem:
type_dict = {
    'xs': ((MyColchian.method1,), (MyColchian.method2,))

Similarly, if you want to validate an optional value against a function or method, you should use (None, function_name) instead of (function_name, None), as that would result in a call to function_name(value, None, strict, keys).

An additional example is available in Try that code and experiment with breaking example_configuration.json in interesting ways.


If you define wildcards in a type dictionary, elements that don't match required keys will be matched against them. If there are multiple wildcards, Colchian will try them all. For example:

type_dict = {
    '*:1': {
        'type': 'car',
        'wheels': 4,
        'engine': float,
        'electric': typing.Optional[bool]
    '*:2': {
        'type': 'bicycle',
        'wheels': 2,
        'electric': bool

Makes this vehicles.json a valid file:

  "Peugeot 208": {
    "type": "car",
    "wheels": 4,
    "engine": 1.4,
    "electric": false
  "Batavus Socorro": {
    "type": "bicycle",
    "wheels": 2,
    "electric": false
  "T-Ford": {
    "type": "car",
    "wheels": 4,
    "engine": 2.9

Note that you can't use the key '*' twice, which is why the wildcards are distinguished as '*:1' and '*:2'.

Dictionary constructor

When you call Colchian.validate() with some dictionary, it constructs a new instance of that dictionary and all of its parts. If you pass it some subclass of a dictionary (as for example, a Conffu DictConfig), you may want to pass specific parameters to its constructor.

from colchian import Colchian

class MyDict(dict):
    # your dict (class or instance) may have some attribute that's important to you
    important = True

    def __init__(self, *args, some_param=True, **kwargs):
        super().__init__(*args, **kwargs)
        # and possibly, you want it set as the object is constructed
        self.some_attribute = some_param

def create_my_dict(md):
    # create a new object of the same type as md, passing parameters to its constructor matching those of md
    result = type(md)(some_param=md.some_param)
    # set other attributes to match the passed object md 
    result.important = md.important
    # return the newly created object
    return result

# create a MyDict with specific settings
some_dict = MyDict({'a': 1}, some_param=False)
some_dict.important = False
# tell colchian to use a factory function when creating new instances of MyDict, instead of just the constructor
Colchian.type_factories[MyDict] = create_my_dict
# validated_dict will have settings matching some_dict
validated_dict = Colchian.validated(some_dict, {'a': int})

Note: this behaviour also means that Colchian will reconstruct lists - it does not check type_factories when doing so. It is assumed Colchian is primarily applied to dictionaries derived from .json documents, and only accounts for data types it would encounter in them.

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