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https://github.com/outbrain-inc/valid_model

valid_model
https://github.com/outbrain-inc/valid_model

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valid_model

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valid_model

===========



The valid_model library is intended to allow declarative modeling of an object schema in a similar way to various ORM and form validation libraries while making as few assumptions as possible about the way it will be used.



Attributes in a `valid_model.Object` instance are guaranteed to have all fields exist with defined restrictions and defaults available. In addition each field can have a mutator function applied to it to enforce uniformity.  An example of this could be make a `String` attribute always be uppercase.



You can nest `Object` classes inside one another using the `EmbeddedObject`, `Set`, `Dict`, and `List` descriptors



The available descriptors are in `valid_model.descriptors` and include:

`Generic`, `String`, `Integer`, `Float`, `Bool`, `DateTime`, `TimeDelta`, `List`, `Set`, `Dict`, and `EmbeddedObject`



When initializing an `Object` all initial values should be passed in as keyword arguments.

When setting an `EmbeddedObject` attribute, it will automatically convert a `dict` to the appropriate `Object` subclass.



`Object` instances have `Object.__json__` defined to be used as a hook to convert objects into `dict` for easy serialization.



```python

class Person(Object):

  name = String(nullable=False)

  homepage = String()



class BlogPost(Object):

  title = String(nullable=False, mutator=lambda x: x.title())

  updated = DateTime(nullable=False, default=datetime.utcnow) # default to time object is created

  published = DateTime() # default value will be None

  author = EmbeddedObject(Person)

  contributors = List(value=EmbeddedObject(Person))

  tags = List(value=String(nullable=False))



  def validate(self):

    super(BlogPost, self).validate()

    if self.published is not None and self.published > self.updated:

      raise ValidationError('a post cannot be published at a later date than it was updated')



post = BlogPost(title='example post', author={'name': 'Josh'}, tags=['tag1', 'tag2'])

"""

post.__json__() would output

{

  'title': 'Example Post',

  'author': {

      'name': 'Josh',

      'homepage': None

   },

   'contributors': [],

   'updated': datetime(2014, 10, 6, 10, 23),

   'published': None,

   'tags': ['tag1', 'tag2']

}

"""

```

##Descriptor Options



|Keyword Arg | Default | Description |

|:-----------|--------|:------------------

|nullable | `True`1 | determines if it is valid for an attribute to be set to None

|mutator | no mutator | allows some mutation to occur on the value. a common use case would be to format a string to always be uppercase

|validator | no validator| a function which returns truthy if the value is valid

|default | `None`2  | a scalar value or function which the attribute will be set to on object initialization if no value is specified at in the constuctor

|value| no descriptor3 | a descriptor to validate values in the container attribute

| key | no descriptor4 | a descriptor to validate keys in the container attribute

1 Not available on `Set`, `List`, and `Dict`. If an attribute with that descriptor is set to `None` it will actually set it to an empty instance of their respective types  

2 Container objects `Set`, `List`, and `Dict` initialize to an empty instance of their respective types  

3 Only available on `Dict`, `List`, and `Set`  

4 Only available on `Dict`  



`EmbededObject` takes one argument which is the `Object` class that is being embedded.



##How Validation Works

Validation occurs whenever an attribute is set.



1. Typechecking and any type coercion implemented occurs

2. The value is checked if it is None  

    * If it is None and nullable == False a ValidationError is raised otherwise it is set to None

3. If mutator function is defined it will run

4. If a validator function is defined it will run  

    * A ValidationError is raised if the validator function returns falsey



### Complex Validation

In addition to validators being defined on individual attributes there is a validate method on Object instances which may be overridden for more complicated validation logic that may include a combination of multiple fields.  By default it will just revalidate all attributes of an `Object` instance.