Python HAL generation/parsing library.
Halogen takes the advantage of the declarative style serialization with easily extendable schemas. Schema combines the knowledge about your data model, attribute mapping and advanced accessing, with complex types and data transformation.
Library is purposed in representing your data in HAL format in the most obvious way possible, but also of the generic web form-like functionality so that your schemas and types can be reused as much as possible.
Schema is the main building block of the serialization. It is also a type which means you can declare nested structures with schemas.
>>> Schema.serialize({"hello": "Hello World"})
>>> {"hello": "Hello World"}Simply call Schema.serialize() class method which can accept dict or any other object.
There's no validation involved in the serialization. Your source data or your model is considered to be clean since it is coming from the storage and it is not a user input. Of course exceptions in the types or attribute accessors may occur but they are considered as programming errors.
Dictionary values are automatically accessed by the schema attributes using their names as keys:
import halogen
class Hello(halogen.Schema):
hello = halogen.Attr()
serialized = Hello.serialize({"hello": "Hello World"})Result:
{
"hello": "Hello World"
}HAL is just JSON, but according to it's specification it SHOULD have self link to identify the
serialized resource. For this you should use HAL-specific attributes and configure the way the
self is composed.
HAL example:
import halogen
from flask import url_for
spell = {
"uid": "abracadabra",
"name": "Abra Cadabra",
"cost": 10,
}
class Spell(halogen.Schema):
self = halogen.Link(attr=lambda spell: url_for("spell.get" uid=spell['uid']))
name = halogen.Attr()
serialized = Spell.serialize(spell)Result:
{
"_links": {
"self": {"href": "/spells/abracadabra"}
},
"name": "Abra Cadabra"
}Similar to dictionary keys the schema attributes can also access object properties:
import halogen
from flask import url_for
class Spell(object):
uid = "abracadabra"
name = "Abra Cadabra"
cost = 10
spell = Spell()
class SpellSchema(halogen.Schema):
self = halogen.Link(attr=lambda spell: url_for("spell.get" uid=spell.uid))
name = halogen.Attr()
serialized = SpellSchema.serialize(spell)Result:
{
"_links": {
"self": {"href": "/spells/abracadabra"}
},
"name": "Abra Cadabra"
}Attributes form the schema and encapsulate the knowledge how to get the data from your model, how to transform it according to the specific type.
The name of the attribute member in the schema is the name of the key the result will be serialized to. By default the same attribute name is used to access the source model.
Example:
import halogen
from flask import url_for
class Spell(object):
uid = "abracadabra"
name = "Abra Cadabra"
cost = 10
spell = Spell()
class SpellSchema(halogen.Schema):
self = halogen.Link(attr=lambda spell: url_for("spell.get" uid=spell.uid))
name = halogen.Attr()
serialized = SpellSchema.serialize(spell)Result:
{
"_links": {
"self": {"href": "/spells/abracadabra"}
},
"name": "Abra Cadabra"
}In case the attribute represents a constant the value can be specified as a first parameter. This first parameter
is a type of the attribute. If the type is not a instance or subclass of a halogen.types.Type it will
be bypassed.
import halogen
from flask import url_for
class Spell(object):
uid = "abracadabra"
name = "Abra Cadabra"
cost = 10
spell = Spell()
class SpellSchema(halogen.Schema):
self = halogen.Link(attr=lambda spell: url_for("spell.get" uid=spell.uid))
name = halogen.Attr("custom name")
serialized = SpellSchema.serialize(spell)Result:
{
"_links": {
"self": {"href": "/spells/abracadabra"}
},
"name": "custom name"
}In some cases also the attr can be specified to be a callable that returns a constant value.
In case the attribute name doesn't correspond your model you can override it:
import halogen
from flask import url_for
class Spell(object):
uid = "abracadabra"
title = "Abra Cadabra"
cost = 10
spell = Spell()
class SpellSchema(halogen.Schema):
self = halogen.Link(attr=lambda spell: url_for("spell.get" uid=spell.uid))
name = halogen.Attr(attr="title")
serialized = SpellSchema.serialize(spell)Result:
{
"_links": {
"self": {"href": "/spells/abracadabra"}
},
"name": "Abra Cadabra"
}The attr parameter accepts strings of the source attribute name or even dot-separated path to the attribute.
This works for both: nested dictionaries or related objects an Python properties.
import halogen
class SpellSchema(halogen.Schema):
name = halogen.Attr(attr="path.to.my.attribute")The attr parameter accepts callables that take the entire source model and can access the neccessary
attribute. You can pass a function or lambda in order to return the desired value which
also can be just a constant.
import halogen
from flask import url_for
class Spell(object):
uid = "abracadabra"
title = "Abra Cadabra"
cost = 10
spell = Spell()
class SpellSchema(halogen.Schema):
self = halogen.Link(attr=lambda spell: url_for("spell.get" uid=spell.uid))
name = halogen.Attr(attr=lambda value: value.title)
serialized = SpellSchema.serialize(spell)Result:
{
"_links": {
"self": {"href": "/spells/abracadabra"}
},
"name": "Abra Cadabra"
}Sometimes accessor functions are too big for lambdas. In this case it is possible to decorate a method of the class to be a getter accessor.
import halogen
class ShoppingCartSchema(halogen.Schema):
@halogen.attr(AmountType(), default=None)
def total(obj):
return sum(
(item.amount for item in obj.items),
0,
)
@total.setter
def set_total(obj, value):
obj.total = valueIn case the schema is used for both directions to serialize and to deserialize the halogen.schema.Accessor
can be passed with both getter and setter specified.
Getter is a string or callable in order to get the value from a model, and setter is a string or callable
that knows where the deserialized value should be stored.
After the attibute gets the value it passes it to it's type in order to complete the serialization.
Halogen provides basic types for example halogen.types.List to implement lists of values or schemas.
Schema is also a Type and can be passed to the attribute to implement complex structures.
Example:
import halogen
from flask import url_for
class Book(object):
uid = "good-book-uid"
title = "Harry Potter and the Philosopher's Stone"
genres = [
{"uid": "fantasy-literature", "title": "fantasy literature"},
{"uid": "mystery", "title": "mystery"},
{"uid": "adventure", "title": "adventure"},
]
book = Book()
class GenreSchema(halogen.Schema):
self = halogen.Link(attr=lambda genre: url_for("genre.get" uid=genre['uid']))
title = halogen.Attr()
class BookSchema(halogen.Schema):
self = halogen.Link(attr=lambda book: url_for("book.get" uid=book.uid))
title = halogen.Attr()
genres = halogen.Attr(halogen.types.List(GenreSchema))
serialized = BookSchema.serialize(book)Result:
{
"_links": {
"self": {"href": "good-book-uid"}
},
"genres": [
{"_links": {"self": {"href": "fantasy-literature"}}, "title": "fantasy literature"},
{"_links": {"self": {"href": "mystery"}}, "title": "mystery"},
{"_links": {"self": {"href": "adventure"}}, "title": "adventure"}
],
"title": "Harry Potter and the Philosopher's Stone"
}Type gets optional validators parameter, which is a list of halogen.validators.Validator objects whose single
interface method validate will be called for the given value during the deserialization. If the value is not valid,
halogen.exceptions.ValidationError should be raised.
Halogen provides basic validators, for example halogen.validators.Range to validate that the values is in certain
range.
If an attribute cannot be taken, provided default value will be used; if default value is
a callable, it will be called to get the default value.
By default, attributes are required, so when an attribute can not be taken during the serialization and default
is not provided, an exception will be raised (AttributeError or KeyError, depending on the input).
It's possible to relax this restriction by passing required=False to the attribute constructor.
For deserialization, the same logic applies, but the exception type will be halogen.exceptions.ValidationError
for human readability (see Deserialization).
Type is responsible in serialization of individual values such as integers, strings, dates. Also type is a base of Schema. It has both serialize() and deserialize() methods that convert the attribute's value. Unlike Schema types are instantiated. You can configure serialization behavior by passing parameters to their constructors while declaring your schema.
Types can raise halogen.exceptions.ValidationError during deserialization, but serialization
expects the value that this type knows how to transform.
Types that are common in your application can be shared between schemas. This could be the datetime type, specific URL type, internationalized strings and any other representation that requires specific format.
The default implementation of the Type.serialize is a bypass.
Serialization method of a type is the last opportunity to convert the value that is being serialized:
Example:
import halogen
class Amount(object):
currency = "EUR"
amount = 1
class AmountType(halogen.types.Type):
def serialize(self, value):
if value is None or not isinstance(value, Amount):
return None
return {
"currency": value.currency,
"amount": value.amount
}
class Product(object):
name = "Milk"
def __init__(self):
self.price = Amount()
product = Product()
class ProductSchema(halogen.Schema):
name = halogen.Attr()
price = halogen.Attr(AmountType())
serialized = ProductSchema.serialize(product)Result:
{
"name": "Milk",
"price": {
"amount": 1,
"currency": "EUR"
}
}In case the accessor returns None and the further serialization by a type or a nested schema is not desired the type can be wrapped into Nullable type.
import halogen
class FreeProduct(object):
"""A free product, that doesn't have a price."""
price = None
class AmountSchema(halogen.Schema):
currency = halogen.Attr(required=True, default="USD")
amount = halogen.Attr(required=True, default=0)
class FreeProductSchema(halogen.Schema):
price_null = halogen.Attr(halogen.types.Nullable(AmountType()), attr="price")
price_zero = halogen.Attr(AmountType(), attr="price")
serialized = FreeProductSchema.serialize(FreeProduct())Result:
{
"price_null": None,
"price_zero": {
"amount": 0,
"currency": "USD"
}
}Hypertext Application Language.
The JSON variant of HAL (application/hal+json) has now been published as an internet draft: draft-kelly-json-hal
Link objects at RFC: link-objects
The "href" property is REQUIRED.
halogen.Link will create href for you. You just need to point to halogen.Link either from where or
what halogen.Link should put into href.
- Static variant
import halogen class EventSchema(halogen.Schema): artist = halogen.Link(attr="/artists/some-artist")
- Callable variant
import halogen class EventSchema(halogen.Schema): help = halogen.Link(attr=lambda: current_app.config['DOC_URL'])
Links can be deprecated by specifying the deprecation URL attribute which points to the document describing the deprecation.
import halogen class EventSchema(halogen.Schema): artist = halogen.Link( attr="/artists/some-artist", deprecation="http://docs.api.com/deprecations#artist", )
CURIEs are providing links to the resource documentation.
import halogen
doc = halogen.Curie(
name="doc,
href="http://haltalk.herokuapp.com/docs/{rel}",
templated=True
)
class BlogSchema(halogen.Schema):
lastest_post = halogen.Link(attr="/posts/latest", curie=doc){
"_links": {
"curies": [
{
"name": "doc",
"href": "http://haltalk.herokuapp.com/docs/{rel}",
"templated": true
}
],
"doc:latest_posts": {
"href": "/posts/latest"
}
}
}Schema also can be a param to link
import halogen
class BookLinkSchema(halogen.Schema):
href = halogen.Attr("/books")
class BookSchema(halogen.Schema):
books = halogen.Link(BookLinkSchema)
serialized = BookSchema.serialize({"books": ""}){
"_links": {
"books": {
"href": "/books"
}
}
}The reserved "_embedded" property is OPTIONAL. It is an object whose property names are link relation types (as defined by [RFC5988]) and values are either a Resource Object or an array of Resource Objects.
Embedded Resources MAY be a full, partial, or inconsistent version of the representation served from the target URI.
For creating _embedded in your schema you should use halogen.Embedded.
Example:
import halogen
em = halogen.Curie(
name="em",
href="https://docs.event-manager.com/{rel}.html",
templated=True,
type="text/html"
)
class EventSchema(halogen.Schema):
self = halogen.Link("/events/activity-event")
collection = halogen.Link("/events/activity-event", curie=em)
uid = halogen.Attr()
class PublicationSchema(halogen.Schema):
self = halogen.Link(attr=lambda publication: "/campaigns/activity-campaign/events/activity-event")
event = halogen.Link(attr=lambda publication: "/events/activity-event", curie=em)
campaign = halogen.Link(attr=lambda publication: "/campaign/activity-event", curie=em)
class EventCollection(halogen.Schema):
self = halogen.Link("/events")
events = halogen.Embedded(halogen.types.List(EventSchema), attr=lambda collection: collection["events"], curie=em)
publications = halogen.Embedded(
attr_type=halogen.types.List(PublicationSchema),
attr=lambda collection: collection["publications"],
curie=em
)
collections = {
'events': [
{"uid": 'activity-event'}
],
'publications': [
{
"event": {"uid": "activity-event"},
"campaign": {"uid": "activity-campaign"}
}
]
}
serialized = EventCollection.serialize(collections)Result:
{
"_embedded": {
"em:events": [
{
"_links": {
"curies": [
{
"href": "https://docs.event-manager.com/{rel}.html",
"name": "em",
"templated": true,
"type": "text/html"
}
],
"em:collection": {"href": "/events/activity-event"},
"self": {"href": "/events/activity-event"}
},
"uid": "activity-event"
}
],
"em:publications": [
{
"_links": {
"curies": [
{
"href": "https://docs.event-manager.com/{rel}.html",
"name": "em",
"templated": true,
"type": "text/html"
}
],
"em:campaign": {"href": "/campaign/activity-event"},
"em:event": {"href": "/events/activity-event"},
"self": {"href": "/campaigns/activity-campaign/events/activity-event"}
}
}
]
},
"_links": {
"curies": [
{
"href": "https://docs.event-manager.com/{rel}.html",
"name": "em",
"templated": true,
"type": "text/html"
}
],
"self": {"href": "/events"}
}
}By default, embedded resources are required, you can make them not required by passing required=False to the
constructor, and empty values will be omitted in the serialization:
import halogen
class Schema(halogen.Schema):
user1 = halogen.Embedded(PersonSchema, required=False)
user2 = halogen.Embedded(PersonSchema)
serialized = Schema.serialize({'user2': Person("John", "Smith")})Result:
{
"_embedded": {
"user2": {
"name": "John",
"surname": "Smith"
}
}
}Schema has deserialize method. Method deserialize will return dict as a result of deserialization
if you wont pass any object as a second param.
Example:
import halogen
class Hello(halogen.Schema):
hello = halogen.Attr()
result = Hello.deserialize({"hello": "Hello World"})
print(result)Result:
{
"hello": "Hello World"
}However, if you will pass object as the second param of deserialize method then data will be assigned on object's
attributes.
Example:
import halogen
class HellMessage(object):
hello = ""
hello_message = HellMessage()
class Hello(halogen.Schema):
hello = halogen.Attr()
Hello.deserialize({"hello": "Hello World"}, hello_message)
print(hello_message.hello)Result:
"Hello World"How you already know attributes launch serialize method from types which they are supported in moment of
serialization but in case of deserialization the same attributes will launch deserialize method. It means that
when you write your types you should not forget about deserialize methods for them.
Example:
import halogen
import decimal
class Amount(object):
currency = "EUR"
amount = 1
def __init__(self, currency, amount):
self.currency = currency
self.amount = amount
def __repr__(self):
return "Amount: {currency} {amount}".format(currency=self.currency, amount=str(self.amount))
class AmountType(halogen.types.Type):
def serialize(self, value):
if value is None or not isinstance(value, Amount):
return None
return {
"currency": value.currency,
"amount": value.amount
}
def deserialize(self, value):
return Amount(value["currency"], decimal.Decimal(str(value["amount"])))
class ProductSchema(halogen.Schema):
title = halogen.Attr()
price = halogen.Attr(AmountType())
product = ProductSchema.deserialize({"title": "Pencil", "price": {"currency": "EUR", "amount": 0.30}})
print(product)Result:
{"price": Amount: EUR 0.3, "title": "Pencil"}On deserialization failure, halogen raises special exception (halogen.exceptions.ValidationError).
That exception class has __unicode__ method which renders human readable error result so user can easily track
down the problem with his input.
Example:
import halogen
class Hello(halogen.Schema):
hello = halogen.Attr()
try:
result = Hello.deserialize({})
except halogen.exceptions.ValidationError as exc:
print(exc)Result:
{
"errors": [
{
"errors": [
{
"type": "str",
"error": "Missing attribute."
}
],
"attr": "hello"
}
],
"attr": "<root>"
}In case when you have nested schemas, and use List, halogen also adds the index (counting from 0) in the list
so you see where exactly the validation error happened.
Example:
import halogen
class Product(halogen.Schema):
"""A product has a name and quantity."""
name = halogen.Attr()
quantity = halogen.Attr()
class NestedSchema(halogen.Schema):
"""An example nested schema."""
products = halogen.Attr(
halogen.types.List(
Product,
),
default=[],
)
try:
result = NestedSchema.deserialize({
"products": [
{
"name": "name",
"quantity": 1
},
{
"name": "name",
}
]
})
except halogen.exceptions.ValidationError as exc:
print(exc)Result:
{
"errors": [
{
"errors": [
{
"index": 1,
"errors": [
{
"errors": [
{
"type": "str",
"error": "Missing attribute."
}
],
"attr": "quantity"
}
]
}
],
"attr": "products"
}
],
"attr": "<root>"
}Note that should ValueError exception happen on the attribute deserialization, it will be caught and reraized
as halogen.exceptions.ValidationError. This is to eliminate the need of raising halogen specific exceptions in
types and attributes during the deserialization.
When serializing or deserializing an object, not all data required for (de)serialization may be available in the object
itself. You can pass this data as separate keyword arguments to serialize or deserialize to provide context.
This context will be available in all nested schema, types and attributes.
Serialize example:
class ErrorSchema(halogen.Schema):
message = halogen.Attr(
attr=lambda error, language: error["message"][language]
)
error = ErrorSchema.serialize({
"message": {
"dut": "Ongeldig e-mailadres",
"eng": "Invalid email address"
}
}, language="dut")
print errorResult:
{"message": "Ongeldig e-mailadres"}Deserialize example:
import halogen
class Book(halogen.Schema):
@halogen.attr()
def title(obj, language):
return obj['title'][language]
class Author(halogen.Schema):
name = halogen.Attr(attr='author.name')
books = halogen.Attr(
halogen.types.List(Book),
attr='author.books',
)
author = Author.deserialize({
"author": {
"name": "Roald Dahl",
"books": [
{
"title": {
"dut": "De Heksen",
"eng": "The Witches"
}
},
{
"title": {
"dut": "Sjakie en de chocoladefabriek",
"eng": "Charlie and the Chocolate Factory"
}
}
]
}
}, language="eng")
print authorResult:
{
"name": "Roald Dahl",
"books": [
{"title": "The Witches"},
{"title": "Charlie and the Chocolate Factory"}
]
}Handling validation and business logic errors are as important as handling HAL responses. Halogen provides support for the vendor error media type, which is fully HAL-compatible.
The vendor error (application/vnd.error+json) has now been published as an internet draft: draft-vnd-error
This mediatype is attempting to standartise the format in which the problem can be represented to many clients so that it can be expressed and understood. Multiple deserialization errors can be mapped to the relevant keys of the payload via the path attribute, which represents the JSON Pointer to the payload key, and therefore to the UI element that is serialized with that key.
import halogen
from halogen.vnd.error import Error, VNDError
class AuthorSchema(halogen.Schema):
name = halogen.Attr(required=True)
class PublisherSchema(halogen.Schema):
name = halogen.Attr(required=True)
address = halogen.Attr()
class BookSchema(halogen.Schema):
title = halogen.Attr(required=True)
year = halogen.Attr(halogen.types.Int(), required=True)
authors = halogen.Attr(halogen.types.List(AuthorSchema), required=True)
publisher = halogen.Attr(PublisherSchema)
try:
BookSchema.deserialize(
dict(
# title is skipped
year="abc", # Not an integer
authors=[dict(name="John Smith"), dict()], # Second author has no name
publisher=dict(address="Chasey Lane 42, Los Angeles, US"), # No name
),
)
except halogen.exceptions.ValidationError as e:
error = Error.from_validation_exception(e)
>>> error.errors
>>>
[
{"path": "/authors/1/name", "message": "Missing attribute."),
{"path": "/title", "message": "Missing attribute."),
{"path": "/year", "message": "'abc' is not an integer"),
{"path": "/publisher/name", "message": "Missing attribute."),
}The errors may or may not be related to the payload, but sometimes to another resources. In this case the about link should be returned within the error.
{
"_links": {
"about": {"href": "/products/1"}
},
"message": "The product is sold out."
}The error messages should be internationalized and respect Accept-Language and Content-Language HTTP headers.
If you have questions, bug reports, suggestions, etc. please create an issue on the GitHub project page.
This software is licensed under the MIT license
See License file
© 2013 Oleg Pidsadnyi, Paylogic International and others.