chili 1.3.1

Chili is a dataclass support library. It is providing simple and fast hydration and extraction interfaces for dataclasses.

Chili

Chili is an extensible dataclass support library. It contains helper functions to simplify initialising and extracting complex dataclasses. This might come in handy when you want to transform your request's data to well-defined and easy to understand objects or when there is a need to hydrate and/or transform database records to desired representation in memory.

Library also ensures type integrity and provides simple interface, which does not pollute your codebase with unwanted abstractions.

Features

• extensible and easy to use
• initialisation and extraction for complex dataclasses
• supports most python's types found in typing package (including generics)
• does not pollute your codebase
• supports adding custom types
• data mapping/transformation with chili.Mapping

Installation

With pip,

pip install chili

or through poetry

poetry add chili

Usage

Initialising a dataclass

from dataclasses import dataclass
from typing import List
from chili import init_dataclass

@dataclass
class Tag:
    id: str
    name: str

@dataclass
class Pet:
    name: str
    tags: List[Tag]
    age: int

pet = init_dataclass({"name": "Bobik", "tags": [{"name": "dog", "id": "12"}]}, Pet)
assert isinstance(pet, Pet)
assert isinstance(pet.tags, List)
assert isinstance(pet.tags[0], Tag)

This example shows how simply cast your dict to given dataclass and that type are ensured by the init_dataclass function.

Transforming dataclass back to a dict

from chili import asdict
from dataclasses import dataclass

@dataclass
class Money:
    currency: str
    amount: float

some_pounds = Money("GBP", "100.00")

some_pounds_dict = asdict(some_pounds)

assert isinstance(some_pounds_dict, dict)
assert isinstance(some_pounds_dict["amount"], float)

Chili works with wide commonly used python types, but not every type can be simply transformed back and forth, so make sure you familiarise yourself with supported types.

Using default values

from dataclasses import dataclass, field
from typing import List
from chili import init_dataclass


@dataclass
class Pet:
    name: str
    tags: List[str] = field(default_factory=lambda: ["pet"])


boo = init_dataclass({"name": "Boo"}, Pet)

assert isinstance(boo, Pet)
assert boo.tags == ['pet']

In the above example tags attribute was not available in the dict object, so default value set in dataclass is being used instead.

Please note dataclasses module does not allow mutable values in default argument of the field function, so this example is using default_factory instead. More details about dataclasses' default and default_factory arguments are available in the python's documentation.

Hiding fields from hydration/deserialisation

There might be scenarios where not all dataclass' fields should be hydrated. In this scenario use built-in python's dataclasses module field function, like in the example below:

from chili import init_dataclass
from dataclasses import dataclass, field
from typing import List


@dataclass
class Pet:
    name: str
    tags: List[str]
    tags_length: int = field(init=False)

    def __post_init__(self):
        self.tags_length = len(self.tags)


boo = init_dataclass({"name": "Boo", "tags": ["hamster", "boo"], "tags_length": 0}, Pet)

assert isinstance(boo, Pet)
assert boo.tags_length == 2

In the above example length of the tags attribute is recalculated everytime we initialise the class and hydrating it might be superfluous.

Hiding fields from extraction/serialisation

To hide attributes of dataclass from being extracted into dict simply use field function with repr attribute set to False

from dataclasses import dataclass, field
from typing import List

from chili import asdict


@dataclass
class Pet:
    name: str
    tags: List[str] = field(repr=False)


boo = Pet(name="Boo", tags=["pet", "hamster", "powerful!"])

boo_dict = asdict(boo)

assert "tags" not in boo_dict

Data mapping

Sometimes you might run into scenarios that data coming from different sources needs to be remapped before you can hydrate it to your dataclass. There might be several reasons for that:

• input data is using camelCase convention
• input data is using different naming
• input data is missing values

In all those cases you can pass mapping attribute to init_dataclass/hydrate or asdict/extract functions to perform mapping before hydration or after extraction dataclass.

Simple mapping

Please consider the following example of simple name mapping:

from dataclasses import dataclass
from typing import List

import chili

input_data = {
    "petName": "Bobik",
    "age": "12",
    "taggedWith": [
        {"tagName": "smart"},
        {"tagName": "dog"},
        {"tagName": "happy"},
    ]
}


@dataclass
class Pet:
    name: str
    age: int
    tags: List[dict]


mapping = chili.Mapper({
    "name": "petName",  # `petName` will be renamed to `name`, which corresponds to `Pet.name` field
    "age": True,  # we just pass true value to include field "as is"
    "tags": chili.KeyMapper("taggedWith", {  # `taggedWith` is a complex structure we want to map, so we have to use KeyMapper 
        "name": "tagName",  # `tagName` will be renamed to `name` which corresponds to `Pet.tags[{index}].name`
    }),
})

bobik = chili.hydrate(input_data, Pet, mapping=mapping)
print(bobik)  # Pet(name='Bobik', age=12, tags=[{'name': 'smart'}, {'name': 'dog'}, {'name': 'happy'}])

Mappings with custom behaviour

We can also use lambdas and functions in mapping to achieve the same result as in the previous example.

from dataclasses import dataclass
from typing import List, Tuple

import chili


def map_pet_tags(value: List) -> List:
    return [{"name": item["tagName"]} for item in value]


input_data = {
    "petName": "Bobik",
    "petAge": "12",
    "taggedWith": [
        {"tagName": "smart"},
        {"tagName": "dog"},
        {"tagName": "happy"},
    ]
}


@dataclass
class Pet:
    name: str
    age: int
    tags: List[dict]


mapping = chili.Mapper({
    "name": "petName",
    "age": lambda value: value["petAge"],  # callables will always receive current's scope data as input 
    "tags": chili.KeyMapper("taggedWith", map_pet_tags)
})

bobik = chili.hydrate(input_data, Pet, mapping=mapping)
print(bobik)  # Pet(name='Bobik', age=12, tags=[{'name': 'smart'}, {'name': 'dog'}, {'name': 'happy'}])

Declaring custom hydrators

If you work with types that are neither dataclasses nor directly supported by Chili, you can define your own hydrator to customise how the type is initialised and how it should be de-initialised by declaring a subclass of chili.hydration.HydrationStrategy and registering it, like below:

from chili import hydrate, registry, extract, HydrationStrategy
import  typing

class MyType:
    def __init__(self, value):
        self.value = value

class MyHydrator(HydrationStrategy):
    def extract(self, value):  # value will be instance of MyType
        return value.value
        
    def hydrate(self, value):
        return MyType(value)

# register our custom type in the hydration registry    
registry.add(MyType, MyHydrator())

# usage
assert isinstance(hydrate("hello", MyType), MyType)
assert isinstance(hydrate("hello", typing.Optional[MyType]), MyType) # this will work as well with optional types

assert extract(MyType("hello")) == "hello"

Working with Generic types

Chili support most of python's generic types like; typing.List, typing.Tuple, typing.Dict, etc. Support is also provided for generic types defined by user (to some extent).

from dataclasses import dataclass
from typing import Generic, List, TypeVar
from chili import init_dataclass

T = TypeVar("T")

@dataclass
class Pet:
    name: str

@dataclass
class Animal:
    name: str

@dataclass
class CustomList(Generic[T]):
    list: List[T]


pet_list = init_dataclass(
    {"list": [
        {"name": "Boo"},
        {"name": "Bobek"},
    ]},
    CustomList[Pet]
)

assert isinstance(pet_list, CustomList)
for pet in pet_list.list:
    assert isinstance(pet, Pet)


animal_list = init_dataclass(
    {"list": [
        {"name": "Boo"},
        {"name": "Bobek"},
    ]},
    CustomList[Animal]
)

assert isinstance(pet_list, CustomList)
for animal in animal_list.list:
    assert isinstance(animal, Animal)

In the above example there are three definitions of dataclasses: Pet, Animal and CustomList. Pet and Animal are just ordinary dataclasses but CustomList is a generic class, parametrised with T parameter. This means we can have subtypes, like: CustomList[Pet], CustomList[Animal] or even CustomList[Dict].

init_dataclass function understands that passed type is a generic type, and can handle it as suspected.

Hydration of dataclass inheriting from another generic dataclasses is also supported, only if that dataclass specifies the parameters:

from dataclasses import dataclass
from typing import Generic, List, TypeVar
from chili import init_dataclass

T = TypeVar("T")

@dataclass
class Pet:
    name: str

@dataclass
class Animal:
    name: str

@dataclass
class CustomList(Generic[T]):
    list: List[T]

@dataclass
class ExtendedGenericList(CustomList, Generic[T]):
    ...

@dataclass
class ExtendedList(CustomList[Pet]):
    ...

# this will work
pet_list = init_dataclass(
    {"list": [
        {"name": "Boo"},
        {"name": "Bobek"},
    ]},
    ExtendedGenericList[Pet]
)

# this will fail
failed_pet_list = init_dataclass(
    {"list": [
        {"name": "Boo"},
        {"name": "Bobek"},
    ]},
    ExtendedList
)

In the above example ExtendedList will fail during initialisation, the reason for that is information required to parametrise this class and probably its subclasses or any other classes aggregated by this class is lost. For now this behaviour is not supported for auto-hydration mode. ExtendedGenericList[Pet] will work as expected.

Supported data types

bool

Passed value is automatically hydrated to boolean with python's built-in bool on hydration and extraction.

dict

Passed value is automatically hydrated to dict with python's built-in dict on hydration and extraction.

float

Passed value is automatically hydrated to float with python's built-in float on hydration and extraction.

frozenset

Passed value is automatically hydrated to frozen set with python's built-in frozenset and extracted to list.

int

Passed value is automatically hydrated to int with python's built-in int on hydration and extraction.

list

Passed value is automatically hydrated to list with python's built-in list on hydration and extraction.

set

Passed value is automatically hydrated to set with python's built-in set and extracted to list.

str

Passed value is automatically hydrated to string with python's built-in str on hydration and extraction.

tuple

Passed value is automatically hydrated to tuple with python's built-in tuple and extracted to list.

collections.namedtuple

Passed value is automatically hydrated to named tuple and extracted to list.

collections.deque

Passed value is automatically hydrated to an instance of collections.deque and extracted to list.

collections.OrderedDict

Passed value is automatically hydrated to an instance of collections.OrderedDict and extracted to dict.

datetime.date

Passed value must be valid ISO-8601 date string, then it is automatically hydrated to an instance of datetime.date class and extracted to ISO-8601 format compatible string.

datetime.datetime

Passed value must be valid ISO-8601 date time string, then it is automatically hydrated to an instance of datetime.datetime class and extracted to ISO-8601 format compatible string.

datetime.time

Passed value must be valid ISO-8601 time string, then it is automatically hydrated to an instance of datetime.time class and extracted to ISO-8601 format compatible string.

datetime.timedelta

Passed value must be valid ISO-8601 duration string, then it is automatically hydrated to an instance of datetime.timedelta class and extracted to ISO-8601 format compatible string.

decimal.Decimal

Passed value must be a string containing valid decimal number representation, for more please read python's manual about decimal.Decimal, on extraction value is extracted back to string.

enum.Enum

Supports hydration of all instances of enum.Enum subclasses as long as value can be assigned to one of the members defined in the specified enum.Enum subclass. During extraction the value is extracted to value of the enum member.

enum.IntEnum

Same as enum.Enum.

typing.Any

Passed value is unchanged during hydration and extraction process.

typing.AnyStr

Same as str

typing.Deque

Same as collection.dequeue with one exception, if subtype is defined, eg typing.Deque[int] each item inside queue is hydrated accordingly to subtype.

typing.Dict

Same as dict with exception that keys and values are respectively hydrated and extracted to match annotated type.

typing.FrozenSet

Same as frozenset with exception that values of a frozen set are respectively hydrated and extracted to match annotated type.

typing.List

Same as list with exception that values of a list are respectively hydrated and extracted to match annotated type.

typing.NamedTuple

Same as namedtuple.

typing.Optional

Optional types can carry additional None value which chili's hydration process will respect, so for example if your type is typing.Optional[int] None value is not hydrated to int.

typing.Set

Same as set with exception that values of a set are respectively hydrated and extracted to match annotated type.

typing.Tuple

Same as tuple with exception that values of a set are respectively hydrated and extracted to match annotated types. Ellipsis operator (...) is also supported.

typing.TypedDict

Same as dict but values of a dict are respectively hydrated and extracted to match annotated types.

`typing.Generic

Only parametrised generic classes are supported, dataclasses that extends other Generic classes without parametrisation will fail.

typing.Union

Limited support for Unions.

API

chili.hydrate(value: typing.Any, type_name: Type[T], strict: bool = False, mapping: chili.Mapper = None) -> T

Hydrates given value into instance of passed type. If hydration fails, it returns passed value as a result, if strict mode is set to True it raises InvalidValueError.

chili.extract(value: typing.Any, strict: bool = False, mapping: chili.Mapper = None) -> typing.Any

Extracts given value into primitive or set of primitives. If extraction fails, it returns passed value as a result, if strict mode is set to True it raises InvalidValueError.

chili.init_dataclass(value: dict, type_name: Type[T], mapping: chili.Mapper = None) -> T

init_dataclass function is instantiating dataclass of specified type_name and will hydrate the instance with values passed in value dictionary. Each of the passed dictionary's keys must correspond to dataclass' attributes in order to be properly interpreted. This rule can be broken if valid mapping is passed to the function.

This function support complex and nested hydration, which means if your dataclass aggregates other dataclasses or defines complex typing, init_dataclass function will respect your type annotations and will cast values to match the defined types.

If attributes in your dataclass do not specify the type value will be hydrated in to a newly created instance as is.

chili.asdict(value, mapping: chili.Mapper = None) -> Dict[str, typing.Any]

asdict is the opposite of init_dataclass function, it takes an instance of dataclass as argument, and extracts its members to a dictionary, so the returned data can be stored as json object or easily serialised to any other format. Additionally, mapping argument allows changing data representation on the fly.

Please note Chili is not a data validation library, although Chili performs some validation and casting behind the scenes it does it only to ensure type consistency.