pymarshaler 0.2.6

Package to marshal and unmarshal python objects

Pymarshaler - Marshal and Unmarshal Python Objects

About

Pymarshaler allows you to marshal and unmarshal any python object directly to and from a JSON formatted string.

Pymarshaler takes advantage of python's new typing support. By reading class init param types, we are able to walk down nested JSON structures and assign appropriate values.

Basic Usage

Declare a class with typing information

class Test:

    def __init__(self, name: str):
        self.name = name

That's it! We can now marshal, and more importantly, unmarshal this object to and from JSON.

from pymarshaler.marshal import Marshal
import json

test_instance = Test('foo')
blob = Marshal.marshal(test_instance)
print(blob)
>>> '{name: foo}'

marshal = Marshal()
result = marshal.unmarshal(Test, json.loads(blob))
print(result.name)
>>> 'foo'

We also use marshal.unmarshal_str(cls, str) if we want to unmarshal directly from the blob source.

This is a pretty trivial example, lets add in a nested class

class StoresTest:

    def __int__(self, test: Test):
        self.test = test

stores_test = StoresTest(Test('foo'))
blob = marshal.marshal(stores_test)
print(blob)
>>> '{test: {name: foo}}'

result = marshal.unmarshal(StoresTest, json.loads(blob))
print(result.test.name)
>>> 'foo'

As you can see, adding a nested class is as simple as as adding a basic structure.

Pymarshaler will fail when encountering an unknown field by default, however you can configure it to ignore unknown fields

from pymarshaler.marshal import Marshal 
from pymarshaler.arg_delegates import ArgBuilderFactory

marshal = Marshal()
blob = {'test': 'foo', 'unused_field': 'blah'}
result = marshal.unmarshal(Test, blob)
>>> 'Found unknown field (unused_field: blah). If you would like to skip unknown fields create a Marshal object who can skip ignore_unknown_fields'

marhsal = Marshal(ignore_unknown_fields=True)
result = marshal.unmarshal(Test, blob)
print(result.name)
>>> 'foo'

Advanced Usage

We can use pymarshaler to handle containers as well. Again we take advantage of python's robust typing system

from pymarshaler.marshal import Marshal
from typing import Set
import json

class TestContainer:

    def __int__(self, container: Set[str]):
        self.container = container

marshal = Marshal()
container_instance = TestContainer({'foo', 'bar'})        
blob = marshal.marshal(container_instance)
print(blob)
>>> '{container: ["foo", "bar"]}'

result = marshal.unmarshal(TestContainer,json.loads(blob))
print(result.container)
>>> '{foo, bar}'

Pymarshaler can also handle containers that store user defined types. The Set[str] could easily have been Set[UserDefinedType]

Pymarshaler also supports default values, and will use any default values supplied in the __init__ if those values aren't present in the JSON data.

from pymarshaler.marshal import Marshal

class TestWithDefault:

    def __init__(self, name: str = 'foo'):
        self.name = name

marshal = Marshal()
result = marshal.unmarshal(TestWithDefault, {})
print(result.name)
>>> 'foo'

Pymarshaler will raise an error if any non-default attributes aren't given

Pymarshaler also supports a validate method on creation of the python object. This method will be called before being returned to the user.

from pymarshaler.marshal import Marshal

class TestWithValidate:

    def __init__(self, name: str):
        self.name = name

    def validate(self):
        print(f'My name is {self.name}!')


marshal = Marshal()
result = marshal.unmarshal(TestWithValidate, {'name': 'foo'})
>>> 'My name is foo!'

This can be used to validate the python object right at construction, potentially raising an error if any of the fields have invalid values

It's also possible to register your own custom unmarshaler for specific user defined classes.

from pymarshaler.arg_delegates import ArgBuilderDelegate 
from pymarshaler.marshal import Marshal


class ClassWithMessage:

    def __init__(self, message: str):
        self.message = message


class ClassWithCustomDelegate:

    def __init__(self, message_obj: ClassWithMessage):
        self.message_obj = message_obj


class CustomDelegate(ArgBuilderDelegate):

    def __init__(self, cls):
        super().__init__(cls)

    def resolve(self, data):
        return {'message_obj': ClassWithMessage(data['message'])}


marshal = Marshal()
marshal.register_delegate(ClassWithCustomDelegate, CustomDelegate)
result = marshal.unmarshal(ClassWithCustomDelegate, {'message': 'Hello from the custom delegate!'})
print(result.message_obj)
>>> 'Hello from the custom delegate!'

The result from any delegate should be a dictionary, however any data inside the returned dictionary should be in their class form.