1

Questions

  • How can I serialize an instance of a data class in a way that saves its state when the state is generated by its meta class?
    • See description and sample code for more clarification.
  • How do I share context in python in a way that replicates the working-case while achieving the modularity of the sample-issue case?
  • Is there an alternative way of populating data classes in an abstract way that doesn't use meta classes?
    • Needs to be abstracted because I have multiple data classes that are representative of request information that are all populated in a similar fashion.

Description

I am currently writing a flask web application that passes processing of tasks off to rq workers via a redis queue (using rq), and I am having trouble with the serialization of the data classes that contain the request information. The issue I am running into happens when rq tries to serialize a data class that uses a meta class to populate all of its fields.

When debugging and diagnosing the issue I have noticed that dill, and cloudpickle are able to serialize the instance of the object correctly when the instance is created and defined in the same file/module, but once I move the definition of the object to a different file/module they are unable to serialize the object in a way that maintains the instances state.

I have added a simplified sample below to replicate my problem.

Environment

Python Version: python 3.7.3
OS: Windows

# File structure
\sample-issue
--> dataclass.py
--> serialization.py
--> deserialization.py
--> __init__.py

\working-case
--> sample.py
--> __init__.py

Sample Issue

# dataclass.py

from dataclasses import dataclass, field
from typing import List, Dict, Any
import json
from collections import defaultdict


class JSONRequest(type):
    def __call__(cls, *args, **kwargs):
        """
        This is a metaclass used to autonomously populate dataclasses

        NOTE: This metaclass only works with dataclasses

        Optional Parameter:
            inp: class attribute that is a callable that produces a dictionary
        """
        inp = cls.__dict__["inp"]()
        cls.__initialize(inp)
        return cls

    def __initialize(cls, inp: dict) -> None:
        """
        Initializes all of the dataclasses fields

        If the field is missing in the JSON request and it does not have a default value in the data class a
        ValueError error will be raised. Additionally if the Json value is [], {}, "" will default to the default
        value, and if the default value is missing an InvalidRequest error will also be raised.

        Parameters:
            inp: Json input
        """
        _json = defaultdict(lambda: None)
        _json.update(inp)

        for name, _ in cls.__dataclass_fields__.items():
            if (not _json[name]) and (name not in cls.__dict__.keys()):
                raise ValueError(f"Request is missing the {name} field")

            value = _json[name] or cls.__dict__[name]
            setattr(cls, name, value)

    def __str__(cls):
        rep = {name: getattr(cls, name) for name, _ in cls.__dataclass_fields__.items()}
        return json.dumps(rep, indent=4)


def generate_input():
    """
    Stub method for generating input
    """
    return {
        "email_list": [f"{name}@yahoo.com" for name in ["peter", "mark", "alysa"]],
        "message": "Foo bar fizzbuzz",
        "subject": "Sample Issue",
        "info": {
            "time": 1619628747.9166002,
            "count": 3,
        }
    }


@dataclass
class EmailChain(metaclass=JSONRequest):
    email_list: List[str] = field(init=False)
    message: str = field(init=False)
    subject: str = field(init=False)
    info: Dict[str, Any] = field(init=False)

    inp = generate_input

# serialization.py

import dill
from sample_issue.dataclass import EmailChain

obj = EmailChain()
data_stream = dill.dumps(obj)
print(data_stream) 

# output: b'\x80\x03csrc.Test\nEmailChain\nq\x00.'

# deserialization

import dill
from sample_issue.dataclass import EmailChain

input = b'\x80\x03csrc.Test\nEmailChain\nq\x00.'
obj = dill.loads(input)
print(obj)

# Results in error since obj is missing data class fields for __str__ method

Working Case

# Working-case

import dill
from dataclasses import dataclass, field
from typing import List, Dict, Any
import json
from collections import defaultdict


class JSONRequest(type):
    def __call__(cls, *args, **kwargs):
        """
        This is a metaclass used to autonomously populate dataclasses

        NOTE: This metaclass only works with dataclasses

        Optional Parameter:
            inp: class attribute that is a callable that produces a dictionary
        """
        inp = cls.__dict__["inp"]()
        cls.__initialize(inp)
        return cls

    def __initialize(cls, inp: dict) -> None:
        """
        Initializes all of the dataclasses fields

        If the field is missing in the JSON request and it does not have a default value in the data class a
        ValueError error will be raised. Additionally if the Json value is [], {}, "" will default to the default
        value, and if the default value is missing an InvalidRequest error will also be raised.

        Parameters:
            inp: Json input
        """
        _json = defaultdict(lambda: None)
        _json.update(inp)

        for name, _ in cls.__dataclass_fields__.items():
            if (not _json[name]) and (name not in cls.__dict__.keys()):
                raise ValueError(f"Request is missing the {name} field")

            value = _json[name] or cls.__dict__[name]
            setattr(cls, name, value)

    def __str__(cls):
        rep = {name: getattr(cls, name) for name, _ in cls.__dataclass_fields__.items()}
        return json.dumps(rep, indent=4)


def generate_input():
    """
    Stub method for generating input
    """
    return {
        "email_list": [f"{name}@yahoo.com" for name in ["peter", "mark", "alysa"]],
        "message": "Foo bar fizzbuzz",
        "subject": "Sample Issue",
        "info": {
            "time": 1619628747.9166002,
            "count": 3,
        }
    }


@dataclass
class EmailChain(metaclass=JSONRequest):
    email_list: List[str] = field(init=False)
    message: str = field(init=False)
    subject: str = field(init=False)
    info: Dict[str, Any] = field(init=False)

    inp = generate_input


obj = EmailChain()
data_stream = dill.dumps(obj)
print(data_stream)

# output: b'\x80\x03cdill._dill\n_create_type\nq\x00(h\x00(cdill._dill\n_load_type\nq\x01X\x04\x00\x00\x00typeq\x02\x85q\x03Rq\x04X\x0b\x00\x00\x00JSONRequestq\x05h\x04\x85q\x06}q\x07(X\n\x00\x00\x00__module__q\x08X\x08\x00\x00\x00__main__q\tX\x08\x00\x00\x00__call__q\ncdill._dill\n_create_function\nq\x0b(cdill._dill\n_create_code\nq\x0c(K\x01K\x00K\x04K\x03KOC\x1a|\x00j\x00d\x01\x19\x00\x83\x00}\x03|\x00\xa0\x01|\x03\xa1\x01\x01\x00|\x00S\x00q\rX\xf5\x00\x00\x00\n        This is a metaclass used to autonomously populate dataclasses\n\n        NOTE: This metaclass only works with dataclasses\n\n        Optional Parameter:\n            inp: class attribute that is a callable that produces a dictionary\n        q\x0eX\x03\x00\x00\x00inpq\x0f\x86q\x10X\x08\x00\x00\x00__dict__q\x11X\x18\x00\x00\x00_JSONRequest__initializeq\x12\x86q\x13(X\x03\x00\x00\x00clsq\x14X\x04\x00\x00\x00argsq\x15X\x06\x00\x00\x00kwargsq\x16h\x0ftq\x17XL\x00\x00\x00C:/Users/739908/Work/systems_automation/report_automation_engine/src/temp.pyq\x18h\nK\nC\x06\x00\t\x0c\x01\n\x01q\x19))tq\x1aRq\x1bc__builtin__\n__main__\nh\nNN}q\x1cNtq\x1dRq\x1eh\x12h\x0b(h\x0c(K\x02K\x00K\x06K\x05KCCvt\x00d\x01d\x02\x84\x00\x83\x01}\x02|\x02\xa0\x01|\x01\xa1\x01\x01\x00xZ|\x00j\x02\xa0\x03\xa1\x00D\x00]L\\\x02}\x03}\x04|\x02|\x03\x19\x00sP|\x03|\x00j\x04\xa0\x05\xa1\x00k\x07rPt\x06d\x03|\x03\x9b\x00d\x04\x9d\x03\x83\x01\x82\x01|\x02|\x03\x19\x00p`|\x00j\x04|\x03\x19\x00}\x05t\x07|\x00|\x03|\x05\x83\x03\x01\x00q"W\x00d\x05S\x00q\x1f(X\xac\x01\x00\x00\n        Initializes all of the dataclasses fields\n\n        If the field is missing in the JSON request and it does not have a default value in the data class a\n        ValueError error will be raised. Additionally if the Json value is [], {}, "" will default to the default\n        value, and if the default value is missing an InvalidRequest error will also be raised.\n\n        Parameters:\n            inp: Json input\n        q h\x0c(K\x00K\x00K\x00K\x01KSC\x04d\x00S\x00q!N\x85q"))h\x18X\x08\x00\x00\x00<lambda>q#K"C\x00q$))tq%Rq&X*\x00\x00\x00JSONRequest.__initialize.<locals>.<lambda>q\'X\x17\x00\x00\x00Request is missing the q(X\x06\x00\x00\x00 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   Stub method for generating input\n    q~h\x0c(K\x01K\x00K\x02K\x04KSC\x16g\x00|\x00]\x0e}\x01|\x01\x9b\x00d\x00\x9d\x02\x91\x02q\x04S\x00q\x7fX\n\x00\x00\x00@yahoo.comq\x80\x85q\x81)hDh4\x86q\x82h\x18X\n\x00\x00\x00<listcomp>q\x83K6C\x02\x06\x00q\x84))tq\x85Rq\x86X"\x00\x00\x00generate_input.<locals>.<listcomp>q\x87X\x05\x00\x00\x00peterq\x88X\x04\x00\x00\x00markq\x89X\x05\x00\x00\x00alysaq\x8a\x87q\x8bX\x10\x00\x00\x00Foo bar fizzbuzzq\x8cX\x0c\x00\x00\x00Sample Issueq\x8dGA\xd8"d\xb2\xfa\xa9\x94K\x03X\x04\x00\x00\x00timeq\x8eX\x05\x00\x00\x00countq\x8f\x86q\x90(hfhuhvhwtq\x91tq\x92))h\x18X\x0e\x00\x00\x00generate_inputq\x93K1C\n\x00\x05\x0c\x01\x02\x01\x02\x02\x02\x01q\x94))tq\x95Rq\x96c__builtin__\n__main__\nh\x93NN}q\x97Ntq\x98Rq\x99h[X\x1b\x00\x00\x00EmailChain(*args, **kwargs)q\x9aX\x14\x00\x00\x00__dataclass_params__q\x9bcdataclasses\n_DataclassParams\nq\x9c)\x81q\x9dN}q\x9e(X\x04\x00\x00\x00initq\x9f\x88X\x04\x00\x00\x00reprq\xa0\x88X\x02\x00\x00\x00eqq\xa1\x88X\x05\x00\x00\x00orderq\xa2\x89X\x0b\x00\x00\x00unsafe_hashq\xa3\x89X\x06\x00\x00\x00frozenq\xa4\x89u\x86q\xa5bh-}q\xa6(hfcdataclasses\nField\nq\xa7)\x81q\xa8N}q\xa9(h4hfh\x02htX\x07\x00\x00\x00defaultq\xaacdataclasses\n_MISSING_TYPE\nq\xab)\x81q\xacX\x0f\x00\x00\x00default_factoryq\xadh\xach\xa0\x88X\x04\x00\x00\x00hashq\xaeNh\x9f\x89X\x07\x00\x00\x00compareq\xaf\x88X\x08\x00\x00\x00metadataq\xb0h\x01X\x10\x00\x00\x00MappingProxyTypeq\xb1\x85q\xb2Rq\xb3}q\xb4\x85q\xb5Rq\xb6X\x0b\x00\x00\x00_field_typeq\xb7cdataclasses\n_FIELD_BASE\nq\xb8)\x81q\xb9}q\xbah4X\x06\x00\x00\x00_FIELDq\xbbsbu\x86q\xbcbhuh\xa7)\x81q\xbdN}q\xbe(h4huh\x02hrh\xaah\xach\xadh\xach\xa0\x88h\xaeNh\x9f\x89h\xaf\x88h\xb0h\xb6h\xb7h\xb9u\x86q\xbfbhvh\xa7)\x81q\xc0N}q\xc1(h4hvh\x02hrh\xaah\xach\xadh\xach\xa0\x88h\xaeNh\x9f\x89h\xaf\x88h\xb0h\xb6h\xb7h\xb9u\x86q\xc2bhwh\xa7)\x81q\xc3N}q\xc4(h4hwh\x02h|h\xaah\xach\xadh\xach\xa0\x88h\xaeNh\x9f\x89h\xaf\x88h\xb0h\xb6h\xb7h\xb9u\x86q\xc5buX\x08\x00\x00\x00__init__q\xc6h\x0b(h\x0c(K\x01K\x00K\x01K\x01KCC\x04d\x00S\x00q\xc7N\x85q\xc8)X\x04\x00\x00\x00selfq\xc9\x85q\xcaX\x08\x00\x00\x00<string>q\xcbh\xc6K\x01C\x02\x00\x01q\xcc))tq\xcdRq\xce}q\xcf(X\x07\x00\x00\x00MISSINGq\xd0h\xacX\x14\x00\x00\x00_HAS_DEFAULT_FACTORYq\xd1cdataclasses\n_HAS_DEFAULT_FACTORY_CLASS\nq\xd2)\x81q\xd3X\x0c\x00\x00\x00__builtins__q\xd4hhX\x08\x00\x00\x00builtinsq\xd5\x85q\xd6Rq\xd7uh\xc6NN}q\xd8Ntq\xd9Rq\xdaX\x08\x00\x00\x00__repr__q\xdbh\x0b(h\x0c(K\x01K\x00K\x03K\tK\x13CDt\x00|\x00\x83\x01t\x01\xa0\x02\xa1\x00f\x02}\x01|\x01\x88\x00k\x06r\x1cd\x01S\x00\x88\x00\xa0\x03|\x01\xa1\x01\x01\x00z\x0c\x88\x01|\x00\x83\x01}\x02W\x00d\x00\x88\x00\xa0\x04|\x01\xa1\x01\x01\x00X\x00|\x02S\x00q\xdcNX\x03\x00\x00\x00...q\xdd\x86q\xde(X\x02\x00\x00\x00idq\xdfX\x07\x00\x00\x00_threadq\xe0X\t\x00\x00\x00get_identq\xe1X\x03\x00\x00\x00addq\xe2X\x07\x00\x00\x00discardq\xe3tq\xe4h\xc9X\x03\x00\x00\x00keyq\xe5X\x06\x00\x00\x00resultq\xe6\x87q\xe7X\'\x00\x00\x00C:\\DevApps\\Python3.7\\lib\\dataclasses.pyq\xe8X\x07\x00\x00\x00wrapperq\xe9M^\x01C\x10\x00\x02\x10\x01\x08\x01\x04\x01\n\x01\x02\x01\x0c\x02\x0c\x01q\xeaX\x0c\x00\x00\x00repr_runningq\xebX\r\x00\x00\x00user_functionq\xec\x86q\xed)tq\xeeRq\xefcdataclasses\n__dict__\nh\xdbNcdill._dill\n_create_cell\nq\xf0h\x01X\x03\x00\x00\x00setq\xf1\x85q\xf2Rq\xf3]q\xf4\x85q\xf5Rq\xf6\x85q\xf7Rq\xf8h\xf0h\x0b(h\x0c(K\x01K\x00K\x01K\nKCC.|\x00j\x00j\x01d\x01|\x00j\x02\x9b\x02d\x02|\x00j\x03\x9b\x02d\x03|\x00j\x04\x9b\x02d\x04|\x00j\x05\x9b\x02d\x05\x9d\t\x17\x00S\x00q\xf9(NX\x0c\x00\x00\x00(email_list=q\xfaX\n\x00\x00\x00, 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\x01\x00\x00X\x0e\x00\x00\x00mark@yahoo.comr!\x01\x00\x00X\x0f\x00\x00\x00alysa@yahoo.comr"\x01\x00\x00ehuh\x8chvh\x8dhw}r#\x01\x00\x00(h\x8eGA\xd8"d\xb2\xfa\xa9\x94h\x8fK\x03uutr$\x01\x00\x00Rr%\x01\x00\x00.'

c = dill.loads(data_stream)
print(c)

"""
output:
{
    "email_list": [
        "peter@yahoo.com",
        "mark@yahoo.com",
        "alysa@yahoo.com"
    ],
    "message": "Foo bar fizzbuzz",
    "subject": "Sample Issue",
    "info": {
        "time": 1619628747.9166002,
        "count": 3
    }
}
"""
Will Udstrand
  • 128
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  • Related question [here (using pickle)](https://stackoverflow.com/questions/4647566/pickle-a-dynamically-parameterized-sub-class/). Yes, this is possible via using a custom metaclass and `copyreg`. See also [issue 7689](https://bugs.python.org/issue7689). – Albert Apr 05 '23 at 20:45

1 Answers1

1

I have figured out the issue dill needed the metaclass type register for marshalling. Below is the updated metaclass and type registration for dill that solved the issue.

Updated Metaclass

from typing import Dict, Tuple

from core.util.errors import OptionalModuleError, InvalidRequest

import json
from collections import defaultdict

try:
    from flask import request
except ImportError:
    raise OptionalModuleError("metaclasses.JSONRequest", ["flask"])


class JSONRequest(type):
    """
    This is a metaclass used to autonomously populate dataclasses with the request data coming in to flask

    NOTE: This metaclass only works with dataclasses

    Optional Parameter:
        inp: **kwargs dict or a class attribute that is a callable that produces a dictionary
    """
    def __call__(cls, *args, **kwargs):
        if "inp" in kwargs:
            inp = kwargs["inp"]
        elif "inp" in cls.__dict__:
            inp = cls.__dict__["inp"]()
        else:
            inp = request.json
        cls.__initialize(inp)
        return cls

    def __initialize(cls, inp: Dict) -> None:
        """
        Initializes all of the dataclasses fields

        If the field is missing in the JSON request and it does not have a default value in the data class a
        InvalidRequest error will be raised. Additionally if the Json value is [], {}, "" will default to the default
        value, and if the default value is missing an InvalidRequest error will also be raised.

        Parameters:
            inp: Json input
        """
        json = defaultdict(lambda: None)
        json.update(inp)

        for name, _ in cls.__dataclass_fields__.items():
            if (not json[name]) and (name not in cls.__dict__.keys()):
                raise InvalidRequest(f"Request is missing the {name} field")

            value = json[name] or cls.__dict__[name]
            setattr(cls, name, value)

    def _get_fields(cls) -> Dict:
        """
        This method returns all of the dataclasses fields
        
        Returns:
             Dict/JSON representation of the dataclasses attributes
        """
        return {name: getattr(cls, name) for name, _ in cls.__dataclass_fields__.items()}

    def get_info(cls) -> Tuple[str, str, Dict]:
        """
        This method returns all the needed information to reconstruct/deserialize the class
        
        Returns:
            Tuple containing the dataclass' module location and name for importing it when deserializing and the 
            input used to repopulate the class to it last state  
        """
        return cls.__module__, cls.__name__, cls._get_fields()

    def __str__(cls) -> str:
        """
        Method to print the request nicely
        
        Returns:
            The str representation of the class
        """
        return json.dumps(cls._get_fields(), indent=4)

Dill Type Registration

def recreate_request(mod_name: str, class_name: str, inp: Dict) -> JSONRequest:
    """
    Method to rebuild the serialized request

    Parameters:
        mod_name: The name of the dataclass' module
        class_name: The name of the dataclass
        inp: The state of the dataclass' member variables
        
    Returns:
        The deserialized JSON Request with its last known state
    """
    mod = __import__(mod_name, fromlist=[class_name])
    klass = getattr(mod, class_name)
    return klass(inp=inp)


@dill.register(JSONRequest)
def reduce_request(pickler, obj):
    """
    This method registers recreate_request for all dataclasses with a metaclass of JSONRequest
    
    Parameter:
        pickler: The pickler from the dill library
        obj: The dataclass being serialized
    """
    args = obj.get_info()
    pickler.save_reduce(recreate_request, args, obj=obj)
Will Udstrand
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