How can I fix the TypeError of my dataclass in Python?

Question

I have a dataclass with 5 attributes. When I give these attributes via a dictionary, it works well. But when the dictionary has more attributes than the class have, the class gives TypeError. I am trying to make that when there is extra values, the class wouldn't care them. How can I make that?

from dataclasses import dataclass

@dataclass
class Employee(object):
    name: str
    lastname: str
    age: int or None
    salary: int
    department: str

    def __new__(cls, name, lastname, age, salary, department):
        return object.__new__(cls)

    def __post_init__(self):
        if type(self.age) == str:
            self.age = int(self.age) or None

    def __str__(self):
        return f'{self.name}, {self.lastname}, {self.age}' 

dic = {"name":"abdülmutallip", 
"lastname":"uzunkavakağacıaltındauzanıroğlu", 
"age":"24", "salary":2000, "department":"İK", 
"city":"istanbul", "country":"tr", "adres":"yok", "phone":"0033333"}

a = Employee(**dic)
print(a)

The error is:

TypeError: __new__() got an unexpected keyword argument 'city'

I want the class works properly in this situation without any error. I don't want to add these extra attributes into the class.

Answer 1

If you want the dataclass to accept arbitrary extra keyword arguments then you either have to define your own __init__ method, or provide a custom __call__ method on a metaclass. If you define a custom __init__ method, the dataclass decorator won't generate one for you; at this point there is no need to use __post_init__ any more either since you already are writing an __init__ method.

Side notes:

• __new__ can't alter what arguments are passed to __init__. The metaclass's __call__ will normally first call cls.__new__(<arguments>) then call instance.__init__(<arguments> on the instance return value from __new__, see the datamodel documentation.
• You can't use int or None, that's an expression that just returns int, it won't let you omit the age parameter. Give the field a default value instead, or use a Union type hint if None is only used to indicate age=0 or a failed int() conversion.
• Fields that have a default defined must come after fields that do not have a default defined, so put age at the end.
• If you also use type hinting beyond dataclasses, and age is meant to be an optional field, then use typing.Optional to properly mark the type of the age field as optional. Optional[int] is equivalent to Union[int, None]; personally I prefer the latter in constructors when there is no default value set and omitting age is not acceptable.
• Use isinstance() to determine if an object is a string. Or just don't test, since int(self.age) just returns self.age unchanged if it already is set to an integer.
• Only use or None in the __post_init__ method if it is okay for an age set to 0 to be set to None.
• If age is to be set to None only if int(age) fails, then you have to use try:...except to handle the ValueError or TypeError exceptions that int() can raise in that case, not or None.

Assuming that you meant for age to be set to None only if conversion fails:

from dataclasses import dataclass
from typing import Union

@dataclass
class Employee(object):
    name: str
    lastname: str
    age: Union[int, None]  # set to None if conversion fails
    salary: int
    department: str

    def __init__(
        self,
        name: str,
        lastname: str,  
        age: Union[int, None],
        salary: int,
        department: str,
        *args: Any,
        **kwargs: Any,
    ) -> None:
        self.name = name
        self.lastname = lastname
        try:
            self.age = int(age)
        except (ValueError, TypeError):
            # could not convert age to an integer
            self.age = None
        self.salary = salary
        self.department = department

    def __str__(self):
        return f'{self.name}, {self.lastname}, {self.age}' 

If you want to go the metaclass route, then you can create one that ignores all extra arguments for almost any class, by introspecting the __init__ or __new__ method call signature:

from inspect import signature, Parameter

class _ArgTrimmer:
    def __init__(self):
        self.new_args, self.new_kw = [], {}
        self.dispatch = {
            Parameter.POSITIONAL_ONLY: self.pos_only,
            Parameter.KEYWORD_ONLY: self.kw_only,
            Parameter.POSITIONAL_OR_KEYWORD: self.pos_or_kw,
            Parameter.VAR_POSITIONAL: self.starargs,
            Parameter.VAR_KEYWORD: self.starstarkwargs,
        }

    def pos_only(self, p, i, args, kwargs):
        if i < len(args):
            self.new_args.append(args[i])

    def kw_only(self, p, i, args, kwargs):
        if p.name in kwargs:
            self.new_kw[p.name] = kwargs.pop(p.name)

    def pos_or_kw(self, p, i, args, kwargs):
        if i < len(args):
            self.new_args.append(args[i])
            # drop if also in kwargs, otherwise parameters collide
            # if there's a VAR_KEYWORD parameter to capture it
            kwargs.pop(p.name, None)
        elif p.name in kwargs:
            self.new_kw[p.name] = kwargs[p.name]

    def starargs(self, p, i, args, kwargs):
        self.new_args.extend(args[i:])

    def starstarkwargs(self, p, i, args, kwargs):
        self.new_kw.update(kwargs)

    def trim(self, params, args, kwargs):
        for i, p in enumerate(params.values()):
            if i:  # skip first (self or cls) arg of unbound function
                self.dispatch[p.kind](p, i - 1, args, kwargs)
        return self.new_args, self.new_kw

class IgnoreExtraArgsMeta(type):
    def __call__(cls, *args, **kwargs):
        if cls.__new__ is not object.__new__:
            func = cls.__new__
        else:
            func = getattr(cls, '__init__', None)
        if func is not None:
            sig = signature(func)
            args, kwargs = _ArgTrimmer().trim(sig.parameters, args, kwargs)
        return super().__call__(*args, **kwargs)

This metaclass will work for any Python class, but if you were to subclass in a built-in type then the __new__ or __init__ methods may not be introspectable. Not the case here, but a caveat that you would need to know about if you were to use the above metaclass in other situations.

Then use the above as a metaclass parameter on your dataclass:

from dataclasses import dataclass
from typing import Union

@dataclass
class Employee(metaclass=IgnoreExtraArgsMeta):
    name: str
    lastname: str
    age: Union[int, None]
    salary: int
    department: str

    def __post_init__(self):
        try:
            self.age = int(self.age)
        except (ValueError, TypeError):
            # could not convert age to an integer
            self.age = None

    def __str__(self):
        return f'{self.name}, {self.lastname}, {self.age}' 

The advantage of using a metaclass should be clear here; no need to repeat all the fields in the __init__ method.

Demo of the first approach:

>>> from dataclasses import dataclass
>>> from typing import Union
>>> @dataclass
... class Employee(object):
...     name: str
...     lastname: str
...     age: Union[int, None]  # set to None if conversion fails
...     salary: int
...     department: str
...     def __init__(self,
...         name: str,
...         lastname: str,
...         age: Union[int, None],
...         salary: int,
...         department: str,
...         *args: Any,
...         **kwargs: Any,
...     ) -> None:
...         self.name = name
...         self.lastname = lastname
...         try:
...             self.age = int(age)
...         except (ValueError, TypeError):
...             # could not convert age to an integer
...             self.age = None
...         self.salary = salary
...         self.department = department
...     def __str__(self):
...         return f'{self.name}, {self.lastname}, {self.age}'
... 
>>> dic = {"name":"abdülmutallip",
... "lastname":"uzunkavakağacıaltındauzanıroğlu",
... "age":"24", "salary":2000, "department":"İK",
... "city":"istanbul", "country":"tr", "adres":"yok", "phone":"0033333"}
>>> a = Employee(**dic)
>>> a
Employee(name='abdülmutallip', lastname='uzunkavakağacıaltındauzanıroğlu', age=24, salary=2000, department='İK')
>>> print(a)
abdülmutallip, uzunkavakağacıaltındauzanıroğlu, 24
>>> a.age
24
>>> Employee(name="Eric", lastname="Idle", age="too old to tell", salary=123456, department="Silly Walks")
Employee(name='Eric', lastname='Idle', age=None, salary=123456, department='Silly Walks')

and of the second approach:

>>> @dataclass
... class Employee(metaclass=IgnoreExtraArgsMeta):
...     name: str
...     lastname: str
...     age: Union[int, None]
...     salary: int
...     department: str
...     def __post_init__(self):
...         try:
...             self.age = int(self.age)
...         except (ValueError, TypeError):
...             # could not convert age to an integer
...             self.age = None
...     def __str__(self):
...         return f'{self.name}, {self.lastname}, {self.age}'
...
>>> a = Employee(**dic)
>>> print(a)
abdülmutallip, uzunkavakağacıaltındauzanıroğlu, 24
>>> a
Employee(name='abdülmutallip', lastname='uzunkavakağacıaltındauzanıroğlu', age=24, salary=2000, department='İK')
>>> Employee("Michael", "Palin", "annoyed you asked", salary=42, department="Complaints", notes="Civil servants should never be asked for their salary, either")
Employee(name='Michael', lastname='Palin', age=None, salary=42, department='Complaints')

If age is meant to be optional (so, have a default value), then move it to the end of the fields, give it Optional[int] as the type, and assign None to it. You'll have to do the same in the __init__ method you specify your own:

from typing import Optional

@dataclass
class Employee(object):
    name: str
    lastname: str
    age: Optional[int] = None
    salary: int
    department: str

    def __init__(
        self,
        name: str,
        lastname: str,  
        salary: int,
        department: str,
        age: Optional[int] = None,
        *args: Any,
        **kwargs: Any,
    ) -> None:
        # ...