When parsing with Union Optional values are swallowing the input

[m-aciek]

Hi, thanks for your work on the library.

Describe the bug

When parsing with union an Optional value in a dataclass doesn't consider the existence of keys in dictionaries when parsing the structure. Please see the reproducer.

To Reproduce
Consider the code below:

from dataclasses import dataclass
from typing import Union, Optional

import dacite


@dataclass
class OptionalDataclass:
    optional: Optional[int]

@dataclass
class Required:
    required: int

@dataclass
class UnionDataclass:
    field: Union[OptionalDataclass, Required]


print(dacite.from_dict(UnionDataclass, {"field": {"required": 2}}))
# UnionDataclass(field=OptionalDataclass(optional=None))

Expected behavior

print(dacite.from_dict(UnionDataclass, {"field": {"required": 2}}))
# UnionDataclass(field=Required(required=2))

I would expect dacite to look into dictionary keys and prefer an object which has the key.

Environment

• Python version: 3.9
• dacite version: 1.8.1

Additional context

Changing the order of classes in the Union annotation helps in this case.

from dataclasses import dataclass
from typing import Union, Optional

import dacite


@dataclass
class OptionalDataclass:
    optional: Optional[int]

@dataclass
class Required:
    required: int

@dataclass
class UnionDataclass:
    field: Union[Required, OptionalDataclass]


print(dacite.from_dict(UnionDataclass, {"field": {"required": 2}}))
# UnionDataclass(field=Required(required=2))