2025-03-20

[MtkN1]

Today I Learned ...

[MtkN1]

pydantic で stdlib dataclass field の metadata alias が使える

pydantic/pydantic#8379 が解決できる。

from dataclasses import dataclass, field

from pydantic import TypeAdapter, ValidationError


@dataclass
class User:
    name: str = field(metadata={"alias": "userName"})


ta = TypeAdapter(User)

# -----------------------------------------
# Case: Non-alias validation is not allowed
# -----------------------------------------
try:
    ta.validate_json('{"name":"John Doe"}')
    raise AssertionError
except ValidationError as e:
    print(e)
# 1 validation error for User
# userName
#   Field required [type=missing, input_value={'name': 'John Doe'}, input_type=dict]
#     For further information visit https://errors.pydantic.dev/2.10/v/missing

# ---------------------------------
# Case: Alias validation is allowed
# ---------------------------------
instance = ta.validate_json('{"userName":"John Doe"}')
print(instance)
# User(name='John Doe')

# ------------------------------------
# Case: Alias serialization is allowed
# ------------------------------------
data = ta.dump_json(instance, by_alias=True)
print(data)
# b'{"userName":"John Doe"}'

# ----------------------------------------
# Case: Non-alias serialization is allowed
# ----------------------------------------
data = ta.dump_json(instance)
print(data)
# b'{"name":"John Doe"}'

# ---------------------------------------------
# Case: Alias class construction is not allowed
# ---------------------------------------------
try:
    User(userName="John Doe")  # typechecker: No parameter named "userName"
    raise AssertionError
except TypeError as e:
    print(e)
# User.__init__() got an unexpected keyword argument 'userName'

# ---------------------------------------------
# Case: Non-alias class construction is allowed
# ---------------------------------------------
print(User(name="John Doe"))
# User(name='John Doe')

[MtkN1]

stdlib dataclass だと TypeAdapter に ConfigDict がエラーになって使えないけど、stdlib dataclass にダンダー変数 __pydantic_config__ を定義すればよい。

https://docs.pydantic.dev/latest/errors/usage_errors/#type-adapter-config-unused