Pydantic: How to pass the default value to a variable if None was passed?

This question asked perfectly so i wanted to provide a wider example, because there are many ways to assign a value dynamically.

Alex’s answer is correct but it only works on when the Field directly inherits a dataclass more specifically something like this won’t work.

class User(BaseModel):
    name: Optional[str] = ""
    password: Optional[str] = ""

    class Config:
        validate_assignment = True

    @validator("name")
    def set_name(cls, name):
        return name or "bar"


user_dict = {"password": "so_secret"}
user_one = User(**user_dict)
Out: name='' password='so_secret'

Validate Always

For performance reasons, by default validators are not called for fields when a value is not supplied. But situations like this when you need to set a Dynamic Default Value we can set that to True

class User(BaseModel):
    name: Optional[str] = ""

    @validator("name", pre=True, always=True)
    def set_name(cls, name):
        return name or "bar"

In: user_one = User(name=None)
In: user_two = User()
Out: name='bar'
Out: name='bar'

But there is a one important catch with always, since we are using always=True pydantic would try to validate the default None which would cause an error.

Setting Pre to True it will call that field before validation error occurs, the default of a validator pre is set to False , in which case they’re called after field validation.

Using Config

But this has some disadvantages.

class User(BaseModel):
    name: Optional[str] = ""

    class Config:
        validate_assignment = True

    @validator("name")
    def set_name(cls, name):
        return name or "foo"

In:  user = User(name=None)
Out: name='foo'

When you set it to None it returns the dynamic value correctly but some situations like it is completely None, it fails.

In:  user = User()
Out: name=''

Again you need to set, to make that work.

pre=True
always=True

Using default_factory

This is mostly useful in cases when you want to set a default value, like UUID or datetime etc. In that cases you might want to use default_factory, but there is a big catch you can’t assign a Callable argument to the default_factory.

class User(BaseModel):
    created_at: datetime = Field(default_factory=datetime.now)

In: user = User()
Out: created_at=datetime.datetime(2020, 8, 29, 2, 40, 12, 780986)

Many ways to assign a default value

Method #1: A required id field with default value

class User(BaseModel):
    id: str = uuid.uuid4()

Method #2 An optional id field with default value

class User(BaseModel):
    id: Optional[str] = uuid.uuid4()

Method #3: A required id field with default value

class User(BaseModel):
    id: str = Field(default=uuid.uuid4())

Method #4: A required id field with default value from callable. This is useful for generating on-demand values such as unique UUIDs or Timestamps.
See @yagiz-degirmenci answer.

class User(BaseModel):
    id: str = Field(default_factory=uuid.uuid4)  # uuid.uuid4 is not executed immediately

Adding an alternative option – convtools models (docs / github). This library doesn’t have implicit type casting, so when only types are defined, it runs validation only.

In your case – keep things separate:

1. you define a field, its type and default value
2. you post-process the resulting model by specifying a model-level validate class method, which returns (data, errors)

from typing import Optional

from convtools.contrib.models import DictModel, build


class User(DictModel):
    name: Optional[str] = ""

    @classmethod
    def validate(cls, data):
        data.name = data.name or ""
        return data, None


user, errors = build(User, {"name": None})
"""
>>> In [26]: build(User, {"name": None})
>>> Out[26]: (User(name=''), None)
>>> 
>>> In [27]: build(User, {"name": ""})
>>> Out[27]: (User(name=''), None)
>>> 
>>> In [28]: build(User, {"name": "123"})
>>> Out[28]: (User(name='123'), None)
>>> 
>>> In [29]: build(User, {"name": 123})
>>> Out[29]: (None, {'name': {'__ERRORS': {'type': 'int instead of NoneType/str'}}})
"""