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

You need to enable validate_assignment option in model config:

from typing import Optional

from pydantic import BaseModel, validator


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

    class Config:
        validate_assignment = True

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


user = User(name=None, password='some_password', )
print("Name is ", user.name)


user.name = None
print("Name is ", user.name)

Name is  foo
Name is  foo

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

Tags:

Python

Pydantic

Fastapi

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