Check if List is not empty with Pydantic in an elegant way

Question

Let’s say I have some BaseModel, and I want to check that it’s options list is not empty. I can perfectly do it with a validator:

class Trait(BaseModel):
    name: str
    options: List[str]

    @validator("options")
    def options_non_empty(cls, v):
        assert len(v) > 0
        return v

Are there any other, more elegant, way to do this?

Asked By: keddad

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Source

Answer 1

In Python, empty lists are falsey, while lists with any number of elements are truthy:

>>> bool([])
False
>>> bool([1,2,3])
True
>>> bool([False])
True
>>> bool([[]])
True

This means that you can simply assert v or assert Trait.options to confirm that the list is non-empty.

Answered By: water_ghosts

Answer 2

If you want to use a @validator:

return v if v else doSomething

Python assumes boolean-ess of an empty list as False

If you don’t want to use a @validator:

In Pydantic, use conlist:

from pydantic import BaseModel, conlist
from typing import List

class Trait(BaseModel):
    name: str
    options: conlist(str, min_length=1)

Answered By: Kirtiman Sinha

Answer 3

Use `Field` with `min_length`:

from typing import List
from pydantic import BaseModel, Field

class Trait(BaseModel):
    name: str
    options: List[str] = Field(min_length=1)

min_length is on the string constraints session but still works for lists.

Answered By: Maicon Mauricio

Check if List is not empty with Pydantic in an elegant way

Question:

Answers:

Use `Field` with `min_length`:

Check if List is not empty with Pydantic in an elegant way

Question:

Answers:

Use Field with min_length:

Use `Field` with `min_length`: