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?

I wasn't that clear: I'm not looking to shrink the size of validator function, I'm looking to find a way not to use it at all. — keddad, Apr 27 '20 at 21:29

score 39 · Accepted Answer · edited Sep 01 '23 at 14:40

39

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_items=1)

edited Sep 01 '23 at 14:40

Mücahit Şenol

87
1
6

answered Apr 27 '20 at 21:23

Kirtiman Sinha

843
8
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Is `return v if v` syntactically correct? What am I missing? – npk Aug 07 '20 at 13:46
it's not, it was meant to imply `else someOtherExpression`. OP I believe, was interested in my `conlist` answer. – Kirtiman Sinha Aug 08 '20 at 22:46

score 2 · Answer 2 · answered Apr 27 '20 at 21:21

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.

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

2 Answers2

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