A test showing how to handle empty string == null in different scenarios.

pydantic_validators_test.py

import typing

import pydantic


class TestBaseModel(pydantic.BaseModel):
    @pydantic.validator('*', pre=True)
    def empty_str_to_none(cls, v):
        if v == "":
            return None
        return v


class TestModel(TestBaseModel):
    str_or_none: typing.Optional[str]
    int_or_none: typing.Optional[int]
    int_or_none_json: typing.Optional[pydantic.Json[int]]

    int_list_or_none: typing.Optional[typing.List[int]]
    int_list: typing.List[int]

    @pydantic.validator("int_list", pre=True)
    def null_or_empty_str_results_in_empty_list(cls, v):
        return v if v else []

    int_list_or_none_json: typing.Optional[pydantic.Json[typing.List[int]]]
    int_list_json: pydantic.Json[typing.List[int]]

    @pydantic.validator("int_list_json", pre=True)
    def null_or_empty_str_results_in_empty_json_list(cls, v):
        return v if v else "[]"


    # Keep in mind you can do:
    a: int = 1
    b: int = 2
    c: int = 3
    @pydantic.validator("a", "b", "c", pre=True)
    def this_applies_to_multiple_fields(cls, v):
        return v


def test_demo_model():


    test_model = TestModel.parse_obj(dict(
        str_or_none="something",
        int_or_none=123,
        int_or_none_json="123",
        int_list_or_none=[1, 2, 3],
        int_list=[1, 2, 3],
        int_list_or_none_json="[1, 2, 3]",
        int_list_json="[1, 2, 3]",
    ))

    assert test_model.str_or_none == "something"
    assert test_model.int_or_none == 123
    assert test_model.int_or_none_json == 123
    assert test_model.int_list_or_none_json == [1, 2, 3]
    assert test_model.int_list_json == [1, 2, 3]


    test_model = TestModel.parse_obj(dict(
        str_or_none="",
        int_or_none="",
        int_or_none_json="",
        int_list_or_none="",
        int_list="",
        int_list_or_none_json="",
        int_list_json="",
    ))

    assert test_model.str_or_none is None
    assert test_model.int_or_none is None
    assert test_model.int_or_none_json is None

    assert test_model.int_list_or_none is None
    assert test_model.int_list == []

    assert test_model.int_list_or_none_json is None
    assert test_model.int_list_json == []