Validate field based on other optional fields

Question

• OS: macOS Mojave
• Python version: 3.7.0
• Pydantic version: 0.21

I’m currently working with pydantic in a scenario where I’d like to validate an instantiation of MyClass to ensure that certain optional fields are set or not set depending on the value of an enum. However, I’m noticing in the @validator('my_field'), only required fields are present in values regardless if they’re actually populated with values.

import enum
from pydantic import BaseModel, validator

class MyEnum(enum.Enum):
    ENUM_VALUE = 'foo'
    OTHER_VALUE = 'bar'


class MyClass(BaseModel):
    required_float: float
    my_enum: MyEnum
    optional_float: float = 0.0

    @validator('my_enum')
    def ensure_stuff_exists(cls, v, values, **kwargs):
        if v == MyEnum.ENUM_VALUE and values['optional_float'] is not None:
            raise KeyError('foo')


MyClass(
    my_enum=MyEnum.ENUM_VALUE,
    required_float=4.0,
    optional_float=22.0,
)
...

In the above example, only required_float and my_enum would be populated in values, I am unable to inspect the value of optional_float, even though it is populated with a value upon instantiation. Is there any way to perform this type of validation? I hope this example makes sense.

[samuelcolvin]

Yes, fields are populated in the order they’re defined in the class, so just move my_enum down one line to below optional_float.

[SethMMorton]

Digging deeper, this comment first appeared in https://github.com/samuelcolvin/pydantic/commit/5efa54d80d8a584e4c369e2ccdf30d723e54db3b, with the note “switch annotation only fields to come first in fields list not last”. It seems like they have always been separated, but before they were after optional, not before.

Based on some of the diffs I saw in that commit, I got an idea to make all my required fields non-annotation-only by adding ....

class ExampleWithOptional(pydantic.BaseModel):
    foo: List[int] = [1, 2]
    bar: int = ...

    @pydantic.validator("bar")
    def add_length_of_foo_to_bar(cls, value, values, **kwargs):
        return value + len(values["foo"])

This got it to work as well.

---

So, there is a workaround! If it is not possible to get what I was originally trying to do to work, it might be nice to add to the docs that you can make it work with = ....