Python Pydantic - how to have an "optional" field but if present required to conform to not None value?

Question

I am trying to validate an object that has "optional" fields in the sense that they may or may not be present. But when they are present, the fields should conform to a specific type definition (not None).

In the example below, the "size" field is optional but allows None. I want the "size" field to be optional, but if present it should be a float.

from pydantic import BaseModel

class Foo(BaseModel):
    count: int
    size: float = None  # how to make this an optional float? 

 >>> Foo(count=5)
 Foo(count=5, size=None)  # GOOD - "size" is not present, value of None is OK


 >>> Foo(count=5, size=None)
 Foo(count=5, size=None) # BAD - if field "size" is present, it should be a float

 # BONUS
 >>> Foo(count=5)
 Foo(count=5)  # BEST - "size" is not present, it is not required to be present, so we don't care about about validating it all.  We are using Foo.json(exclude_unset=True) handles this for us which is fine.

Answer 1

It's possible to do with a validator.

from pydantic import BaseModel, ValidationError, validator

class Foo(BaseModel):
    count: int
    size: float = None

    @validator('size')
    def size_is_some(cls, v):
        if v is None:
            raise ValueError('Cannot set size to None')
        return float(v)

This works as intended:

>>> Foo(count=5)
Foo(count=5, size=None)

>>> Foo(count=5, size=1.6)
Foo(count=5, size=1.6)

>>> Foo(count=5, size=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "[venv]/lib/python3.6/site-packages/pydantic/main.py", line 283, in __init__
    raise validation_error
pydantic.error_wrappers.ValidationError: 1 validation error for Foo
size
  Cannot set size to None (type=value_error)