[Python] Python Pydantic - how to have an "optional" field but if present required to conform...

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.

Continue reading...