Possible fields values to depend on other values

I have those 3 fields: event, category, subcategory Depending on the name I allow different categories and depending on the category I allow different subcategories. Example:

• If name is "foo" then category can be "foo_1" or "foo_2".
• If category is foo_1 then subcategory can be foo_11 or foo_111.
• If category is foo_2 then subcategory can be foo_22 or foo_222.
• If name is "goo" then category and subcategory can only be none.

How can I do that? I need it for FastAPI validation and if there is a way of doing it more efficiently that would be great.

I couldn't find an easy way to implement this dependence with the Pydantic schema itself. I used model_validator (A newer version of validator suggested in the comments) with a dictionary schema.

Example

Schema defining the the allowed name, category and subcategory combinations:

events_schema = {
    """
    Example:

    "name": {
        "category": ["subcategory", "subcategory"],
        "category": ["subcategory", None], # None If we want to allow a category without a subcategory.
        None: [None]  # If we want to allow events without a category and subcategory.
    },
    """
    
    "a": {
        "aa": ["aaa", "aab"],
        "ab": ["aba", "abb"],
        None: [None]

    },
    "b": {
        "ba": ["bba", "bbb"],
        "bb": ["bca", "bcb"],
        "bc": [None]
    },
    # More events can be added here
}

Use Pydantic's model_validator to validate the request data:

class EventRequest(BaseModel):
    name: str
    category: Optional[str]
    subcategory: Optional[str]

    @model_validator(mode="after")
    def verify_event_hierarchy(self) -> "EventRequest":
        # Check if name exists in the hierarchy
        if self.name not in events_schema:
            raise ValueError(f"Invalid name: '{self.name}'")

        # Check if category exists for the given name
        if self.category not in events_schema[self.name]:
            raise ValueError(f"Invalid category '{self.category}' for name '{self.name}'.")

        # Check if subcategory exists for the given category under the name
        if self.subcategory not in events_schema[self.name].get(self.category, []):
            raise ValueError(
                f"Invalid subcategory '{self.subcategory}' for category '{self.category}' under name '{self.name}'."
            )

        return self