[Python] Pydantic V2 @field_validator is not executed

I try to parse and validate a CSV file using the following Pydantic (V2) model.

from typing import Optional
from pydantic import BaseModel, field_validator

class PydanticProduct(BaseModel):
fid: Optional[float]
water: Optional[float]

class ConfigDict:
from_attributes = True

@field_validator('fid', 'water', mode='before')
def check_empty_fields(cls, value) -> float:
if value == '':
return None
try:
float_value = float(value)
except ValueError:
raise ValueError("Unable to parse string as a number. Please provide a valid number.")
return float_value


If the CSV file contents look like this

fid,water
1.0,81.3
1.25,26.3
3.0,31.5


the fields will get converted from strings to float correctly.

On the other hand if the CSV file contents have empty strings

fid,water
1.0,
,26.3
3.0,31.5


then I will get the following error

Input should be a valid number, unable to parse string as a number [type=float_parsing, input_value='', input_type=str]
​

I tried to use the @field_validator with the mode="before" but the problem is that the validation is not executed.

In addition, I noticed that the validation is not executed even if there are no errors (aka. the case without empty strings)

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