Validators in Python

field_validator for one field, model_validator for the whole object.

from pydantic import BaseModel, field_validator, model_validator


class Session(BaseModel):
    lang: str
    started: int
    finished: int

    @field_validator("lang")
    @classmethod
    def lowercase(cls, value: str) -> str:
        return value.strip().lower()

    @model_validator(mode="after")
    def check_order(self):
        if self.finished < self.started:
            raise ValueError("finished before it started")
        return self


print(Session(lang="  PYTHON ", started=1, finished=5).lang)
try:
    Session(lang="rust", started=9, finished=2)
except ValueError as exc:
    print(str(exc).splitlines()[1].strip())

How it works

1. A field validator returns the cleaned value or raises.
2. mode=\u0022after\u0022 runs once the type has been coerced.
3. A model validator can compare fields against each other.

Keywords and builtins used here

• Session
• as
• check_order
• class
• cls
• def
• except
• if
• int
• lowercase
• print
• raise
• return
• self
• str
• try

The run, in numbers

Lines

25

Characters to type

561

Tokens

147

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 84 seconds.

Type this snippet

Step 3 of 5 in Pydantic models, step 3 of 19 in Web services & data access.

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