A python crash course demo for how to parse, validate, convert fields data, convert field names and serialize json/dicts using pydantic.

pydantic_crash_course.py

from datetime import datetime, timedelta
from pydantic import BaseModel, Field, validator, ValidationError

# Random json data or something
person_json = {
    "name": "John Doe",
    "age": 30,
    "address": {
        "street": "Main Street",
        "city": "New York",
        "state": "NY",
        "zip": "10001",
    },
    "field_idc_about": "This is a field",
    "field_idc_about2": "This is another field",
    "phone": "123-456-7890",
}

# Pydantic model for address
class Address(BaseModel):
    street: str
    city: str
    state: str
    zip: str

# Pydantic model for person
class Person(BaseModel):
    name: str
    age: int
    address: Address
    phone: str


person = Person(**person_json)
print(f"{person=}")
print(f"{person.name=}")
print(f"{person.address=}")
print(f"{person.address.city=}")
print()

# Notice the fieds_idct_about* are not in the model

# Here is how you can let them be and anything else, allow extra arbitrary fields
class OpenPerson(Person):
    class Config:
        extra = "allow"  # can be: allow, ignore, forbid   --> meaning will let extra fields in

extra_fields_person = OpenPerson(**person_json)
print(f"{extra_fields_person=}")
print(f"{extra_fields_person.field_idc_about=}")  # This can be unpredictable and problematic though
print()


# Example to rename fields
class NamePerson(BaseModel):
    othername: str
    otheraddress: Address
    class Config:
        fields = {
            "othername": "name",  # Field in object: Field in json
            "otheraddress": "address",  # Field in object: Field in json
        }

renamed_field_person = NamePerson(**person_json)
print(f"{renamed_field_person=}")
print(f"{renamed_field_person.othername=}")
print()

# You can also rename using Fiel(..., alias="name")
class AliasPerson(BaseModel):
    othername: str = Field(alias="name")
    otheraddress: Address = Field(alias="address")

aliasperson = AliasPerson(**person_json)
print(f"{aliasperson=}")
print(f"{aliasperson.otheraddress=}")


# You can also use validators
class ValidatorPerson(BaseModel):
    name: str
    age: int
    address: Address
    phone: str

    @validator("age")
    def age_must_be_positive(cls, v):
        if v < 0:
            raise ValueError("Age must be positive")
        return v

validator_person = ValidatorPerson(**person_json)
print(f"{validator_person=}")
print(f"{validator_person.age=}")

# You can convert objects back to dicts
dict_person = person.dict()
print(f"{dict_person=}")
# And then back to objects
try:
    dict_person['age'] = -10
    person2 = ValidatorPerson(**dict_person)
except ValidationError as e:
    print(e)  # But the validation error is raised for negative ages
print()

# You can also convert fields using validators
class BirthDatePerson(BaseModel):
    name: str
    birthdate: datetime
    address: Address
    phone: str

    @validator("birthdate", pre=True)  # pre means before the field is validated with datetime type
    def convert_birthdate(cls, v):
        return datetime.now() - timedelta(days=v * 365)

    class Config:
        fields = {
            "birthdate": "age",
        }


birthdate_person = BirthDatePerson(**person_json)
print(f"{birthdate_person=}")
print(f"{birthdate_person.birthdate=}")
print()

# You can copy objects modifying fields you want
birthdate_person2 = birthdate_person.copy(update={"name": "Jane Doe", "birthdate": datetime.now()})
print(f"{birthdate_person2=}")
print()

# You can also apply conversions and serialise to json strings
class ExportPerson(BaseModel):
    name: str
    birthdate: datetime
    address: Address
    phone: str

    @validator("birthdate", pre=True)
    def convert_birthdate(cls, v):
        return datetime.now() - timedelta(days=v * 365)

    class Config:
        fields = {
            "birthdate": "age",
        }
        json_encoders = {
            datetime: lambda dt: dt.isoformat(),
        }

export_person = ExportPerson(**person_json)
person_json = export_person.json()
print(f"{type(person_json)=}")
print(f"{person_json=}")


# And thousands of other things: https://pydantic-docs.helpmanual.io/