With a Pydantic class as follows, I want to transform the foo field by applying a replace operation:
from typing import List from pydantic import BaseModel class MyModel(BaseModel): foo: List[str] my_object = MyModel(foo="hello-there") my_object.foo = [s.replace("-", "_") for s in my_object.foo] How can I do the replace operation right within the class, when the object is created? Without Pydantic I would simply do that within __init(self, foo) but since Pydantic creates its own __init__ implementation I'm not sure how to proceed exactly.
2 Answers
Using a pydantic BaseModel
It seems as you would have to override the basemodels init method, something like this:
from typing import List from pydantic import BaseModel class MyModel(BaseModel): foo: List[str] def __init__(self, **data): data["foo"] = [s.replace("-", "_") for s in data["foo"]] super().__init__(**data) my_object = MyModel(foo=["hello-there"]) print(my_object) # Outputs foo=['hello_there'] Using a Pydantic dataclass
... or you could also turn it into a pydantic dataclass and use the post init dunder provided by pydantic to do other things upon instantiation. e.g:
from typing import List from pydantic.dataclasses import dataclass @dataclass class MyModel(): foo: List[str] def __post_init__(self): self.foo = [s.replace("-", "_") for s in self.foo] my_object = MyModel(foo=["hello-there"]) print(my_object) # Outputs foo=['hello_there'] Use the @validator decorator (in this case with the option each_item=True, since we want the validator to look at each item in the list rather than the list itself):
from typing import List from pydantic import BaseModel, validator class MyModel(BaseModel): foo: List[str] @validator('foo', each_item=True) def replace_hyphen(cls,v): return v.replace("-","_") my_object = MyModel(foo=["hello-there"]) print(my_object) # Outputs foo=['hello_there'] or if you prefer dataclasses:
from typing import List from pydantic import validator from pydantic.dataclasses import dataclass @dataclass class MyModelDC(): foo: List[str] @validator('foo', each_item=True) def replace_hyphen(cls,v): return v.replace("-","_") my_objectDC = MyModelDC(foo=["hello-there"]) print(my_objectDC) # Outputs foo=['hello_there'] 
