["python","fastapi","python-dataclasses","pydantic"]
How to give a Pydantic list field a default value?

I want to create a Pydantic model in which there is a list field, which left uninitialized has a default value of an empty list. Is there an idiomatic way to do this? For Python's built-in dataclass objects you can use field(default_factory=list), however in my own experiments this seems to prevent my Pydantic models from being pickled. A naive implementation might be, something like this: from pydantic import BaseModel class Foo(BaseModel): defaulted_list_field: Sequence[str] = [] # Bad! But we all know not to use a mutable value like the empty-list literal as a default. So what's the correct way to give a Pydantic list-field a default value?
For pydantic you can use mutable default value, like: class Foo(BaseModel): defaulted_list_field: List[str] = [] f1, f2 = Foo(), Foo() f1.defaulted_list_field.append("hey!") print(f1) # defaulted_list_field=['hey!'] print(f2) # defaulted_list_field=[] It will be handled correctly (deep copy) and each model instance will have its own empty list. See "Fields with non-hashable default values" from the documentation. Pydantic also has default_factory parameter. In the case of an empty list, the result will be identical, it is rather used when declaring a field with a default value, you may want it to be dynamic (i.e. different for each model). from typing import List from pydantic import BaseModel, Field from uuid import UUID, uuid4 class Foo(BaseModel): defaulted_list_field: List[str] = Field(default_factory=list) uid: UUID = Field(default_factory=uuid4)