How 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.

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)